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WO2018033584A1 - Nanocristaux auto-assemblés - Google Patents

Nanocristaux auto-assemblés Download PDF

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Publication number
WO2018033584A1
WO2018033584A1 PCT/EP2017/070791 EP2017070791W WO2018033584A1 WO 2018033584 A1 WO2018033584 A1 WO 2018033584A1 EP 2017070791 W EP2017070791 W EP 2017070791W WO 2018033584 A1 WO2018033584 A1 WO 2018033584A1
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Prior art keywords
phase
particle
nanocrystal
nanocrystals
droplet
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English (en)
Inventor
Silvia Vignolini
Richard Parker
Bruno FRKA-PETESIC
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Cambridge Enterprise Ltd
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Cambridge Enterprise Ltd
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Priority to EP17752392.5A priority Critical patent/EP3500651A1/fr
Publication of WO2018033584A1 publication Critical patent/WO2018033584A1/fr
Anticipated expiration legal-status Critical
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    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K19/00—Liquid crystal materials
    • C09K19/04—Liquid crystal materials characterised by the chemical structure of the liquid crystal components, e.g. by a specific unit
    • C09K19/38—Polymers
    • C09K19/3804—Polymers with mesogenic groups in the main chain
    • C09K19/3819—Polysaccharides or derivatives thereof
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K2219/00—Aspects relating to the form of the liquid crystal [LC] material, or by the technical area in which LC material are used

Definitions

  • the present invention provides particles, such as microparticles, having self-assembled nanocrystals in a chiral nematic phase, and the use of the particles as dyes, for example to dye articles such as paper, card and clothes. Also provided are methods for preparing the particles by fluidic methods.
  • cellulose has attracted a growing interest due to its abundance and versatility when processed on the nanoscale in the form of cellulose nanocrystals.
  • cellulose nanocrystals can be extracted from a variety of natural sources, producing stable aqueous suspensions that exhibit cholesteric liquid crystalline behaviour at higher concentrations.
  • a cellulose nanocrystal suspension on a flat substrate results in the formation of a solid film with a periodic chiral structure that can reflect visible light.
  • Revol et al. J. Pulp Paper Science 1998, 24, 1466 have described the preparation of reflective films prepared from a suspension of cellulose nanocrystals that was deposited on a Teflon surface. The films were shown to have structural colour in the infra-red, ultraviolet and visible ranges.
  • WO 95/21901 describes a solidified liquid crystal cellulose film prepared from a dispersion containing a chiral nematic phase of the cellulose. However, these is no description of a particle with the chiral nematic phase in a radial alignment. Although WO 95/21901 describes particulates, these are apparently prepared by disruption of the solid film, for example by milling.
  • the self-assembly of colloidal liquid crystals systems has been studied almost exclusively in planar geometries as solid films.
  • the films may be used in a wide variety of applications including pressure or temperature sensors, amongst others.
  • a shell of material is created by flowing a mixture of the molecular liquid crystal RO-TN 615 with chiral dopant CB14 between water-glycerol mixtures to form inner and outer phases, in a nested system.
  • the authors show that polymerization of the shell components maintains optically useful properties in the material, whilst also improving robustness.
  • cholesteric shells may be assembled without any degradation of order, and with a fully retained optical quality. Particles are said to contain a considerable number of defects, which manifest after the polymerization of the assembled shell, to give a product having a reduced reflection.
  • the authors point to the use of very thin shells, having a thickness of 10 helical pitches (5 ⁇ in the system reported), as the most useful, as this allows for complete Bragg reflections of the wavelength and polarisation that match the cholesteric helix.
  • the authors say there is no benefit in making the shell any thicker than this.
  • the work of Geng et al. seeks to optimize the photonic cross communication between crystal shells for the purpose of secure authentication in a sample, and the optimal shell structures are those that give rise to unique optical patterns resulting from that cross communication.
  • the authors point to the use of a randomly generated spatial distribution of those shells to achieve a non-reproducible cross-communication pattern leading to the desired unclonable optical pattern.
  • Musevic reviews earlier work on the formation of a microlaser where a chiral nematic liquid crystal is dispersed in an isotropic insoluble medium. Chiral nematic microdroplets are formed, with a diameter of around 40 ⁇ . The presence of a surfactant anchors the liquid crystals to the phase boundary.
  • the internal structure of the droplet is such that there is helical modulation of the chiral nematic liquid crystal, extending from the centre of the droplet to the surface.
  • the product gives rise to a birefringent onion Bragg microresonator. This droplet is said to emit light in all directions, and is therefore said to be useful source of monochromatic light.
  • the systems described here make use of an assembly generated from molecular liquid crystals.
  • Li et al. (Nat. Commun. 2016, 7, 12520) describe the confinement of cholesteric suspensions of cellulose nanocrystals into droplets using microfluidic flow-focusing methods. The authors observe the reorganization of the liquid crystal phase into a monodomain Frank Pryce structure in some circumstances, recognizable between crossed polarizers as a concentric fingerprint pattern superimposed with a Maltese cross. The recorded pitch values were typically about 6 ⁇ . Li et al. was published after the priority date of the present case.
  • Cho et al. (Angew. Chemie Int. Ed. 2016, 55, 14014) report the preparation of droplets and microgels from cellulose nanocrystals using microfluidic flow-focusing methods. The recorded pitch values were in the range 5-6 ⁇ (for 126 ⁇ droplets) and 6-9 ⁇ (for 20 ⁇ droplets). Cho et al. was also published after the priority date of the present case.
  • Li et al. (Proc. Natl. Acad. Sci. 2017, 114, 2137) describe the assembly of nanoparticle arrays in the defects and disinclinations found within cholesteric cellulose nanocrystal droplets. The methods of preparation and analysis are the same as those described in Li et al. ⁇ Nat. Commun. 2016, 7, 12520). Li et al. was published after the priority date of the present case.
  • Cipparrone et al. (Adv. Mater. 2011 , 23, 5773) describe the formation of polymeric microparticles via photopolymerization of a micro-emulsion of molecular liquid crystal droplets.
  • the microspheres were shown to contain a variety of internal configurations depending on the preparation conditions, including a Frank Pryce structure.
  • the pitch was controlled by the addition of a chiral dopant allowing the use of the polymeric microparticles as, for example, a microlaser, when a dye was also incorporated.
  • Cipparrone et al. later describe that the pitch of such a system can be tuned to reflect visible colours (Liquid Crystals Reviews. 2016, 4, 59).
  • the present case provides alternative structures that are based on the hierarchical self-assembly of nanocrystals (colloidal liquid crystals) in a confined geometry such as a droplet.
  • the present invention provides a particle having self-assembled nanocrystals.
  • the nanocrystals form a chiral nematic phase, otherwise known as a cholesteric phase, and this phase has a radial alignment in the particle.
  • the chiral nematic phase is a helical nematic phase.
  • the particles have structural colour, of which the colour may be visible colour, infrared colour or ultraviolet colour.
  • the structural colour derives from the presence of the chiral nematic phase, which gives rise to strong reflections at specific wavelengths.
  • the periodicity of the cholesteric structure influences the structural colour as the cholesteric pitch may be varied by changes to the method of preparation.
  • a particle having a self-assembly of a nanocrystal where the particle has a chiral nematic phase, and the chiral nematic phase has a radial alignment within the particle.
  • the chiral nematic phase may have radial alignment within the particle.
  • the radial alignment of a particle may be determined from the polarization micrograph for example when the particle is viewed through cross-polarizers, optionally with a first-order tint plate.
  • the chiral nematic phase extends throughout the particle, and is not confined to a shell of material.
  • the chiral nematic phase is also preserved during shrinkage, for example where water is at least partially removed from the system.
  • the particles may have a very low size distribution, which is derived from their method of preparation, which allows for the uniform preparation of precursor droplets containing the nanocrystals.
  • the inventors have found that it is possible to reliably measure the cholesteric pitch in particles at concentrations that are usually inaccessible from traditional pitch diagrams owing to the problem of kinetic arrest. Consequently, it is relatively straightforward to study the effects of changes to the methods of preparation on cholesteric pitch, thereby allowing for optimization of the pitch characteristics.
  • the methods of the invention also permit the formation of self-assemblies overall several hours rather than several days or weeks, thereby allowing desirable architectures to be prepared rapidly. This relatively quick synthesis is important in many systems, such as those based on cellulose nanocrystals, where structural errors induced by desulfation of the cellulose nanocrystals becomes prominent over longer assembly routes.
  • the cholesteric pitch in the particles of the invention also matches well with the cholesteric pitch found in films prepared from similar nanocrystal staring materials.
  • the particles may be used in place of films in various applications.
  • the cholesteric pitch in the particles of the invention departs from the cholesteric pitch found in films prepared from similar nanocrystal staring materials.
  • the particles provide alternative structures to those films, and therefore the particles also provide alternative structural colour.
  • a dye composition comprising a particle of the first aspect of the invention.
  • the dye composition may further comprise agents, such as solvents, for dyeing an article, such as food and beverage products, paper or card, or a clothing item, or for coating the composition onto a surface.
  • agents such as solvents
  • the dye composition may be for dyeing an article, or the dye composition may be an ink or a paint, for providing decorative colour to a surface.
  • a method of preparing a particle having a self-assembly of a nanocrystal, where the particle has a chiral nematic phase, such as a particle of the first aspect of the invention comprising the steps of:
  • the method may be a microfluidic method.
  • the methods of the invention allow for the preparation of particles at high volume and with a very small size distribution.
  • the methods of the invention allow the formation of the nanocrystal self-assembly to be followed using standard optical instrumentation, thereby permitting a localized, quantitative investigation of the complex dynamic interaction of nanocrystals in suspension.
  • the methods provide a practical route to obtaining highly hierarchical structures in a confined geometry from the nanometre to the macroscopic scale, using readily available nanocrystals.
  • a method of preparing a particle having a self-assembly of a nanocrystal, where the particle has a chiral nematic phase, such as a particle of the first aspect of the invention comprising the steps of:
  • nanocrystals thereby to generate the particle having a self-assembly of a nanocrystal, where the particle has a chiral nematic phase.
  • Methods for the preparation of the particles of the invention may be performed using simple bulk dispersion methods.
  • Particles prepared by the methods of the invention may be subsequently processed, for example to stabilise the cholesteric phase.
  • Such methods are well known in the art for the processing of films having self-assembled nanocrystals with a chiral nematic phase.
  • a particle of the first aspect of the invention as a dye.
  • a method of dyeing an article using the particle such as within a dye composition, and also provided is an article containing the particle of the invention, which article may be referred to as a dyed article.
  • the particles of the invention are particularly suitable for use as dyes given their structural colour, which includes visible and infrared colours.
  • the particles are not subject to bleaching and do not suffer from angular dependent colour shifts or spatial dispersity of colour wavelength or intensity.
  • Figure 1 shows (a) the phase behavior of cellulose nanocrystal suspensions of increasing concentration, as imaged under cross-polarizers, where a clear transition from pure isotropic to anisotropic phase is observed from left to right; (b) the calculated ratio of anisotropic phase present for each concentration investigated in (a) to compile the phase diagram (crosses). The specific concentrations investigated within microdroplets are indicated by the colored circles; and (c) a polarization micrograph of the generation of microfluidic water-in-oil droplets from a 14.5 wt % suspension of cellulose nanocrystals, as imaged under cross- polarizers (right) and with a first-order tint plate (left), illustrating the initial radial assembly.
  • Figure 2 shows a comparison between (a) theoretical and (b) experimental images obtained from the confinement of cholesteric suspension of CNC within a spherical geometry, when viewed through cross-polarizers (top row) and upon addition of a first-order tint plate (bottom row). Upon loss of water the Maltese cross is retained, until the onset of buckling upon final drying.
  • the CNC was used at an initial concentration of 7.3 wt %.
  • Figure 3 shows (a) a scheme of an evolution pitch diagram.
  • the cholesteric pitch measured in the droplets (blue circles) is compared against a macroscale capillary measured by laser diffraction (red circles) and microscopy (red triangles).
  • the pitch below 2 ⁇ was not measured due to the optical resolution limit.
  • the capillary error bars correspond to the gradient in pitch observed as a function of position within the anisotropic phase; and (b) Schematics illustrating the effect on the helicoidal cholesteric structure upon three-dimensional contraction when confined within a sphere, as occurs after c g (p ⁇ 1 3 , top), compared to unidirectional contraction in a planar geometry (p oc c "1 , bottom).
  • the cellulose nanocrystal was used at an initial concentration of 7.3 wt % in water.
  • Figure 4 shows (a) an image of a dried cellulose nanocrystal microparticle, as imaged in transmission (left) and under cross polarizers with a first-order tint plate (right); and (b-d) SEM images of a dry, buckled cellulose nanocrystal microparticle, showing: (c) the clear ordering of cellulose nanocrystals on the surface and (d) the helicoidal assembly of cellulose nanocrystals with a defined pitch, p, within the particle.
  • Figure 5 shows the polarization optical micrographs of particles developed from droplets containing 14.5 wt % suspension of cellulose nanocrystals in water (left), where the droplet and the particles are viewed through cross-polarizers (top row) and upon addition of a first- order tint plate (bottom row).
  • the initial interference colour pattern is retained upon evaporation to form dry microparticles (right), with no evidence of radial ordering observed during this process.
  • Figure 6 shows the polarization optical micrographs of (a) developing droplets containing 10.9 wt % suspension of cellulose nanocrystals in water (top), where the droplet is viewed through cross-polarizers (right) and upon addition of a first-order tint plate (left); and (b) partially concentrated particles developed from the droplets, where the particle is viewed through cross-polarizers (bottom) and upon addition of a first-order tint plate (top).
  • An initial Maltese cross pattern is observed in (a), which then disappears as the microdroplets exhibit an uncontrolled complex arrangement of the interference colour, which does not improve its order upon concentration of the suspension in the concentration step.
  • Figure 7 shows the polarization optical micrographs of (a) developing droplets containing 7.3 wt % of suspension of cellulose nanocrystals in water, as imaged under cross-polarizers (right) and upon addition of a first-order tint plate (left) showing the initial formation, and subsequent rapid loss, of a Maltese cross pattern; and (b) the resultant microdroplets contain factoids within a predominantly isotropic phase, that upon slow concentration can reorganize into (c) a radially ordered cholesteric phase, as observed under a first order tint plate.
  • Figure 8 shows the polarization optical micrographs of three different partially concentrated particles, all prepared from developing droplets having an initial diameter of 140 ⁇ containing 7.3 wt % of suspension of cellulose nanocrystals in water, as imaged under cross-polarizers (bottom) and upon addition of a first-order tint plate (top), where there is (a) radial order throughout the entire diameter of the droplet, (b) significant radial ordering but with an isotropic region, and (c) a chiral nematic shell containing discrete factoids.
  • Figure 9 shows the polarization optical micrographs of (a) particles developed from droplets containing 5.8 wt % suspension of cellulose nanocrystals in water (left), where the droplet and the particles are viewed through cross-polarizers (bottom row) and upon addition of a first-order tint plate (top row).
  • a chiral nematic shell is formed without formation of intermediate factoids; and
  • the formation of the chiral nematic shell without formation of intermediate factoids leads to an improved yield in radially-assembled particles.
  • Figure 10 shows the change in pitch ( ⁇ ) with the change in cellulose nanocrystal concentration (v/v) as exemplified by the evaporation of water from droplets with an initial diameter of 140 ⁇ (circles) and 50 ⁇ (diamonds) with an initial cellulose nanocrystal concentration of 5.8 wt % (ca. 4% v/v).
  • the diameter of the droplet does not affect the trend in measured cholesteric pitch.
  • Figure 11 shows example SEM of the cross-section of (a) a cast film and (b) dry
  • FIG. 12 shows the change in droplet diameter ( ⁇ ) (and consequentially CNC
  • Figure 13 is a pair of SEM images of a microparticle prepared from a suspension having an increased ionic strength. The images shows a particle in cross-section (left image) with an expansion of the observed helical pitch (right image).
  • Figure 14 is a microscope image of the particles of Figure 13 that have been subsequently washed with ethanol and anisotropically collapsed.
  • the present invention provides particles having self-assembled nanocrystals.
  • the nanocrystals are provided in a chiral nematic phase.
  • the particles of the invention may be reliably and reproducibly prepared by fluidic methods.
  • the fluidic methods are easily adapted and allow for the preparation of particles having defined cholesteric pitches, thereby allowing for the formation of particles having alternative structural colours.
  • the inventors have studied the earlier work of Jativa et al. (Soft Matter 2015, 11, 5364), which is said to describe the confined self-assembly of cellulose nanocrystals in a shrinking droplet.
  • the work in the present case differs in significant aspects from the work of Jativa et al.
  • the particles of the present case are distinguishable from the particles of Jativa et al.
  • the fluidic methods of the present case are also distinguishable from the particles of Jativa et al.
  • Jativa et al. study a shrinking droplet containing a suspension of cellulose nanocrystals in water.
  • the droplets prepared by Jativa et al. are considerably larger than the droplets used in the exemplified methods of the present case.
  • Jativa et al. say that they did not observe any pitch lines or reflection colours in the assembled droplets, where such are expected in the corresponding cellulose nanocrystal films.
  • the article explains that "[t]he absence of black extinction lines in the polarized images of the factoids suggests that the carboxylated cellulose nanocrystals used in this work do not develop a helical arrangement under the dynamic conditions of this study.”
  • the TEM images do show a particle cross section.
  • the authors describe regions of ordering either parallel or perpendicular to the image plane.
  • this at best shows the presence of multiple domains with a nematic-like structure, and it does not show evidence of the helicoidal structure that is required for structural colour from this material.
  • the particles of the present case are therefore clearly distinguishable over the structures described by Jativa et al.
  • Bardet et al. describe the formation of particles that are prepared from cellulose nanocrystal films.
  • a film having a chiral nematic phase and visible light structural colour is prepared, and the film is subsequently ball-milled to give a highly heterogeneous collection of particles.
  • the particles are observed to have structural colour, which is attributable to the retention of the chiral nematic order within the particles.
  • the particles of the present case are distinguishable from those of Bardet et al. in that the present particles have a chiral nematic order with a radial alignment within the particle. Such an ordering cannot be expected from the dry grinding of a nanostructure film.
  • the particles of the invention may be obtained or are obtainable by the methods of preparation described herein.
  • the particles have a chiral nematic phase, and more specifically the particles have a cholesteric order.
  • a particle of the invention has structural colour.
  • the chiral nematic phase permits
  • the colour may be ultraviolet colour, visible colour or infrared colour, and it is preferably visible colour.
  • Visible colour refers to a colour with a wavelength in the range 400 to 700 nm.
  • Infrared colour refers to a colour with a wavelength in the range 700 to 1 mm, most preferably 700 to 5 ⁇ .
  • Ultraviolet colour refers to a colour with a wavelength in the range 100 to 400 nm, most preferably 200 to 400, such as 300 to 400 nm.
  • a reference to structural colour is a reference to the wavelength of light having a maximum reflectivity when normal incident light is directed onto the particle.
  • a particle is distinguishable from a film.
  • the particle has a spherical or substantially spherical structure, including a collapsed spherical structure, or the particle has an elliptical or substantially elliptical structure, including a collapsed elliptical structure.
  • the film has a planar morphology. The structures of the particle and a film are dictated by their methods of manufacture.
  • the particles are formed within a typically spherical droplet, and the particle takes the form of the droplet in which it is prepared, which may then be modified by the subsequent concentration (shrinking) of the droplet.
  • Films are prepared by deposition of a suspension of nanocrystals onto a planar surface, such as described by Revol et al., where a nanocrystal suspension is deposited on a Teflon surface, Park et al., where a nanocrystal suspension is drop-cast onto a glass slide, and Dumanli et al., where the film is generated within a Petri dish.
  • a particle is typically a nanoparticle or a microparticle.
  • the largest dimension of the particle may be in the nanometer or the micrometer range.
  • the particle is a microparticle.
  • the particle may have an average diameter of at most 50, 100, 150, 200, 250 or 500 ⁇ .
  • the particle may have an average diameter of at least 1 , 5, 10 or 20 ⁇ .
  • the average diameter may be in a range selected from the upper and lower limits given above.
  • the average diameter may be within the range 10 to 200 ⁇ .
  • Diameter may refer to the largest cross-section of the particle.
  • the diameter may be determined from a transmission micrograph or SEM images.
  • the diameter of larger particles such as those with an average diameter of 1 ⁇ or more, may be determined from simple optical micrographs.
  • the particles of the invention are generally formed in droplets, which are allowed to shrink during a concentration process. Control of the droplet size therefore provides control of the particle size.
  • a particle refers to a droplet containing the nanocrystals that has been permitted to shrink, thereby forming a self-assembly having a chiral nematic phase. Thus, the particle is a product where the nanocrystals are not entirely in suspension. The self- assembly process is associated with the solidification of the nanocrystals.
  • a particle of the invention may be substantially free of water. However, the particle may also contain some water.
  • the inventors have shown that the concentration of the particle, achieved by removal of a dispersed phase, such as water, alters the pitch of the chiral nematic phase. Thus, concentrating the nanocrystals within the particle has the effect of decreasing the pitch, and thereby shifting the structural colour to shorter wavelengths.
  • the water content of the particle may be 98% v/v or less, 95% v/v or less, such as 90% v/v or less, 80% v/v or less, 70% v/v or less, 60% v/v or less, 50% v/v or less, 40% v/v or less, 30% v/v or less, 20% v/v or less, 10% v/v or less, 5% v/v or less, or 1 % v/v or less.
  • the water content may be 10% v/v at most, 20% v/v at most, or 30% v/v at most.
  • the content of the particle may be expressed as the nanocrystal content, where high nanocrystal contents are preferred.
  • the nanocrystal content may be expressed as a volume percentage of the total volume of the particle.
  • the nanocrystal content of the particle may be at least 2% v/v, at least 5% v/v, at least 6% v/v, at least 7% v/v, at least 8% v/v, at least 9% v/v, at least 10% v/v, at least 20% v/v, at least 30% v/v, at least 40% v/v, at least 50% v/v, at least 60% v/v, at least 70% v/v, at least
  • the nanocrystal content of the particle may be at least 3 wt %, at least 5 wt %, at least
  • the nanocrystal may make up substantially all of the particle (100% v/v or 100% wt %).
  • the particles of the present invention have a chiral nematic phase throughout the internal structure.
  • the particle does not possess a hollow interior.
  • the particle is not a capsule having only a shell of nanocrystals in a chiral nematic phase (a "hollow particle").
  • the distribution of the chiral nematic phase throughout the particle may be seen from the polarized optical microscopy images and the SEM images of the internal contents of the particle.
  • the presence of chiral nematic organization throughout the entire diameter of the particle is seen from the appearance of concentric dark and bright rings within the polarized optical microscopy images.
  • the arrangement of the helical axis is also seen from the characteristic Maltese-cross pattern in the polarized images, which is indicative of the radial alignment of the helical axes.
  • the inventors have found that the formation of the particles of the invention is favoured where the nanocrystal is provided in a suspension having a low level of anisotropy, for example at 20% or below, and most preferably where the level of anisotropy is very low, such as about 0%.
  • the formation of particles having an assembly of nanocrystals in a chiral nematic phase is regularly observed at the preferred nanocrystal concentrations.
  • the particles of the invention may be formed from mixtures where the level of anisotropy is greater than 0%, as the work in the present case shows.
  • the higher anisotropic levels are associated with products having a shell of self-assembled material and an internal space that is occupied by tactoid structures. Thus, it is preferable to use mixtures having lower level of anisotropy.
  • the particles of the present case may be contrasted with the shells described by Geng et al.
  • fluidic techniques are used to develop a shell of molecular liquid crystals, and there is no crystal material provided in the internal space.
  • the particle has a cholesteric structure, which is also referred to as a chiral nematic phase.
  • the particles have a self-assembly that is not a non-helical chiral nematic.
  • the nanocrystals within the particle are in a helicoidal assembly.
  • the helicoidal assembly may have a defined pitch within the particle.
  • the particle of the invention is formed from the self-assembly of nanocrystals within a confined space, such as a discrete region, which may be a droplet.
  • the subsequent concentration of the particle may involve a change in the pitch of the chiral nematic phase.
  • a gelation may be seen, where there is a transition from a liquid-like behaviour to a solid-like behaviour, which is seen from the change in pitch rate of change during the concentration step.
  • the cholesteric pitch, p may be at most 3.0, at most 4.0, at most 5.0, at most 6.0, at most 7.0, at most 8.0, at most 9.0, or at most 10 ⁇ .
  • the cholesteric pitch, p may be at most 2.0 ⁇ .
  • the cholesteric pitch, p may be at least 0.1 , at least 0.2, at least 0.4, at least 0.5, at least 0.6, at least 0.7, at least 0.8, at least 0.9, at least 1.0 or at least 2.0 ⁇ .
  • the cholesteric pitch, p may be in a range selected from the upper and lower limits given above.
  • the cholesteric pitch, p may be within the range 0.1 to 5 ⁇ , such as 0.4 to 4.0 ⁇ , such as 0.4 to 2.0 ⁇ .
  • a particle having a lower cholesteric pitch, p may be associated with a particle having structural colour in the visible range.
  • the cholesteric pitch, p may be within the range 0.2 to 0.7 ⁇ (200 to 700 nm), such as 0.3 to 0.5 ⁇ and such as 0.4 to 0.7 ⁇ .
  • the cholesteric pitch may be controlled by selection of appropriate conditions in the methods of preparation.
  • the pitch may be altered by changes in the nanocrystal concentration within the particle in the second (dispersed) phase.
  • an increase in the nanocrystal concentration is associated with a decrease in the pitch.
  • the increase in concentration may be affected by at least partially removing the dispersed phase, typically water, from the particle, which consequently shrinks the droplet and effectively concentrates the nanocrystals, resulting in a decrease in the pitch.
  • the cholesteric pitch in a shrunken particle is dictated by the cholesteric pitch in the large particle, which is in turn influenced by the nanocrystal mixture used in the preparation methods.
  • the change in cholesteric pitch in a particle during the concentration step follows a relationship of ⁇ 1 at lower nanocrystal concentrations within the particle, and then ⁇ ⁇ 3 at higher nanocrystal concentrations within the particle.
  • Figure 3 in the present case shows the change in pitch with the change in concentration of the nanocrystal in the particle.
  • the pitch change follows the trend p ⁇ c 1 at lower concentration before transitioning to p ⁇ c 1/3 at higher nanocrystal concentrations.
  • the trend line shows a decrease in pitch with an increase in the nanocrystal concentration (as measured by volume percentage).
  • a particle having a reduced pitch such as would give rise to structural colour in the visible range
  • Concentrating the particle having a lower nanocrystal concentration will provide a particle having the described reduced pitch.
  • the intent is to reduce the trend line for the pitch decrease such that the pitch at a particular level of concentration is at the level needed to provide the desired structural colour.
  • a change in the pitch of an at least partially concentrated particle may be modified by altering the concentration at which the change in pitch transitions from ⁇ 1 to ⁇ 1/3 . Where this transition is moved to higher nanocrystals concentration within the particle, the pitch will be reduced, and the wavelength of structural colour will also be reduced (for the reason that the more rapid drop in pitch following p ⁇ c 1 is sustained until greater nanocrystal concentrations, and the slower drop in pitch following ⁇ 1/3 is delayed).
  • the cholesteric pitch may be measured, in a dried particle for example, from SEM images of the particle, where the helicoidal assembly of the nanocrystals is visible, and the spacing between layers can be measured. An example SEM image is provided in Figure 4(d).
  • Polarized optical microscopy may also be used to determine pitch values, as described herein, and is suitable for use with those particles having a water content, and that have not yet been subject to collapse.
  • the particles of the invention are formed from the self-assembly of nanocrystals within the confined geometry of a droplet. This self-assembly is observed during the shrinking of the droplet in the methods of preparation.
  • any nanocrystalline material that is known to form cholesteric structures may be used in the present case.
  • the nanocrystal is a nanocrystal of a chiral compound, such as cellulose.
  • a nanocrystal is typically rod-shaped.
  • the crystal may be elongate with a length dimension considerably greater than the width dimension.
  • a nanocrystal for use in the present case such as a cellulose nanocrystal, may be have a length that is at most 200, at most 500, at most 1 ,000, or at most 1 ,500 nm.
  • the nanocrystal for use in the present case such as a cellulose nanocrystal, may have a length that is at least 50, at least 70, or at least 100 nm.
  • a nanocrystal for use in the present case such as a cellulose nanocrystal, may be have a width that is at most 20, at most 30, at most 50 nm.
  • the nanocrystal for use in the present case such as a cellulose nanocrystal, may have a width that is at least 1 , at least 3, at least 5, or at least 10 nm.
  • the aspect ratio for the nanocrystal may be at least 5, 10, 15 or 20.
  • the aspect ratio for the nanocrystal may be at most 40, 50, 100, 150 or 200.
  • nanocrystals having a low aspect ratio give rise to particles having a buckled, rather that wholly spherical, structure.
  • Such structures have been found to have a reduced pitch compared to unbuckled particles, and the inventors have established that red-coloured particles may be prepared from cellulose nanocrystals when the aspect ratio is low.
  • nanocrystals having a low aspect ratio is not an essential feature for obtaining a particle having structural colour
  • structural colour may be obtained in other ways, such as adaptations to the method of particle synthesis and work-up, as described herein, for example through the use of an additional washing and dry step.
  • Different nanocrystals for use in the present case may have different liquid crystal volume fractions for a set nanocrystal concentration in, for example, water. Below certain concentrations of the nanocrystal in, for example, water, there may be no visible liquid crystal volume. That is, the anisotropic phase may be absent at certain low concentrations of the nanocrystal.
  • the nanocrystals are provided as a mixture, such as a suspension, for example in a second phase, which may be water. Droplets of the mixture are generated in a continuous phase, and these droplets are then concentrated which permits the formation of particles having a chiral nematic phase with a radial alignment.
  • a nanocrystal for use in the present invention may be selected from any nanocrystal that is known to form a chiral nematic phase.
  • Many nanocrystals are known in the art, and have been shown to form chiral nematic phases within, for example, films.
  • nanocrystals for use in the present case include cellulose nanocrystals, chitin nanocrystals, amyloid fibres, fd-viruses and organic colloidal rods, including phytosterol particles, amongst others.
  • a nanocrystal for use in the present case may be a nanocrystal that is known to form a chiral nematic phase, for example within a film, that gives rise to structural colour, and preferably structural colour that is visible or ultraviolet colour, such as ultraviolet colour. These nanocrystals may then be used to develop particles having a visible structural colour. As discussed in further detail below, the inventors have found that particles generally exhibit a reflectance maximum that is at a greater wavelength than the reflective maximum of the corresponding film (that is a film made from the same nanocrystal). Thus, selecting a film having ultraviolet structural colour, may allow formation of a particle having visible structural colour. Selecting a film having visible structural colour, may allow formation of a particle having infrared structural colour.
  • the nanocrystal may be a nanocrystal of a polysaccharide, such as glucose-containing polysaccharides, such as cellulose or a cellulose derivative, or chitin or a chitin derivative.
  • the polysaccharide may be cellulose or a cellulose derivative.
  • nanocrystals of cellulose are used.
  • Methods for the preparation of cellulose nanocrystals (CNCs) are well known in the art.
  • Many types of cellulose are well known in the art.
  • nanocrystals are known, and examples includes those cellulose nanocrystals obtained from different biological sources as well as those nanocrystals prepared in different ways from the same source.
  • a cellulose nanocrystal may be prepared from bacterial cellulose, animal cellulose, cotton cellulose, paper, such as filter-paper cellulose, or wood pulp cellulose. Cellulose for processing into nanocrystals may also be obtained from cotton, tunicate and valonia, for example. Typically, the cellulose material is hydrolysed, such as with acid, in the
  • a preparation process or the cellulose material is oxidized, such as with TEMPO-oxidized cellulose material.
  • a preparation of a cellulose nanocrystal may include sonication of the cellulose sample.
  • Sonication such as tip-sonication
  • a nanocrystal suspension is typically used to improve the stability of that suspension, and the dispersion of the individual nanocrystals. It is also known that such techniques may be used to alter, such as red-shift, the pitch of the chiral nematic phase, although the mechanism for this is not known.
  • the present inventors prepared a particle without an initial sonication of the nanocrystal suspension. Although an initial reduction in pitch was observed, of around 50% compared with the pitch recorded for the particles in the worked examples of the present case, the radial alignment of the chiral nematic phase in the particle was not seen.
  • the second phase may be sonicated, such as tip-sonicated, prior to use in the methods of the invention.
  • the sonication may be varied in terms of the input energy and the duration of sonication, as will be understood to those of skill in the art.
  • the step of sonicating the nanocrystal suspension is not essential, and it is understood by the inventors that changes to the nanocrystal suspension, and changes to the particle drying steps may be made in order to avoid the need for a sonication step.
  • particles having a radial alignment of the chiral nematic phase may be obtained without sonication.
  • a sonication step may be performed when needed, and may be performed where it is desirable to red-shift the pitch of the chiral nematic phase.
  • the processing procedures typically involve purification of the biological source material followed by the separation of the resulting cellulose material into its nanocrystalline components.
  • Known methods for producing cellulose nanocrystals are described by Lagerwall et al. (NPG Asia Materials 2014, 6, e80), the contents of which are hereby incorporate by reference.
  • the cellulose nanocrystal is obtained by sulfuric acid hydrolysis of filter-paper.
  • the cellulose may have sulfate groups, such as sulfate esters, on the surface of the cellulose backbone. Modifications to the cellulose surface by covalent or non-covalent functionalization may be used to modify the self-assembly process. For example, grafting polymers to the nanocrystal may improve colloidal stability without loss of cholesteric liquid crystal behavior, as recently demonstrated by Azzam et al.
  • nanocrystal which may be used to alter the physical and chemical properties of the product particle.
  • Additives may also be added to the nanocrystal, such as acid, base, buffer or salt, to influence the self-assembly of the nanocrystals. These components are discussed in further detail below with respect to the second phase holding the nanocrystals during the methods of preparation. Adaptations to nanocrystals, and adaptations to the compounds for use in forming such nanocrystals, are described in the art. Such adaptations are made with a view to
  • the particles of the invention may be prepared from droplets containing nanocrystals.
  • the particles may be prepared using fluidic methods. Such methods allow for the preparation of a large number of particles having a very small size distribution. However, the particles may be formed in other ways, and the methods of preparation are not limited to fluidic methods.
  • nanocrystals thereby to generate the particle.
  • the particle has a self-assembly of the nanocrystal in a chiral nematic phase.
  • the nematic phase is a helical nematic phase.
  • the nanocrystals may be provided as a suspension in an aqueous second phase.
  • other non-aqueous phases may be used, and the nanocrystals may be provided in an organic solvent second phase, for example in an apolar organic solvent second phase.
  • Step (ii) may be the at least partial contortion of the droplet contents, thereby to form a particle having a chiral nematic phase, where the chiral nematic phase has radial ordering within the particle.
  • concentrating may refer to a drying step.
  • Step (ii) may include evaporation of the second phase into the atmosphere from the discrete region, such as from the droplet. This is less preferred as a concentrating method, as it is difficult to maintain close control of the concentrating rate.
  • Step (ii) may include concentrating the discrete region to dryness.
  • the resulting particle is substantially free of the fluid used as the second phase.
  • Step (ii) may include initially collecting the outflow from the channel, thereby to collect the discrete regions, preferably droplets, prior to concentrating.
  • a dispersion of the second phase is created within the continuous first phase.
  • the second phase is an aqueous phase and the other phase is a water immiscible phase.
  • the first phase is substantially free of an alcohol solvent, such as an alkyl alcohol, such as substantially free of methanol and/or ethanol.
  • an alcohol solvent such as an alkyl alcohol, such as substantially free of methanol and/or ethanol.
  • the second phase is substantially free of an alcohol solvent, such as alkyl alcohol, such as substantially free of methanol and/or ethanol.
  • an alcohol solvent such as alkyl alcohol, such as substantially free of methanol and/or ethanol.
  • the nanocrystal is provided in a flow that is an aqueous phase.
  • the aqueous phase is typically the second phase.
  • the second phase is an aqueous phase.
  • the first phase is a water immiscible phase, for example an organic solvent or an oil phase.
  • the flow of the second phase is brought into contact with the flow of the first phase substantially perpendicular to the first phase.
  • the channel structure may be a T-junction geometry.
  • the path of the channel may follow the path of the flow of the first phase, in which case the second flow will be substantially perpendicular to the resulting combined flow in the channel.
  • the path of the channel may follow the path of the flow of the second phase, in which case the first phase flow will be
  • Methods utilising a T-junction geometry provide discrete regions, typically droplets, of the second phase in the first phase as a result of induced shear forces within the two phase system.
  • the shearing interactions developed in the flow system appear to disrupt radial ordering of nanocrystals at the interface of the two phases, as the discrete region travels along the channel.
  • an additional flow of the first phase is provided.
  • the first phase flows are brought into contact with each side of the second phase flow in a channel, and the flow of phases is then passed through a region of the channel of reduced cross-section (an orifice) thereby to generate a discrete region, preferably a droplet, of the second phase in the channel.
  • Such methods which have an inner second phase flow and two outer first phase flows, are referred to as flow-focussing configurations.
  • Methods using flow-focussing techniques provide discrete regions, typically droplets, of the second phase in the first phase as a result of the outer first phase applying pressure and viscous stresses to the inner second phase, thereby generating a narrow flow of that phase. This narrowed flow then separates into discrete regions, typically droplets, at the orifice or soon after the combined flow has passed through the orifice.
  • the discrete region is a droplet.
  • the discrete region is a slug.
  • the droplet is substantially spherical.
  • the average size of the droplet is at least 0.1 , 0.5, 0.6, 1 , 5, 10, 20, 30, or 40 ⁇ in diameter.
  • the average size of the droplet at most 1 ,000, 600, 400, 200, 150, or 100 ⁇ in diameter.
  • the average size of the droplet is in a range where the minimum and maximum rates are selected from the embodiments above.
  • the average size is in the range 40 to 400 ⁇ .
  • the methods of the invention may be used to form droplets having a desired volume.
  • the methods of the invention are suitable for developing droplets having a very small volume.
  • the average volume of the droplet is at least 1 , at least 5, at least 10, at least 50, at least 100, or at least 500 fl_.
  • the average volume of the droplet is at most 1 , at most 5, at most 10, at most 50, at most 100, at most 500 or at most 1 ,000 pL.
  • the average volume of droplets described by Jativa et al. is in the range 1 to 10 ⁇ _.
  • the discrete region may be passed along the channel to a collection area.
  • the residence time of the discrete region in the channel is not particularly limited. In one embodiment, the residency time is sufficient to allow the formation of self-assemblies within the droplet.
  • Discrete regions of second phase are generated in the channel as the immiscible first phase shears off the second phase. The frequency of shearing is dependent on the flow rate ratio of the two phases.
  • the flow rate is selected so as to provide a set number of discrete regions per unit time (discrete regions, such as droplets, per second).
  • the discrete regions may be prepared at a rate of at most 10,000, at most, 5,000, at most 1 ,000 or at most 500 Hz.
  • the discrete regions may be prepared at a rate of at least 1 , at least 10, at least 50, at least 100, or at least 200 Hz.
  • the discrete regions may be prepared at a rate that is in a range where the minimum and maximum rates are selected from the embodiments above.
  • the rate is in range 100 to 500 Hz.
  • the method comprises the step of (ii) concentrating the discrete region. This step at least partially removes solvent (which may be water) from the discrete region and may be referred to as desolvation.
  • the droplets obtained may simply be allowed to stand at ambient conditions, and the solvent (such as water) permitted to evaporate.
  • the droplets may also be heated to evaporate the solvent.
  • the second phase solvent is permitted to diffuse into the first phase, thereby resulting in the effective concentration of the nanocrystals in the droplet.
  • the first phase has some capacity to absorb and hold the solvent of the second phase.
  • the method of concentrating may also be referred to as the shrinkage of the droplet.
  • the change in droplet size which is a reduction in the droplet size, may be expressed as a change in the droplet diameter over time.
  • the diameter of the droplet may be reduced at a rate of at least 1 , 2, 5, 10 ⁇ /h.
  • the diameter of the droplet may be reduced at a rate of at most 25, 30, 40, 50 or 100 ⁇ /h.
  • the observed decrease in the droplet diameter for large droplets will be less than the observed decrease in the droplet diameter for smaller droplets.
  • a droplet having an initial diameter of around 139 ⁇ shrank at a rate of around 10 to 20 ⁇ /h whilst a droplet having an initial diameter of around 50 ⁇ shrank at a rate of around 25 ⁇ /h.
  • the diameter of the droplet may be reduced at a rate of at most 1 , 2, 5, 10 ⁇ /h.
  • the droplet may be reduced at a very slow rate until the nanocrystal has self-assembled.
  • the inventors have found that a very slow rate of shrinkage, where the solvent loss from the droplet over time is very low, allows for the use of a nanocrystal suspension at a higher concentration. Under such conditions, the homogeneity of the internal particles structure may be improved. A change in the pitch of the produced microparticle was not observed with change in the initial concentration of the nanocrystal, coupled with a change in the drying rate.
  • the rate of solvent loss, typically water loss, from the droplet over time may be altered by changing the concentration of a surfactant contained in the continuous phase.
  • a surfactant contained in the continuous phase.
  • the surfactant concertation is reduced, the water loss from the droplet over time is reduced. It is believed that the surfactant aids transport of water through the oil. If water cannot be removed it locally saturates the continuous phase and further losses from the aqueous droplet are inhibited.
  • the inventors have used this approach to prepare droplets from an almost entirely anisotropic suspension, which can self-assemble over several hours ( ⁇ 24 h) with minimal loss of volume.
  • the inventors noted that the use of higher concentrations of nanocrystal in the nanocrystal suspension (such as the worked example where the CNC concentration was 10.9 wt %) is accompanied by gelation before significant self-assembly could occur. Altering the drying rate, for example by reducing the surfactant concentration in the continuous phase, permits self-assembly for higher concentrations of the nanocrystal.
  • the methods of the invention are not restricted to methods where the formed droplets are predominantly isotropic in initial composition.
  • the methods of the invention typically simply need droplets where the contained suspension has not yet reached kinetic arrest (gelation). It is appreciated that the higher the concentration of the nanocrystals in the suspension, the higher the viscosity of that suspension, and the time necessary for the self-assembly of the nanocrystals also increases.
  • the drying step may be separated into two phases, where the first phase is a slow, preferably very slow, drying step where the nanocrystal is permitted to self-assemble at near constant concertation.
  • the drying rate may be change in a second phase, to allow for the evaporation of the second phase, typically water, from the forming particle. It is believed that the rate of evaporation in this second phase may be less important, as the self-assembly step has already occurred.
  • the method optionally comprises a washing step, whereby the particles obtained are washed with a solvent.
  • the purpose of the washing step may be to remove surfactant (where used) or any other component used in the particle-forming step.
  • a reference to a size of a droplet is also a direct reference to a particle that forms in the droplet, prior to the concentration of the nanocrystal. However, during the concentration process the particle size decreases and the size of the particle ultimately differs from, and is smaller than, the size of the droplet that is generated.
  • the droplet is a droplet formed in a channel of a fluidic device or a droplet that is collected from the channel of such a device.
  • a droplet formed directly after preparation is substantially spherical, and a particle that is formed at this stage is also substantially spherical. Desolvation may result in the collapse of the structure as the spherical edge becomes distorted.
  • the flow rate of the first phase and/or the second phase may be varied to allow preparation of droplets, and therefore particles, of a desired size. As the flow rate of the first phase is increased relative to the second phase, the average size of the droplet decreases, and the formed particle size decreases also.
  • the dimensions and the geometry of the fluidic device may also be used to control the size of the droplets that are generated. Such considerations are familiar to those of skill in the art.
  • the flow rate of the first phase is at least 1.5, 2, 3, 4, 5 or 10 times greater than that of the second phase.
  • the flow rates are typically selected to provide droplets of the second phase in the first phase.
  • the flow rates of the first and the second phases may be selected so as to provide droplets having a desired average diameter.
  • the average particle size may be determined from measurements of a sample of droplets collected from the flow channel using simple microscopy techniques.
  • each droplet is a nanodroplet. That is a droplet have a diameter measureable in nanometres.
  • each droplet is a microdroplet. That is a droplet have a diameter measureable in micrometres.
  • the droplet formed from the fluidic preparation has a narrow size distribution. This may be gauged empirically by observation of the packing of collected droplets. A hexagonal close packing arrangement of collected droplets is indicative of a low monodispersity value, for example.
  • the droplets have a relative standard deviation (RSD) of at most 1.5%, at most 2%, at most 4%, at most 5%, at most 7%, or at most 10%.
  • RSD relative standard deviation
  • the worked examples in the present case show that the droplets have a coefficient of variation of less than 2%. It follows that the particles produced from the droplets may also have the same relative standard deviation if the droplets are concentrated in a uniform manner in step (ii).
  • the relative standard deviation may be a measurement of the largest dimension of the droplets or the resulting particles.
  • Fluidic techniques and more specifically microfluidic techniques, are often considered not amenable to the large-scale preparation of products, such as particles.
  • fluidics particularly microfluidics
  • a massive parallel flow-focussing device may contain 100 or more, such as 500 or more, flow focussing nozzles, each for the preparation of individual streams of a dispersed second phase in a continuous first phase.
  • Such parallel devices are described by, for example, Amstad et al. (Lab chip 2016, 16, 4163) and may be referred to as millipede devices.
  • particles may be prepared by the bulk dispersion of a second phase containing the nanocrystals in a continuous first phase. For example, shaking or vortexing a mixture of the second phase and the first phase, will generate a dispersion of discrete regions of the second phase in the first phase. The nanocrystals are permitted to self-assemble in the discrete regions thereby to form particles having a chiral nematic phase. The discrete regions may then be concentrated.
  • a method of preparing a particle having a self-assembly of a nanocrystal, where the particle has a chiral nematic phase comprising the steps of:
  • nanocrystals thereby to generate the particle having a self-assembly of a nanocrystal, where the particle has a chiral nematic phase.
  • the methods of the invention encompass methods for the generation of emulsions, such as microemulsions, using standard methods of emulsification, such as those methods that are based on simple dispersion and homogenisation, and membrane emulsification.
  • the suspension of the nanocrystal in the second phase is selected for formation of a particle having a chiral nematic phase.
  • the continuous first phase may be selected for the most appropriate concentration rate.
  • mechanical stirring of a bulk mixture typically moderate or rapid stirring, can develop a dispersion of discrete regions of a second phase, such as a dispersion of droplets, at an equilibrium size range in a continuous first phase.
  • This size range is defined by the stirring rate as well as the solvents used in the continuous and dispersed phases, together with any surfactants that are present in those phases.
  • the use of bulk mechanical stirring methods to generate a suitable dispersion of cellulose nanocrystals is described by
  • membrane emulsification methods may also be used as a bulk method to generate a dispersion of discrete regions in a continuous phase.
  • membrane emulsification allows for a large-scale preparation of a dispersion, but does so with some decrease in the dispersed phase homogeneity.
  • the second phase is passed through a porous, such as a microporous, membrane into a flow of the first phase, which passages across the surface of the membrane.
  • the size and distribution of the dispersed discrete regions of the second phase in the continuous first phase depends on a number of factors, including the nature and size of the membrane pores and the degree of coalescence at the membrane surface and within the bulk composition.
  • An increase in the concentration of the nanocrystals in the second phase may be associated with an increase in anisotropy in the mixture.
  • a decrease in the concentration of the nanocrystals in the second phase may be associated with a decrease in anisotropy in the mixture.
  • the cellulose nanocrystals used in the worked example show complete anisotropy in water at around 15 wt % and above. Complete loss of anisotropy is seen at around 6 wt % and below.
  • the concentration of the nanocrystals in the second phase is chosen to provide a mixture where the second phase has low anisotropy (high isotropy).
  • the present inventors have found that the use of nanocrystal mixtures, such as aqueous suspensions, where there is high anisotropy (low isotropy) do not provide particles having a substantial cholesteric phase. Rather, the particles have a disordered anisotropic structure without any particular re-ordering.
  • the use of a completely anisotropic suspension of cellulose nanocrystals in water was found to give a particle lacking order, and the particle remained in a disordered anisotropic state.
  • the suspension of cellulose nanocrystals in water has some isotropic character, with a predominant anisotropy, the particle that results still has a disordered anisotropic state.
  • the inventors have seen that the initial stages of the droplet formation involve an immediate radial ordering of the nanocrystal. However, this arrangement is rapidly disrupted by chaotic advection caused by the shearing interactions in the flow methods. The shear forces are believed to induce multiple topological defects into the forming self-assembly, thereby preventing the formation of a radial geometry. However, when the level of isotropy is increased further (and therefore the level of anisotropy is decreased further) there is a change in the behaviour of the nanocrystals within the droplet. At higher levels of isotropy, the formation of radial order throughout the droplet is observed, and this radial order is carried through into the product particle.
  • the nanocrystals do not form particles where there is radial order throughout the entire diameter. Instead, under very low concentrations shell formation is observed, and these shells have a transient cholesteric order. The shell is disrupted during the concentration process.
  • Changes in anisotropy can be readily achieved by, for example, concentrating or diluting the nanocrystals.
  • the amount of nanocrystal present, to achieve the preferred level of anisotropy, will depend upon on the nanocrystal in question and the liquid that it is suspended in.
  • Lagerwall et al. has shown that the anisotropy levels for a range of cellulose nanocrystals differs for a particular set nanocrystal concentration in water.
  • the nanocrystal concentration is selected appropriately to the nanocrystal for use.
  • cellulose nanocrystals in water are preferably used in a concentration range of from 4 to 10 wt %.
  • the cellulose nanocrystals may be used at a higher concentration whilst still providing a low level of anisotropy.
  • the cellulose nanocrystal concentration may be in the range 15 to 40 wt %.
  • the nanocrystal may be present in the mixture at an amount of at most 7, at most 8, at most 10, most 15, at most 20, at most 25, at most 30, at most 35, at most 40 wt %.
  • the nanocrystal may be present in the mixture at an amount of at least 1 , at least 2, at least 3, at least 4, at least 5, or at least 6 wt %.
  • the nanocrystal may be present in the mixture in an amount selected from a range with the upper and lower limits selected from the values given above.
  • the nanocrystal may be present in the mixture in an amount selected from 4 to 10 wt %.
  • wt % values that are chosen will depend upon the level of anisotropy that results from the use of a particular nanocrystal.
  • a concentration from 4 to 10 wt % is suitable of forming particles having radial order throughout the particle.
  • the amount of nanocrystal used may be expressed in terms of the level of anisotropy within the nanocrystal suspension.
  • the nanocrystal may be present in a mixture where the level of anisotropy is at most 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 or 60 %. Additionally, or alternatively, the nanocrystal may be present in a mixture where the level of anisotropy is at most 65, 70, 75, 80, 85, 90 or 95%.
  • the nanocrystal may be present where the mixture is substantially all anisotropic
  • the nanocrystal may be present in a mixture where the level of anisotropy is at least 0%, at least 0.1 , at least 0.5, at least 1 , at least 2, at least 5 %.
  • the nanocrystal may be present in the mixture where the level of anisotropy selected from a range with the upper and lower limits selected from the values given above.
  • nanocrystal may be present in the mixture where the level of anisotropy is selected from 5 to 60 %.
  • the nanocrystal may be present as a suspension where the isotropic phase dominates.
  • the level of isotropy required for a particular nanocrystal mixture may also be gauged experimentally by preparing droplets using the methods described herein, and the appropriate concentration and levels of isotropy may be determined from the droplets and particles that result having a radial order throughout the entire diameter.
  • the level of anisotropy may be gauged by visualisation of a nanocrystal mixture under cross- polarizers.
  • the relative amounts of isotropic and anisotropic phases may be taken as the relative volumes of each phase in the nanocrystal mixture. See, for example Figure 1 (a) in the present case, where visual images of samples with different levels of anisotropy are shown.
  • particles having a very high level of anisotropy do not give rise to particles having a chiral nematic phase extending through the particle.
  • the fluidic methods yield a particle having a disordered anisotropic structure throughout the particle.
  • a particle of the invention has a chiral nematic phase extending through the particle, as can be seen the polarization micrograph of the particle product.
  • the fluidic methods sometimes yield a capsule having a shell of material only.
  • the internal space is not occupied by a self-assembly having a chiral nematic phase.
  • this occurs only when the concentration of the nanocrystal is very low, and therefore the effect is associated not with the level of anisotropy as such.
  • the second phase is substantially homogenous. Prior to use the second phase may be homogenised, for example, by vortexing a suspension of the nanocrystal in the second phase. Alternatively, the sample suspension may be sonicated. In one embodiment, the discrete region is formed at ambient temperature.
  • the discrete region is formed at about 5, 10, 15, 20, 25, or greater than
  • the particle may be further processed in a further step, for example to stabilise the chiral nematic phase.
  • steps are known in the art for stabilising films having self-assembled nanocrystals with a chiral nematic phase.
  • the preferred nanocrystals for use in the present case are cellulose nanocrystals.
  • Particles prepared from the self-assembly of these nanocrystals may be processed by vacuum treatment or treatment with base, such as potassium hydroxide, in order to remove the sulfate functionality from the cellulose.
  • base such as potassium hydroxide
  • Suitable processing methods for films, which may be adapted for the particles of the present case, are described by Bardet et al. and Giese et al.
  • a particle prepared by the methods of the invention may be washed, for example with an organic solvent, and subsequently dried. It has been found that washing with certain solvents may be used as a method to reduce pitch length in a particle of the invention.
  • a substantially spherical particle may be swelled in a non-dissolving solvent, such as ethanol, and then subsequently dried. The resulting dried particle has a slightly flattened appearance and the recorded pitch is less than that recorded for the spherical particle, and this reduction in pitch may be in the region of 100 to 200 nm.
  • a cellulose nanocrystal particle having a minimum pitch of about 0.7 ⁇ may be washed with ethanol, and then dried, to yield a particle having observable red-coloured regions, which are indicative of a reduction in the pitch length into the visible region.
  • the pitch may be further reduced by washing the particle in a method of solvent-induced compression, as described above.
  • buckling of the particle induced by any means, such as those methods described herein, may be used as a mechanism to blue-shift the pitch.
  • the methods of the present invention call for a flow of a second phase and a flow of a first phase, which is immiscible with the second phase, to be brought together in a channel, thereby to generate a dispersion of the second phase in the first phase.
  • Methods for the generation of a flow of a first phase and a second phase are well known in the art.
  • each flow may be generated from a syringe under the control a programmable syringe pump.
  • Each syringe is loaded with an appropriate second phase or water-immiscible phase.
  • droplets may be collected only when the flows are at the required flow rate.
  • the channel in which the second phase and first phase flows are contacted is not particularly limited.
  • the channel is a microfluidic channel.
  • the channel has a largest cross-section of at most 1 ,000, at most 500, at most 200, at most 100 or at most 50 ⁇ .
  • the channel has a largest cross-section of at least 0.1 , at least 1 , at least 10 or at least 20 ⁇ .
  • the channel may be provided in an appropriate substrate.
  • the substrate is one that will not react with the components used in the formation of the particle, such as the nanocrystals, and such as the first and second phases.
  • the substrate may be a PDMS-based substrate.
  • the channels may be prepared or treated such that they are not wettable by the second phase.
  • the channels may be hydrophobic, including hydrofluoric.
  • the channel may have a depth of at least 0.1 , at least 1 , at least 10 or at least 20 ⁇ .
  • the channel may have a depth of at most 50, at most 80, at most 100 or at most 200 ⁇ .
  • the second phase is immiscible with the first phase.
  • the second phase may be referred to as a dispersed phase, particularly once it has contacted the first phase and is separated into discrete regions, such as droplets.
  • the second phase is an aqueous phase. Therefore, the first phase is water immiscible.
  • the nanocrystal is provided in an aqueous phase, such as a suspension in the aqueous phase. This is the second phase.
  • the second phase is a non-aqueous phase.
  • the first phase is immiscible with this non-aqueous phase.
  • the self-assembly of nanocrystals, such as cellulose nanocrystals, in non-aqueous liquid is known.
  • organic, such as apolar organic, liquids may be used, and the nanocrystal may be provided as a suspension in these fluids.
  • Elazzouzi-Hafraoui et al. describe the self-assembly and chiral nematic properties of cellulose nanocrystals from suspensions in cyclohexane. Cheung et al.
  • a second phase may be a combination of liquids, such as water and a second liquid.
  • the second phase may be a combination of water and an alcohol, such as butanol, methanol or ethanol, although this is less preferred.
  • the combination of water and alcohol is typically used where the first phase is an organic oil.
  • the flow rate of the second phase is at most 1 ,000, at most 500, at most 250, at most 200, at most 150 or at most 100 ⁇ ⁇ .
  • the flow rate of the second phase is at least 0.05, at least 0.1 , at least 0.5, at least 1 , at least 5, at least 10, or at least 50 ⁇ ⁇ .
  • the flow rate of the second phase is in a range where the minimum and maximum rates are selected from the embodiments above.
  • the flow rate of the second phase in the range 50 to 150 ⁇ ⁇ .
  • the flow rate of the second phase refers to the flow rate of that phase before the phase is contacted with the first phase.
  • a flow of the second phase at 80 ⁇ -Jh is used.
  • the second phase may be provided with additional components to modify the formation of the particle during the concentration step and/or to modify the physical or chemical properties of the particle product.
  • a component that is present in the second phase may be become present, such held, within the particle product.
  • the second phase may additionally hold a label, such as a dye, such as a fluorescent dye, to give a labelled particle.
  • a label such as a dye, such as a fluorescent dye
  • Other components such as carbon black, such as particles, such as nanoparticles, of carbon black, may be used to enhance or supplement the structural colour of the particle.
  • colloidal particles such as nanoparticles
  • examples include graphene, gold, silver and quantum dot particles. Such particles may be used where they do not disrupt the chiral nematic phase.
  • the second phase may additionally hold polymers or non-volatile solvents, such as glycerol, whose presence is capable of influencing the behaviour of the particle during the concentration process, for example to reduce the bucking of the particle.
  • polymers or non-volatile solvents such as glycerol
  • these components may also be used to modify, such as increase, the cholesteric pitch in the particle product.
  • Other components, such as surfactants may be provided in the second phase, for example on the surface of the nanocrystals, to improve mechanical properties.
  • the additional components may be added into the second phase by simple admixing of the component with the nanocrystals and the second phase fluid.
  • components that are added to the second phase may preferentially locate to the core of a particle.
  • Li et al. (Nat. Commun. 2016, 7, 12520) note that particles of, for example, metal or carbon may partition to the core of a cholesteric droplet.
  • the droplets and particles described herein may include an additional components, such as microparticles, that are predominantly located in the core of the droplet or particle.
  • the first phase comprises a component that is immiscible with the second phase.
  • the first phase may be referred to as a continuous or carrier phase.
  • the flow rate of the first phase is at most 10,000, at most 5,000, at most 1 ,000, at most 500, or at most 250 ⁇ ⁇
  • the flow rate of the first phase is at least 10, at least 50, or at least 100 ⁇ ⁇ .
  • the flow rate of the first phase is in a range where the minimum and maximum rates are selected from the embodiments above.
  • the flow rate of the first phase in the range 100 to 250 ⁇ _/ ⁇ .
  • the flow rate of the first phase refers to the flow rate of that phase before the phase is contacted with the second phase.
  • the flow rates of the two first phases may be the same.
  • the values given above may refer to the combined flow rate of the two first phases, to the flow rate of each first phase.
  • the first phase may be less dense than the second phase.
  • the second phase will sink within the first phase, and will subsequently not be exposed to the atmosphere. In this situation the rate of loss of the fluid of the second phase, such was water, may be reduced.
  • the inventors have found that a slower concentration of the droplet contents is helpful in controlling the structure of the particle product.
  • the first phase may additionally comprise a surfactant.
  • the surfactant is provided in the first phase in order to stabilise the micro-emulsion that is formed in the fluidic preparation methods.
  • the step of forming the discrete region (such as a droplet) may require the presence of a surfactant.
  • the presence of a surfactant is useful in limiting or preventing the coalescence of the droplets collected.
  • changes in the concentration of the surfactant may be used to alter the rate of water loss from the droplet and particle during the concentration process, and this may be used to alter the structure of the final product particle.
  • the surfactant chosen is not particularly limited, and encompasses any surfactant that is capable of promoting and/or stabilising the formation of discrete regions, such as droplets, of the second phase in the first phase.
  • a surfactant for use in the present case is a non-ionic surfactant, such as the sorbitan oleate surfactant which is available as Span 80 (RTM) from Sigma-Aldrich.
  • Such surfactants are especially useful for stabilising water-in-hydrocarbon oil emulsions, such as water-in hexadecane, for example.
  • the surfactants comprise an oligomeric perfluorinated polyether (PFPE) linked to a polyethyleneglycol.
  • PFPE perfluorinated polyether
  • Such surfactants are especially useful for stabilising water-in- fluorocarbon oil emulsions, for example.
  • the choice of fluid and surfactant for the first phase is taken with a view to the preferred rate of liquid loss from the droplet and the particle during the concentration step.
  • the droplet and the particle may not be exposed to atmosphere during the concentration process, and any loss of the second phase fluid, such as water, from the droplet and the particle is into the first phase only. It is under these circumstances that there is closest control of the liquid loss from the droplet and particle.
  • a droplet or particle may be exposed to the atmosphere, and the liquid loss may be to the atmospheres under controlled conditions, for example in a controlled humidity chamber.
  • arrangements of this type are more complex, and are less preferred.
  • the surfactant is present at most 0.1 %, at most 0.2%, at most 0.5%, at most 0.75%, at most 1 %, at most 2%, at most 5% w/w to the total phase.
  • the surfactant is present at least 0.05% or at least 0.07% w/w to the total phase.
  • second phase has a limited solubility in the first phase.
  • the concentration step may involve loss of liquid, such as water, from the droplet or the particle into the first phase.
  • second phase has a solubility in the first phase of at most 50, at most 20, at most 10, or at most 5 ppmw.
  • the first phase has a solubility in the second phase of at most 50, at most 20, at most 10, or at most 5 ppmw.
  • the present invention calls for the use of an aqueous phase as the dispersed phase in the methods of the invention.
  • Methods for the preparation of suitable aqueous mixtures comprising nanocrystals will be apparent to those of skill in the art, and examples preparations are given in the worked examples of the present case.
  • the nanocrystals are provided as a suspension in water.
  • the aqueous phase may have an ionic strength or a pH that is selected for a particular pitch value in the particle of the invention.
  • changes to the ionic strength or the pH can be used to alter, such as decrease the pitch.
  • an increase in ionic strength may be used to decrease the pitch of the chiral nematic phase in the product particle.
  • the ionic strength may be altered, such as increased, by the addition of a salt and/or an acid and/or a base to the aqueous phase, by the increased concentration of a salt in the aqueous phase, or by the change in acidity of the aqueous phase.
  • the salt is not particularly limited, and it may be an inorganic salt such as NaCI.
  • the acid is not particularly limited, and it may be an inorganic acid such as HCI or H2SO4.
  • the inventors believe that the observed changes in the recorded pitch occur at the limits of the electrolyte concentration. At lower ionic strengths, which is the lower limit in the system (near zero), electrostatic repulsions are dominant and very long-range, promoting kinetic arrest. At low concentrations of the nanocrystal this is generally referred to as a repulsive Wgner glass. At higher ionic strengths, the upper limit in the system, the colloidal stability is disrupted and this leads to either a slow evolution of the particle into an attractive colloidal gel, or to flocculation and precipitation. The specific ionic strength needed to produce a desired result (a desired pitch) will depend upon the nanocrystal, and particular the surface charge of that nanocrystal, and the nanocrystal suspension itself, including the relative concertation of the suspension.
  • the amount of a monovalent salt (such as NaCI) to the mass of nanocrystal is kept below 200 mmol/kg, such as below 100 mmol/kg.
  • the present invention calls for the use of a phase that is immiscible with water.
  • That phase may be an oil-based phase (oil phase) or an organic solvent-based phase (organic phase), or a combination of the two.
  • the water immiscible phase is a liquid phase.
  • the water immiscible phase has as a principal component an organic solvent.
  • the organic solvent is selected from chloroform, octane and hexadecane, such as hexadecane.
  • the oil phase has, as a principal component, an oil.
  • the oil is a liquid at ambient temperature.
  • the oil is an organic oil, such as a hydrocarbon-based oil.
  • a hydrocarbon-based oil for use in the invention is hexadecane.
  • the oil is a mineral oil.
  • the oil is a fluorinated hydrocarbon oil.
  • the oil is a perfluorinated oil.
  • a perfluorinated oil is FC-40 (Fluorinert as available from 3M).
  • the oil is a silicone oil.
  • the water immiscible phase is inert. That is, it does not react with the nanocrystals, or any other component used to form a particle of the invention.
  • the water immiscible phase may be selected for its limited ability to absorb water from the developing particle during the concentration step, for example to a maximum water content as discussed above.
  • a particle of the invention may be analysed by simple bright field microscopy to determine the shape of the particle surface.
  • the images obtained may also be used to determine the cross-section, typically the diameter, of the particle.
  • the particle may also be analysed for shape and cross-section, amongst others, thickness using scanning electron microscopy and atomic force microscopy, such as described in the worked examples of the present case.
  • Polarized optical microscopy may be used to analyse phases within the particles, such as the presence of isotropic and anisotropic phases of the nanocrystal.
  • the worked examples describe the use of polarized optical microscopy with analyses performed in transmission mode with crossed polarizers.
  • SEM images may be used to ascertain the presence of the chiral nematic phase, and may be used to measure the pitch of the helicoidal structures.
  • the particles of the invention are suitable for use as dyes, owing to their ability to provide structural colour, including visible, infrared and ultraviolet colour.
  • the spacing of the layers within the particle may be controlled in order to provide a particle having a desired structural colour.
  • the particles of the invention may be use as dyes or pigments in place of traditional dyes or pigments, for example, in food and beverage colouring, publishing and clothing manufacture.
  • the invention also provides a dye composition comprising the particle of the invention.
  • composition optionally further comprises agents, such as solvents, for dyeing an article, such as paper or card, or a clothing item.
  • agents such as solvents, for dyeing an article, such as paper or card, or a clothing item.
  • Suitable agents for use in a dye composition are well known to those of skill in the art.
  • the composition may be an ink or a paint and the composition may further comprise agents suitable for use in coating the surface to which the ink or paint is to be applied.
  • the composition may a colouring additive for food or beverage, and the additive may comprise further ingestible ingredients for use in the preparation of the food or beverage.
  • the invention also provides an article, such as a foodstuff, beverage, paper or card, or a clothing item, comprising the particle of the invention.
  • the particles of the invention provide the advantage over traditional dyes in that they are not subject to bleaching. Furthermore, particles having structural colour, in contrast with films having structural colour, do not suffer from angular dependent colour shifts, such as those visible to the eye, or spatial dispersity of colour wavelength or intensity. Also provided by the present invention is a method of dyeing an article, such as paper or card, or a clothing item, the method comprising the step of contacting the article with a dye composition comprising the particle of the invention.
  • the particles of the invention may also find use in applications that are unrelated to structural colour. For example, the particles, such as microparticles, may find use as beads, such as microbeads, in consumer and healthcare products, including cleaning compositions and cosmetic compositions.
  • the particles of the invention may be prepared from
  • biopolymers such as polysaccharides, and these may be used to generate beads that have biocompatible and biodegradable properties. This provides an alternative to current microbeads, which are prepared from synthetic polymers, and which suffer from the problem of bio-accumulation, most particularly in the marine environment.
  • Hexadecane (99%) and Span 80 were purchase from Sigma Aldrich and Fluka respectively and were used without further purification.
  • the initial suspension of 14.5 wt % cellulose nanocrystals was prepared from filter paper, as described below, with subsequent formulations diluted with deionized water (Millipore Milli-Q Gradient A10, resistivity
  • Microdroplets were imaged in transmission using a Vision Research Phantom Miro ex4-M fast camera, attached to an Olympus IX-71 inverted microscope (10x-64x objectives).
  • Polarized optical microscopy was performed in transmission with crossed polarizers.
  • a sensitive tint-plate (Olympus U-TP530) was additionally inserted between the crossed polarizers.
  • Atomic Force Microscopy (AFM) images were acquired with an Agilent 5500, collected in tapping mode (OTESPA-R3 tip) and at room temperature over a 25 ⁇ 2 area.
  • AFM samples were prepared by drop casting 10 ⁇ _ of a diluted CNC
  • Cellulose nanocrystals were obtained from the hydrolysis of 'Whatman No.1' cellulose filter paper (30 g) with sulfuric acid (64 wt %, 420 ml_) at 64 °C for 30 min, before quenching using Milli-Q ice and water. Soluble cellulose residues and acid were removed by
  • the suspension was concentrated by heating at 60 °C in a water bath for 12 h, resulting in a 14.5 wt% (approx. 9.4% v/v) suspension of CNC.
  • the initial 14.5 wt % CNC suspension was diluted with deionized water using a high precision scale, vortexed and transferred to a flat capillary of sufficiently large inner dimensions to eliminate any confinement effects (1.00 x 10.00 x 50 mm) and sealed with glass plates and nail polish.
  • the self-assembly was observed after 4 days and later after 95 days with no noticeable change.
  • the pitch was measured as twice the period of the fingerprint pattern, taking either an average over 10 pitch distances or using a FFT of the image processed with ImageJ.
  • (ii) Laser diffraction performed using a laser ( ⁇ 531.8 nm) and observing the diffraction pattern in transmission (images not shown).
  • the pitch was derived using Bragg's law, as adapted by Kahn to include Snell law correction. In order to account for possible sample inhomogeneity in the vertical dimension, the pitch was measured at regular intervals throughout the anisotropic phase.
  • Monodisperse water-in-oil microdroplets were generated within a hydrophobic flow-focusing microfluidic device. These were manufactured from PDMS via soft lithography, whereby: (i) the microchannel network was designed in silico (AutoCAD), (ii) printed as a negative photomask and (iii) transferred onto a silicon wafer spin-coated with SU-8 photoresist via
  • the continuous oil phase and the discrete aqueous phase were injected into the microfluidic device via two syringe pumps (PHD 2000, Harvard Apparatus) with controlled flow rates of 200 ⁇ _. ⁇ 1 and 80 ⁇ _. ⁇ 1 , respectively.
  • PLD 2000 Harvard Apparatus
  • the continuous phase comprised of the organic oil, hexadecane, with 2.0 wt% Span 80 surfactant.
  • the aqueous phase was allowed to slowly diffuse into the oil at RTP, until solid
  • microparticles were formed. During droplet shrinkage a linear decrease in the droplet diameter was observed (Figure 12). This was typically in the range of 10 - 20 ⁇ . ⁇ 1 for the large droplets (0A) and increasing to 25 ⁇ . ⁇ 1 for the smaller droplets (0B) reported in Figure 4 (see ESI). Microparticle Analysis
  • Residual surfactant was removed from dry microparticles by washing with n-hexane, prior to imaging by SEM. To image the interior of a droplet, it was fractured using the following protocol: the particles (on a substrate) were first placed in a nitrogen atmosphere, cooled in liquid nitrogen and finally mechanically crushed. The low temperature made the droplets more brittle, while the low humidity inhibited condensation of water.
  • the coexistence regime follows a higher slope at lower concentrations, with a transition around 9.7 wt % in the present case, and related to the change of pH and ionic strength as the sample concentration varies.
  • the cholesteric pitch in the glass capillaries was determined using both polarized optical microscopy and laser diffraction, as described below.
  • n calculated as the average of the effective optical indices given by Bruggeman modeling (described below).
  • the diffracted light is mainly linearly polarized along the helix axis of the diffracting domains. This contrasts with the case of smaller pitches comparable with the wavelength of visible light. In the latter case, the first order diffraction is observed in reflection. At the limit of normal incidence, the diffracted light is bound in wavelengths (photonic band-gap) and is fully circularly polarized, with no existence of higher order diffraction peaks as long as the cholesteric helix profile remains sinusoidal.
  • Microdroplets of different size (-140 and -50 ⁇ ) both with a low starting concentration of 7.3 wt % were prepared and their change in pitch upon concentration compared in
  • the concentration dependence of the pitch in both cases overlap, with a change of power law (matching previous reports), initially closer to approx. ⁇ 1 and then to cr 5/3 .
  • both these droplets and the capillaries display a polydomain structure, which reduces topological constraint on the pitch relaxation (i.e. the local nematic director on each end of a large cholesteric domain has to rotate fast to accommodate for the creation of more cholesteric bands).
  • aqueous microdroplets are submerged beneath a thin layer of hexadecane oil, and consequentially water loss from the droplets to the air is dependent upon diffusion through this barrier.
  • the rate of water loss was found to be most dependent on two parameters that dominated over any variation in the droplets themselves: (i) the thickness of the oil barrier and (ii) the amount of surfactant present within the oil.
  • the first factor is readily apparent when a large number of droplets are dispersed across a single large oil droplet, with those near the edge shrinking in a matter of minutes compared to hours for those nearer the center.
  • the second factor is attributed to the surfactant acting as a micellar carrier for lost water; if the surfactant is diluted it was observed that the rate of water loss could be massively retarded.
  • a constant surfactant concentration of 2.0 wt % span 80 was used throughout experiments and microdroplets near the edge of the macro-scale oil droplet (where complete loss of water in under an hour prevents radial ordering occurring prior to kinetic arrest) were discounted from all studies.
  • V(t+dt) V(t) - Drate * S(t) * dt
  • V and S represent the volume and the surface of the droplet respectively
  • Drate is the diffusion rate through the interface (in m.s "1 , i.e. homogeneous to a diffusion coefficient multiplied by an interface thickness).
  • the liquid crystal phase is described as an effective birefringent medium with the optical axis parallel to the local nematic axis.
  • Light propagation is then only determined by the phase shift between orthogonally polarized waves and can be numerically implemented with the Mueller matrices method. Images were obtained assuming collimated monochromatic incident light and plotting the intensity of light transmitted through the second polarizer in different positions, using a color scale.
  • ⁇ is the CNCs volume fraction
  • e ⁇ n ⁇ 2
  • e w n w 2
  • the mass fraction [CNC] in wt% and its volume fraction ⁇ are related using the formula:
  • aqueous suspension of cellulose nanocrystals was prepared as described above. To characterize the lyotropic properties of this suspension, it was diluted to give a series of concentrations from 14.5 to 4.7 wt % CNC and the proportion of anisotropic phase was evaluated at each concentration (see Figure 1 (a)). This enabled the construction of a traditional phase diagram, as show in Figure 1 (b). This phase diagram allows for determination of the critical values of CNC concentration for this specific suspension at which the transition from isotropic to anisotropic phase occurs.
  • Microdroplets were generated in a single step as an aqueous emulsion in hexadecane oil within a polydimethylsiloxane (PDMS) flow-focusing microfluidic device, as described in the Experimental Methods.
  • PDMS polydimethylsiloxane
  • Microdroplets with a typical diameter of 140 ⁇ were prepared from a series of CNC concentrations across the phase transition, as indicated by the circles in Figure 1 (b).
  • the optical anisotropy of the suspension allowed for the ordering of CNC domains to be visualized during droplet formation by polarized optical microscopy, as described in the Experimental Methods. This is exemplified with a 14.5 wt % suspension of CNC, as denoted by the uppermost circle in the phase diagram ( Figure 1 (b)).
  • Figure 1 (b) radial ordering of the liquid crystalline structure is observed upon generation of the microfluidic droplets, however mixing within the droplet as it flows along the channel results in microdroplets in a predominantly isotropic phase, containing clearly defined factoids (Figure 7).
  • Figure 7 Upon the loss of water from the droplet the factoids re-arrange, resulting in the formation of an ordered chiral nematic shell, growing inwards from the water- oil interface ( Figure 2).
  • Figure 8 Depending on the number and dimensions of the factoids (which is influenced by the individual composition of each microdroplet), either a chiral nematic shell containing free factoids, or a radial order throughout the entire diameter of the droplet is obtained (Figure 8,).
  • the superimposed Maltese-cross pattern is due to the isoclines of the radial cholesteric helix axis aligned with the axes of the crossed polarizers, in agreement with a planar anchoring of the CNC local director with the droplet interface.
  • Low viscosity and a homogeneous composition are expected to increase the proportion of droplets retaining the radial chiral nematic order, with the in situ formation of a chiral nematic phase expected to reduce the generation of shear-induced topological defects.
  • Figure 4 shows the morphology of CNC micro particles after the complete loss of water.
  • smaller microdroplets 50 ⁇ in diameter
  • the chiral nematic nature can be maintained, evidenced by the clear helicoidal structure observed in the scanning electron microscopy (SEM) image reported in
  • a particle was prepared having a measured pitch of around 1.3 ⁇ (see Figure 3).
  • the preparation was repeated, but with the addition of further acid to the aqueous suspension.
  • the initial suspension contained 7.25 wt% CNC with 8 mM H2SO4 (1 10 mmol/kg CNC) and the pitch of the dried particle prepared from the suspension was determined to be about 0.7 ⁇ .
  • Figure 13 is an SEM cross-section of the microparticle product prepared from this suspension. The particle was embedded in a polymer matrix to allow for cross-sectioning.
  • the added electrolyte was sulfuric acid, but salt (such as NaCI) was seen to act similarly.
  • changes in ionic strength may be use to change the pitch in the particle product.
  • the particles prepared in the method above were swollen in ethanol and subsequently dried, thereby anisotropically collapsing the particle. This method can produce spots of visible colour in the particle. The observed red colour shows in some areas of the particles shows that the pitch has been compressed to around 0.4 ⁇ by this post-treatment.
  • Figure 14 is a microscope image of the particles produced in this method. Red colour spots are clearly visible in the imagery across all the particles produced.
  • a particle was prepared in a continuous phase having a surfactant at a relatively high concertation (2 wt % Span 80 in hexadecane).
  • concentration of the surfactant in the continuous phase was reduced during the particle preparation. It was found that a droplet stored in a dish where the surrounding oil phase containing low surfactant concentration (0.5 wt % Span 80 in hexadecane) evaporated much more slowly than one stored in higher concentration (2 wt % Span 80 in hexadecane), with a linear contraction of the diameter by 1 ⁇ /h and 5 ⁇ /h respectively.

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Abstract

L'invention concerne une particule ayant des nanocristaux auto-assemblés, la particule ayant une phase nématique chirale et la phase nématique chirale ayant un alignement radial à l'intérieur de la particule, une composition de colorant comprenant la particule, et des procédés de préparation de la particule à l'aide de techniques fluidiques.
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WO2020122951A1 (fr) * 2018-12-14 2020-06-18 Hewlett-Packard Development Company, L.P. Puce d'analyse
IT201900022908A1 (it) 2019-12-04 2021-06-04 Univ Della Calabria Micro-fingerprint fotoniche come dispositivi anticontraffazione
WO2021111349A1 (fr) * 2019-12-04 2021-06-10 Universita' Della Calabria Micro-empreintes photoniques en tant que dispositifs anti-contrefaçon
CN115397384A (zh) * 2019-12-12 2022-11-25 太阳化学品色彩与效果有限公司 用于化妆品应用的纤维素纳米晶体效应颜料
CN114250070A (zh) * 2020-09-21 2022-03-29 香港科技大学 仿生手性超结构荧光复合膜及其制备方法和应用
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GB2610186A (en) * 2021-08-24 2023-03-01 E Droguet Benjamin Method for preparing structurally coloured films and pigments
WO2023025863A1 (fr) 2021-08-24 2023-03-02 Silvia Vignolini Procédé de préparation de films et de pigments structurellement colorés
GB2610186B (en) * 2021-08-24 2023-11-29 Sparxell Uk Ltd Method for preparing structurally coloured films and pigments
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CN116925429A (zh) * 2022-04-06 2023-10-24 中国科学院大连化学物理研究所 一种手性圆偏振荧光复合膜及其制备和应用
WO2025168936A1 (fr) 2024-02-05 2025-08-14 Cambridge Enterprise Limited Fabrication de pigments photoniques assistée par pulvérisation

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