Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N11/00—Investigating flow properties of materials, e.g. viscosity, plasticity; Analysing materials by determining flow properties
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N11/00—Investigating flow properties of materials, e.g. viscosity, plasticity; Analysing materials by determining flow properties
  • G01N2011/006—Determining flow properties indirectly by measuring other parameters of the system
  • G01N2011/008—Determining flow properties indirectly by measuring other parameters of the system optical properties

Definitions

• the present invention relates generally to devices and techniques for the measurement of rheology. More specifically, it relates to microrheology of surfactant interfaces using microfabricated probes and to microrheology of bulk materials in very small volumes.
• Fluid interfaces are ubiquitous in industry, technology and life. Interfaces separate and organize biological systems, from organelles to cells to organs, and enable gas exchange in respiration. High-interface foams and emulsions find wide use in industry, food and personal care products: emulsified oil droplets give both taste and texture to a fine espresso.
• Surfactants including traditional amphiphilic fatty acids and phospholipids and non-traditional surfactants, such as colloids, block copolymers and nanoparticles, lower the free energy of interfaces and introduce a kinetic barrier to coalescence, thereby stabilizing these high-interface systems by creating novel microstructures and phases.
• Bo ⁇ drag from interface / drag from subphase ⁇ 8 ⁇ ⁇ ⁇ ⁇
• ⁇ 8 and ⁇ are the viscosities of the interface and the bulk, and P c and A c are the contact perimeter and wetted surface area of the probe.
• Bo establishes a practical lower limit, that can be measured with a given probe.
• High- aspect ratio probes such as knife-edges or magnetic needles maximize sensitivity, and even larger P c /A c ratios can be achieved with micron-scale colloidal spheres or ferromagnetic nanorods.
• surfactants provide better stability in emulsions and foams, while also influencing breakup, coalescence, and drainage processes.
• both static and rheological properties of complex interfaces have seen intense interest in both fundamental science and industrial applications in recent years.
• a clear example where both static and dynamic properties of surfactant monolayers matter is the lung surfactant, which must meet several demands simultaneously.
• a characteristic scale for the surface viscosity r ⁇ s mm ⁇ r ⁇ A c /P c can be defined where Bo ⁇ 1. This represents an approximate scale for the minimum surface viscosity that can be clearly distinguished with a given probe from the viscosity of the bulk.
• the Boussinesq number for a given system depends on material parameters, which are dictated by the properties of the interface itself, and geometric factors, over which one has control. In particular, increased sensitivity can be achieved by maximizing the perimeter to area ratio of the probe, which has been accomplished in two typical ways.
• a second technique is to employ small, interfacially active probes, often driven by their own Brownian motion.
• Brownian motion of fluorescent proteins or lipids which represent perhaps the smallest probes possible, can be related to the viscoelasticity of the interface in which they are located via the Saffman-Delbruck model.
• diffusivities have been measured with both florescence recovery after photobleaching (FRAP) and single molecule tracking.
• microrheology Relating rheological properties to responses of micron-scale (or smaller) probes is called microrheology.
• Two broad classes of microrheology have been developed: passive microrheology, in which the probe is driven by thermal fluctuations, and active microrheology, in which the probe is driven by external forces such as optical tweezers and magnetic tweezers. Active microrheology exerts an external force directly on the probe, while measuring its response. Measurements of the probe resistance encode the rheological response of the material.
• linearresponse measurements one can extract the frequency-dependent, linear viscoelastic moduli by generalizing the Stokes resistance for the probe, following the correspondence principle. For nonlinear forcing, however, the quantitative interpretation is uncertain.
• microrheology can be extended to interfacial rheometry: both passive and active microrheology effectively measure the hydrodynamic resistance of the probe, whether the probe lies at an interface or is fully submerged in a bulk fluid.
• the appropriate hydrodynamic equation must be solved in order to extract the rheological properties (bulk and/or interfacial) of the material from the measured resistance.
• colloids provide critical model systems for a variety of fundamental phenomena in materials and soft matter, and enable a broad range of technological applications from optics to biotechnology.
• the ability to fabricate large quantities of colloids with high uniformity and specified shape and function has thus been greatly desired.
• magnetic colloids can be externally manipulated and controlled, and have been used in applications ranging from photonic crystals, cell sorting, biosensors, drug delivery, biomedical applications, and single-molecule biophysics. Magnetic colloids that can be remotely controlled by applied fields have also been exploited in m-ink, microrheo logical probes, and a variety of self- organizing systems.
• ferro fluids and magneto-rheological fluids are frequently used to make useful materials because of their tuneable dynamic response.
• Another important direction for designing colloids involves shape anisotropy.
• Anisotropic particles offer additional control of light propagation for photonic crystals and resistance against efficient phagocytosis in drug delivery applications. They can also give rise to interesting and unique behaviors that are not found with spherical particles, with examples including self-assembly at fluid/fluid interfaces, novel interactions when dispersed in nematic liquid crystals, and non-Newtonian rheo logical properties.
• Janus particles can be used for drug delivery, where one side is designed to bind specifically to a cell surface, while the other side incorporates a second functionality that binds a particular drug.
• the prior art does not appear to teach particles combining all three features: magnetic, anisotropic and Janus functionality.
• the present invention provides Janus micro-particles of defined, variable shape that exhibit either ferromagnetic properties.
• the applications where such particles open new capabilities in areas of growing interest are the rheology of complex fluid interfaces, and the reversible directed assembly of particles at interfaces.
• the invention provides techniques in which ferromagnetic Janus microdisks serve as sensitive probes for the active, interfacial microrheology of complex fluid/fluid interfaces.
• Surface-active molecules and particles are known to modify the surface energy of fluid interfaces; additionally, they can affect the dynamic behavior of such interfaces materials by imparting a viscosity (or viscoelasticity) to the interface itself.
• the invention provides powerful new techniques to measure the rheological properties of complex fluid interfaces.
• the techniques use probes with all three functionalities: ferromagnetism, amphiphilicity, and two-dimensional shape anisotropy.
• the microprobes 1) are small, yet visible under optical microscopy (1 to 100 ⁇ ); 2) ferromagnetic, so as to enable external forces or torques to be applied; 3) amphiphilic, to ensure the probes physically absorb onto fluid/fluid interfaces; and 4) anisotropic, to enable the orientation of a rotating probe to be tracked optically.
• the invention provides methods to fabricate ferromagnetic, Janus microdisks with two 'buttonholes' which may be circular or non-circular.
• a planar (2D) fabrication strategy enabled by photo-lithography, is ideally suited for these structural properties, as it allows essentially any planar shape to be designed with various functionalities incorporated through layer-by-layer deposition. While there have been recent reports of micrometer-sized particles fabricated by photolithography, the novel introduction of magnetic properties and chemical anisotropy imparts significantly greater utility for the widespread use of these microparticles in a variety of fields.
• the inventors have discovered that a photolithographic process along with evaporative metal deposition offers an extremely versatile route to the synthesis of multifunctional microparticles possessing magnetic, amphiphilic, and anisotropic structural features.
• the ability to control various aspects of these structural features allows micro-fabricated ferromagnetic disks to be fabricated and used as tiny probes of the rheo logical properties of complex fluid interfaces.
• the modularity of this design strategy also allows the corresponding paramagnetic microtriangles to be prepared for the analysis of 2D suspensions at fluid/fluid interfaces.
• the 2D nature of interfaces present unique challenges and opportunities: coupling between the 2D films and the bulk fluids complicates the measurement of surface dynamic properties, but allows the interfacial microstructure to be directly visualized during deformation.
• the present invention provides a novel technique that combines active microrheology with fluorescence microscopy to visualize fluid interfaces as they deform under applied stress, allowing structure and rheology to be correlated on the micron-scale in monolayer films.
• the inventors have discovered that even simple, single-component lipid monolayers can exhibit viscoelasticity, history dependence, a yield stress and hours-long time scales for elastic recoil and aging. Simultaneous visualization of the monolayer under stress shows that the rich dynamical response results from the cooperative dynamics and deformation of liquid-crystalline domains and their boundaries .
• New active microrheology techniques are sensitive to the surface viscoelasticity of a wide range of interfaces.
• Photolithography is used to fabricate micron-scale, ferromagnetic 'microbuttons', whose surface chemistry is tuned to render them amphiphilic (Fig. 3).
• a known torque is applied to the microbuttons by electromagnets to rotate a microbutton at the interface, and the 'buttonholes' are tracked to record the angular displacement as a function of time.
• the result is a micron-scale, two-dimensional (2D) Couette rheometer with P c /A c ⁇ 1/R ⁇ 1/(10 ⁇ ) that can measure surface viscosities of order ⁇ 8 ⁇ 0(1(T 8 ) Nsrrf 1 (
• the technique provides high sensitivity with simultaneous visualization. Such direct correlation is extremely challenging in traditional, three-dimensional (3D) rheology.
• the invention provides, in one aspect, a new technique that combines the advantages of microrheology with the versatility and dynamic range of actively-driven probes.
• inventions have developed a technique that combines the advantages of microrheology with the versatility, dynamic range and control of macroscopic rheology.
• embodiments of the invention use microfabricated ferromagnetic "buttons" as probes, which are externally torqued using electromagnets, and whose orientation are tracked while they rotate.
• rotation rather than translation, the technique avoids many of the issues of prior techniques, and introduces a variety of advantages.
• rotating disks establish a deformation field that is, in principle, pure shear, avoiding complications due to extensional, mixed and compressible flows. The interpretation of measurements using this technique is thus more straightforward, as the hydrodynamic problem is relatively simple, avoiding contact line motion and translation-rotation coupling.
• micron-sized probes provide additional advantages.
• a relatively small sample area is required for experiments: areas are as small as 1 mm 2 are possible for Gibbs monolayers.
• the PJA C ratio for a 10 ⁇ radius probe gives 2-3 orders of magnitude more sensitivity than macroscopic techniques.
• a minimum surface viscosity scale r ⁇ s mm ⁇ 1(T 8 N s m "1 can be achieved.
• Rotating probes hold advantages for nonlinear measurements as well. Arbitrarily large strain can be imposed, and a fully-developed, Lagrangian- steady deformation field can be established. By contrast, the deformation field around translating probes is inherently unsteady in the Lagrangian sense, viewed from the reference frame of material elements. Moreover, for a given B field, a greater probe velocity is established for rotation than translation.
• a simple phospholipid monolayer exhibits a rich variety of dynamical responses, including a linear viscoelastic solid response, yielding, aging and recoiling.
• Direct visualization reveals these phenomena to reflect the cooperative dynamics of individual, interlocked liquid-crystalline domains.
• the invention provides, in one aspect, a device and technique for the measurement of the rheology (visco-elasticity, shear-dependent viscosity and yield stress) of surfactant interfaces using microfabricated probes.
• rheology visco-elasticity, shear-dependent viscosity and yield stress
• These properties are relevant for the dynamics of any multiphase material and systems with fluid interfaces, including foams and emulsions (stability, coarsening, drainage), froth floatation and enhanced oil recovery, food science (stabilizing emulsions), biological systems (lung surfactant monolayers, tear film in the eyes, lipid bilayers for cells and organelles), pharmaceutical materials (protein aggregation at interfaces), suspensions and coatings (formation of surface skin layers), etc.
• Another aspect of the invention enables the measurement of the linear and non-linear rheology (visco-elasticity, shear-dependent viscosity and yield stress) of bulk soft materials, while requiring microliter or smaller sample volumes.
• the ability to make these measurements is important for industries that work with small quantities of precious sample; for example, newly-synthesized or purified formulations that must be characterized to determine whether large-scale production is worthwhile. Examples include pharmaceutical materials (injectibility of drug formulations, early- stage protein aggregation), suspensions and coatings, high-throughput characterization in the chemical or other industries, etc.
• the device uses a microfabricated, permanent magnetic probe with tunable surface chemistry.
• the small size of the probe affords it extra sensitivity over commercial devices, the precise shape enables quantitative extraction of linear and nonlinear rheological properties; the active driving enables measurements in a wide range of interfaces (unlike passive probes, which are in principle sensitive to even weaker interfaces, but which are sensitive to a very limited range of interfaces); the small size also enables it to be used with exceedingly small sample volumes, so that measurements can be made with minute quantities of precious sample, or in parallel arrays for high-throughput characterization and screening.
• Various aspects of the invention may include the following: 1) Microfabricated microbutton probes with different magnetic moments, sizes, shapes, and surface functionalities in order to probe different surfactant layers at fluid/fluid interfaces, or bulk materials with different visco-elastic characteristics. This would be analogous to providing different rheometer bobs for a rheometer. 2) Service whereby microbutton probes are custom-designed with different surface chemistry, shape, magnetic moment, size, etc. 3) An electromagnet array for microrheometry: two- or four-pole electromagnet array designed to allow a surfactant monolayer, a solution of soluble surfactant, a surfactant monolayer between immiscible liquids, or a sample volume of bulk material into the center of the poles.
• microbutton designs e.g. circular probes with 'teeth' - may be designed and used to facilitate other types of measurements, much like vane rheometers are designed to prevent slip between the material and the rheometer surface.
• range of magnetic moments and microbutton materials may be expanded to make them compatible with a greater range of interfaces (both in terms of stiffness and solvents).
• alternate probe shape/chemistry may be used to enable its inclusion into lipid bilayers.
• a ferromagnetic and amphiphilic microbutton probe may be fabricated by a particular novel combination of using photolithography to make structures out of photoresist and evaporating magnetic metal layers onto these photoresist structures.
• the ferromagnetic particles are made and lifted off the wafer, without all the other evaporated magnetic material also lifting off. Furthermore, the fabrication method is able to make these disks with "buttonholes" that enable the disk orientation to be tracked during use. The details of these two significant features are described in the attached appendices.
• shape anisotropy via buttonholes
• ferromagnetism without junk nickel around that causes aggregation, etc.
• amphiphilicity via surface chemistry
• the button probe is torqued into rotation, rather than forced into translation, which avoids compression/expansion in the front/rear of translating probes.
• the button probe may, of course, be forced into translation as well by imposing a magnetic field gradient rather than a uniform field.
• Linear viscoelastic moduli small oscillations at different frequencies
• nonlinear rheological measurements large or infinite strain, at different strain rates, and "creep" (constant torque) to measure yield stresses, strain hardening or softening, shear-thickening or thinning
• Embodiments of the present invention overcome various problems in the art. For example, others have made ferromagnetic nanorods, which can be torqued and used as microrheo logical probes. This does not have the "pure shear" advantage that the present microbutton probe does, its quantitative interpretation is far more complex (and perhaps impossible, depending on exactly what torque is used), it introduces compression and extensional deformations into the monolayer, and its use of large amplitude rotations can disrupt and destroy the interfacial meso-structure responsible for the rheology of interest.
• the invention provides a rheological microprobe consisting of a microdisk which may be circular or non-circular, less than 500 microns in diameter, has a shape anisotropy created by holes through the microdisk, and is composed of a ferromagnetic material.
• the surface of the microdisk is amphiphilic due to surface chemical modifications.
• the invention provides a method for fabricating a rheological microprobe.
• the method includes using photolithography to create microdisk structures out of a photoresist layer, where each of the microdisk structures is circular or non-circular, less than 500 microns in diameter and has a shape anisotropy created by holes which may be circular or non-circular.
• the method also includes evaporatively depositing on the microdisk structures ferromagnetic metal layers, and depositing surface chemical modification layers on the microdisk structures to make the microdisk structures amphiphilic.
• the invention provides an apparatus for interfacial microrheometry of insoluble or soluble surfactants and for the microrheometry of microliter (or smaller) sample volumes of bulk materials.
• the apparatus includes a sample holder suitable for containing a sample and microprobe disk (with different sample holders for insoluble surfactants, which are integrated into a Langmuir Trough, than for soluble surfactant or bulk materials, which require small volumes with a planar interface), an electromagnet array comprising at least one pair of electromagnets positioned on opposite sides of the sample holder, a microscope imager for bright-field visualization of the microprobe disk, and a microprocessor connected to the electromagnet array and microscope imager for driving the electromagnets such that a torque is exerted on the microprobe disk and for processing images from the microscope to determine an orientation of the microprobe disk.
• Fig. 1 shows a schematic of a method for fabrication of microbuttons and images thereof.
• A) Photolithographic process for the microfabrication of ferromagnetic particles SU- 8 particles are initially made photolithographically atop a sacrificial layer. An 0 2 plasma etch removes the bare sacrificial layer between the particles. A ferromagnetic nickel layer is evaporatively deposited on SU-8 particles, followed by a gold layer, and the entire wafer is immediately dunked into a solution of thiol-terminated molecules to form a self-assembled monolayer. Chemical etching and sonication removes the sacrificial layer, releasing the particles but not the metal between the particles.
• Fig. 2. Graphs showing properties of microdisks.
• Ferromagnetic Janus (two-faced) 'microbutton' probes are fabricated photolithographically from SU-8 photoresist, evaporatively coated with ferromagnetic (here nickel, but may be any ferromagnetic material that can be deposited) and gold layers, and made amphiphilic by depositing alkane-thiol or fluorocarbon-thiol monolayers on the gold.
• Fig. 4 A schematic drawing of an apparatus for implementing the measurement technique: a Janus ferromagnetic microbutton is placed within a surfactant layer, where two orthogonal pairs of computer-controlled electromagnets exert a defined torque (stress) on the microbutton, whose rotation (strain) is recorded with bright- field microscopy, along with simultaneous fluorescence video microscopy of the monolayer.
• Fig. 5 Schematic flow chart of a measurement technique: A computer using a LabVIEW program generates a programmed voltage sequence, which is either amplified through a linear amplifier (Oscillatory or steady rotation mode) or a DC power supply (Creep mode) to drive high electric currents ( ⁇ 1 A) through electromagnet coils. The resulting field B(t) exerts a torque on a ferromagnetic microbutton probe, which exerts a shear stress on the interface.
• a CCD camera records the rotational response of the microbutton to the applied torque in order to determine the orientation by tracking two buttonholes in real-time using a LabVIEW program. Fluorescent video camera records simultaneous images of an interface as it is being sheared.
• Fig. 6 illustrates an apparatus according to an embodiment of the invention.
• (C) A aluminum cone with two side slits to suppress stray convective flows. These thin slits allow surfactants to freely move in and out of the cone, while pinning the air- water interface along the rim.
• Fig. 7 Sample holder for soluble surfactants and liquid-liquid interfaces.
• An aluminum cone with 5 -mm circular hole is inserted between two pairs of electromagnets and used to form a planar interface, either for liquid-air systems (soluble surfactants and bulk solutions) or liquid-liquid systems (e.g. oil-water).
• Fig. 8 A Schematic diagram of a custom-built microscope that is capable of simultaneously visualizing florescent molecules at the interfaces and bright-field visualization of microbuttons.
• Fig. 9 Output from a program written in Lab VIEW to track two buttonholes on the microbutton in real-time.
• FIG. 10 illustrates the orientational displacement of a microbutton following a suddenly applied magnetic field.
• the orientation of the microbutton exponentially approaches the direction of the magnetic field.
• Fig. 11 illustrates aspects of rheological measurement according to an embodiment of the invention, showing the theoretical computation used to determine surface rheological quantities from the measured rotational resistance of the microbutton..
• Fig. 13 illustrates history-dependent linear viscoelasticity.
• Fig. 14 illustrates how a fractured monolayer heals and highlights the ability of ferromagnetic microbutton probes to measure aging and recovery of rheological properties.
• Fig. 15 illustrates the ability of ferromagnetic microbutton probes to measure surface yielding and yield stresses.
• (c) shows rotation at 30 Hz.
• the monolayer is divided into two regions: an inner region that flows with the microbutton, deforms continuously and appears domain-free; and an outer region with domains that do not deform significantly.
• the radius Ry of the yielded region (white arrows) is set by the surface yield stress, T s y .
• the viscoelastic moduli of the colloidal monolayer can be determined from the microbutton's rotation in response to an externally imposed oscillatory torque (B- C).
• Fig. 17 Ferromagnetic microbutton probes measure linear viscoelastic moduli of bulk soft materials that quantitatively match traditional macroscopic rheometry.
• the viscoelastic moduli of bulk solutions of xanthan gum are measured using microrheologically, using the ferromagnetic microbutton probes described here (empty symbols) and a traditional cone -plate rheometer (solid symbols).
• the measurements using the microbutton are in excellent agreement with those by a traditional cone-plate rheometer.
• Fig. 18 Evolving visco-elastic properties of the interface of a Bovine Serum Albumin solution, which adsorbs from solution onto the interface and aggregates to form a surface layer whose rheology stiffens with time. This shows
• A Isotherm of palmitic acid (PA) exhibits three different phases: gas and liquid expanded phases for extremely low pressures, liquid condensed phase (tilted) for low pressures (up to ⁇ 24 mN/m), and solid phase (untilted) for high pressures.
• B Rheological properties of PA as a function of surface pressures, comparing with measurements using macroscopic scale needles. For a liquid condensed phase, our microbutton measures ⁇ 8 even for very low surface pressure ⁇ , whereas macroscopic needles cannot measure such low ⁇ 8 . The viscoelasticity changes discontinuously at the tilt-untilt phase transition. For pressures above 24 mN/m (solid phase), the microbutton is capable of measuring the elasticity, whereas a steadily translating needle can not.
• Fig. 20 shows creep compliance measurement of a DPPC monolayer. Using real-time measurements of the orientation of the microbutton, two pairs of electromagnets are used to apply a magnetic field that rotates with the disk, to be perpendicular to the magnetic moment of the disk and therefore to impose a constant torque.
• (A) shows a creep recovery measurement.
• the creeping strain (rotation) of a microbutton in a DPPC monolayer is measured in response to a constant imposed torque; at 300 s the torque is turned off and the microbutton counter-rotates due to the elastic nature of the monolayer.
• FIG. 21 illustrates data related to the characterization of the electromagnets.
• Fig. 22 is a graph illustrating LED light intensity measured by applying 10 Hz oscillatory currents as a function of time to find the time lag between video microscopy and electric current.
• To find maximum intensity we fit the intensity of the light, emitted by LED, with the Gaussian, and fit the current with a sine wave function. We find a 1.6 ms time lag between them.
• the ferromagnetic microbutton probes play the central role in the microrheology techniques of the present invention.
• the active interfacial microrheology technique of the present invention makes use of probes that are 1) small, yet visible under optical microscopy (10 to 100 ⁇ ); 2) ferromagnetic, so as to enable external forces or torques to be applied; 3) amphiphilic, to ensure the probes physically absorb onto fluid/fluid interfaces; and that 4) have tracers to enable optical tracking of the orientation of the probe.
• a 10 nm gold layer is then directly deposited onto the magnetic layer, which allows us to modify their hydrophilicity of the top surface with a self-assembled monolayer of thiol-terminated ligands. (We typically use thiol-terminated fluorocarbons). In this way, we can tune surface chemistry to ensure an amphiphilic character for any particular interface.
• photolithography is used to fabricate micron-scale, ferromagnetic, amphiphilic 'microbutton' probes. Briefly, a 4 inch diameter silicon wafer is cleaned with piranha solution, and a 200 nm sacrificial layer (Omnicoat, Microchem) is spin-coated onto the wafer at 1,000 r.p.m. for 30 s, followed by 1-m thick photoresist (SU8-2001) at 3,000 r.p.m. for 30 s. After baking the photoresist at 95 °C for 1 min, ultraviolet light is exposed through a Chrome mask using a 5X stepper (GCA Autostep 6300 i-line).
• GCA Autostep 6300 i-line 5X stepper
• the wafer After developing the photoresist, the wafer is exposed to 0 2 plasma for 2 min to remove the sacrificial layer. A 150-nm nickel layer is then evaporatively deposited onto the photoresist, followed by a 10-nm gold layer. The wafer is then soaked in lH,lH,2H,2H-perfluorooctanethiol (Sigma) for 8 h to promote the formation of a self-assembled monolayer on the gold surface. Finally, gentle sonication in water releases the microbuttons by dissolving the sacrificial layer.
• Fig. 1 The schematic photolithographic process for production of multifunctional microparticles according to one embodiment of the invention is shown in Fig. 1.
• a 200 nm sacrificial layer (Omnicoat, Microchem) is spin-coated on a 4-inch silicon wafer, followed by a one -micrometer layer of photoresist (SU-8, Microchem).
• the bilayer structure is then baked at 95 °C for 1 min, and photoresist exposed to UV light through a patterned chrome photomask for 3 sec.
• the wafer is exposed to an oxygen plasma at 0.19 Torr for 2 min, which removes the exposed sacrificial layer but not the sacrificial layer buried under the photoresist structures.
• Ferromagnetic functionality is then imparted to the microstructures by depositing a magnetic layer, typically 10-300 nm of nickel, cobalt or iron, but can be any ferromagnetic material that can be deposited.
• a 10 nm gold layer is then directly deposited on the magnetic layer which gives rise to a Janus character and allows for facile functionalization with self-assembled monolayers (SAMs) of a wide variety of thiol-terminated molecules.
• SAMs self-assembled monolayers
• the wafers were submerged in a 1 mM solution of lH,lH,2H,2H-perfluorooctanethiol (Sigma) in ethanol for 8 hours.
• lH,lH,2H,2H-perfluorooctanethiol Sigma
• a wafer of fabricated microdisks was diced after metal deposition into 3 mm by 3 mm sections, each of which contains approximately 10 4 ferromagnetic microdisks.
• the in- plane magnetic properties of microdisks were then measured using a SQUID (MPMS 5XL, Quantum design).
• Fig. 2(a) shows the ferromagnetic properties of a representative batch of 20 um-diameter microdisks incorporating a 150 nm ferromagnetic layer of nickel. It should be noted that the ferromagnetic properties of the microparticles are retained, with hysteresis and saturation of magnetization being observed.
• the microrheological application described above requires a fixed permanent magnetic moment that does not reorient under externally applied fields.
• the coercivity represents the magnetic field required to demagnetize materials via reorientation of the magnetic moments of the domains.
• bulk nickel has extremely low coercivity ( ⁇ 1 Oe).
• the large internal stresses generated during evaporative deposition of Ni films however, increase coercivity by up to two orders of magnitude (Fig. 2(b)).
• the inventors have experimentally studied how the in-plane saturation magnetization of the fabricated thin Ni films changes with thickness. Unlike bulk materials, whose saturation magnetization (Ms) depends linearly on their volume, the saturation magnetization of thin films shows different behavior (Fig. 2(c)).
• an advantage of monolayers of insoluble surfactant at fluid interfaces is that the surfactant concentration can be controlled by using barriers to change the surface area.
• the large interfacial areas typical in Langmuir troughs allow strong convective flows at the interface. Such convective flow rapidly remove any probes from the ⁇ 200 x 200 ⁇ 2 field of view of the microscope, making measurements impossible.
• one embodiment of the invention provides a Langmuir trough (Fig. 6A) to control surfactant concentration while also incorporating several mechanisms to reduce this flow.
• a small aluminum conical cylinder in the center that pins the interface at the 5 mm diameter rim of the cone (Fig. 6C).
• Narrow slits (- 0.5 mm) are notched into cylindrical walls to allow surfactant to flow in and out, with the cylinder wall effectively suppressing convective flows.
• the subphase is kept to a depth of less than 5 mm to further suppress subphase flows.
• the viewing area of the trough is itself isolated from the larger reservoirs and barriers by two narrow 5 mm width channels.
• Two pairs of aligned, independently controlled electromagnets are used to generate magnetic fields of specified magnetic fields in any planar direction.
• Two independent signals are generated for each set of electromagnets, using two channels of a digital analog converter (National Instrument, PCI-6933).
• the signals are passed through a linear audio amplifier (Sony, HDMI 259), which amplifies the voltage by a factor of two, and secures up to 4A in current.
• This current is passed through the electromagnet coils, wrapped around 5 mm diameter pure iron (> 99 %) core, generating a uniform magnetic field between the two electromagnets.
• the current passing through each set of electromagnet coils is recorded with two data acquisition boards (National Instruments, USB-6009).
• Fig. 21a To relate the current to the magnetic field, we measure the magnetic field as a function of current using Hall probe (F.W. Bell Inc.). As seen in Fig. 21a, the magnetic field increases linearly with electric current for all currents imposed. This linear relation is used to relate the actual magnetic field from measured electric current. Having determined the relationship between the static magnetic field and applied current, we determined the response time of the electromagnets. Fig. 21b shows the response of the electric current of the electromagnets following a step change of applied voltage. The inductance time of the electromagnetic circuit is seen to be 1.52 ms, so that frequencies below ⁇ 100 Hz can be accurately imposed. As a complementary technique to confirm this response time, we applied a random signal and measure the power spectral density.
• Fig. 22 shows 10 Hz oscillatory electric currents measured as described above, along with the intensity of light emitted by LED.
• the phase difference between current and light intensity is ⁇ 2 ms, which gives the maximum possible phase lag is ⁇ 4 ms.
• a key feature of our technique is that we can visualize the interface while it deforms.
• a microscope (Fig. 8) that employs a dichroic mirror, where light at the emission frequency of a fluorophore is passed to a fluorescent camera, and the frequency of bright field light for visualizing the buttonholes is reflected to a CCD camera.
• light from a mercury source is passed through an excitation filter, and reflected by a dichroic mirror, and illuminate the interface under investigation.
• Emitted fluorescence light passes through the first dichroic mirror, is reflected by a mirror, and passes through the second dichroic mirror to be imaged on a fluorescence camera (Andor Ixon).
• a second light source whose wavelength does not overlap with the emission spectra of fluorescent dyes, is sent through a color filter. This bright-field light passes the interface, and reflected at the second dichroic mirror to be imaged on a CCD camera (BFC).
• the dichroic filters are interchangeable if needed to change fluorophores.
• Fig. 9 shows the procedure for the tracking algorithm. Starting with a digital image, wherein each pixel has a brightness between 0 and 255, we begin by locating the darkest pixel (assumed to be on the microbutton). We then crop a square twice as big as the disk diameter, centered at the darkest pixel to restrict the search area. To find the center of the disk, we convolute a black square whose side is 0.75 times the disk diameter with this cropped image. The maximum of this convolution represents the disk center.
• Fig. 10 shows the response of a representative microbutton in glycerol to a suddenly applied constant magnetic field.
• a microbutton at glycerol/air interface, and measure the angular displacement of the microbutton as a function of time following the known DC magnetic field.
• a response of the probe to constant field is governed by
• a Janus microbutton is placed within a surfactant layer, where two orthogonal pairs of electromagnets are used to exert a torque on it (Fig. 4).
• the applied magnetic field and thus torque, or stress
• the rotational displacement and thus strain
• the relationship between torque and rotation gives the rotational resistance, from which rheological properties can be computed.
• Fig. 5 shows a general flow diagram for the experimental procedure.
• ) and the direction ⁇ of the magnetic field are imposed externally, and the orientation ⁇ of the probe is determined.
• the direction of the magnetic moment m of the microbutton must be known prior to measurements. We thus initialize all measurements by using an external field to align m, or by using two pairs of electromagnets to impose the magnetic field in a specified direction relative to the direction of the magnetic moment..
• the rotational drag coefficient ⁇ ⁇ ( ⁇ ) is generally complex and frequency-dependent, and depends upon the visco(elastic) moduli of the surface and bulk phases.
• F D 67ir
• the interface surface viscosity ⁇ 8 ) is
• Figs. 13-15 20 highlight experiments on insoluble phospholipid surfactants, spread as a monolayer at the water-air interface. They reveal that the monolayer dynamics of even a single-component monolayer of dipalmitoyl-phosphatidylcholine (DPPC), one of the primary lipids in lung surfactant and ubiquitous in cell membranes, can be far richer than ever expected.
• DPPC dipalmitoyl-phosphatidylcholine
• a small-amplitude, oscillatory magnetic field, Be i ⁇ wi applied perpendicular to the magnetic moment, m, of a microbutton suspended at the interface (Fig.
• the DPPC monolayer had a primarily elastic response (G s ' ⁇ 150 nN rrf 1 ) down to 0.1 Hz, indicating that the monolayer stored elastic energy, without appreciable relaxation, over 10-s time scales (Fig. 13a- b). Above 4Hz, however, G s '(oo) and G s "(oo) crossed, and the monolayer response was primarily viscous (r
• DPPC monolayers in the LC phase respond to a weak applied stress with small elastic deformations of the domains, rather than rearrangement of the domains or the domain boundaries. Large-amplitude deformations, however, drive the monolayer out of its equilibrium microstructure (Fig. 13d).
• the domains deform enough that the boundary forms a continuous, almost circular slip line that effectively fractures the material.
• the microbutton and domains within the slip line rotate freely, with minimal deformation of the domains inside or outside of the slip line, eliminating the elastic response (Fig. 13c,d).
• Fig. 20 shows an alternate method for measuring yield stress, using a constant-torque (creep) mode that does not require epifluorescent or Brewster-Angle Microscopy visualization of the surfactant monolayer.
• our new technique provides an unprecedented ability to correlate structural deformations with rheological response. More generally, our technique can interrogate the dynamical response of a wide variety of fluid/fluid interfaces, of scientific, biological, industrial and technological relevance. For example, lipids, proteins and fatty acids can be added to systematically construct model monolayers of biological relevance, such as the lung surfactant monolayer.
• Fig. 16 highlights an example measurement of the rheology of a particulate monolayer at the interface between two immiscible liquids.
• a ferromagnetic, Janus microbutton probe is made to rotate in an oscillatory fashion within a monolayer of polystyrene colloids. Both the microbutton probe and the colloids are adsorbed at the planar interface between clean, immiscible solvents (pure water and decane). Colloids within the monolayer experience a long-range electrostatic repulsion mediated through the decane, which gives rise to hexagonal crystalline order within the monolayer.
• FIG. 16B shows the frequency-dependent surface visco-elastic shear moduli of the colloidal monolayer itself— a low-frequency elastic plateau is observed, along with increasing visco-elasticity at frequencies above the diffusive relaxation time of colloids within their well. Additionally, Fig. 16C shows the weakening— and eventual yielding— of the colloidal monolayer for increasing applied strain, which can be directly correlated with the onset of lattice hopping by particles.
• Viscoelastic bulk material Xanthan gum
• BSA Bovine Serum Albumin
• a monolayer in addition to Gibbs monolayers is a Langmuir monolayer, which is not soluble in water. It spreads and stays at the interface when the surfactant solution is spread using organic solvents.
• PA palmitic acid
• PA palmitic acid
• We prepare a 1 mg/ml solution of PA in chloroform which we spread using a microsyringe on a clean air/water interface in a Langmuir trough.
• the surface concentration ⁇ is controlled by moving teflon barriers to change the area of the monolayer, and the surface pressure is measured as a function of the concentration using Wilhelmy plate (area/molecule).
• Fig. 19 shows the equilibrium phase behavior of PA at 20 ⁇ 2 °C.
• PA exhibits three phases at room temperature as pressure increases: a gas for ⁇ ⁇ 0, a condensed liquid (tilted) L 2 " phase for ⁇ ⁇ 24 mN/m, and solid phase (untilted) for ⁇ >24 mN/m.
• G s ' and G s " of the PA monolayer at 20 ⁇ 2 °C as a function of surface pressures.
• PA shows a primarily viscous response at 1 Hz.
• G s " increases exponentially with surface pressure, as expected from the free area model. This is the two-dimensional analog of the (three-dimensional) free volume model, which postulates that the viscosity of a liquid increases exponentially with the inverse of the free area available to each molecule.

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