WO2006027602A1

WO2006027602A1 - Liquid mixing/reactor device and method

Info

Publication number: WO2006027602A1
Application number: PCT/GB2005/003484
Authority: WO
Inventors: Sebastien Courty; Laurent Costier; Guillaume Lagubeau; Thomas Tixier; Guillaume Fichet
Original assignee: Cambridge University Technical Services Ltd CUTS
Current assignee: Cambridge University Technical Services Ltd CUTS
Priority date: 2004-09-10
Filing date: 2005-09-09
Publication date: 2006-03-16
Anticipated expiration: 2007-03-10
Also published as: GB0420155D0

Abstract

A device and method for mixing or reacting two or more components within a droplet are provided. The droplet (1) is placed on a substrate (2), and excitation means (3) are used to cause the droplet to resonate at one or more excitation frequencies, which comprise one or more eigenfrequencies of the droplet. A generator (4) generates the one or more excitation frequencies. A device and method for detecting one or more eigenfrequencies of a droplet is also provided.

Description

LIQUID MIXING/REACTOR DEVICE AND METHOD

The present invention relates to a device and method for mixing or reacting liquids within a small volume, such as a droplet, by generating an oscillating perturbation which propagates within the droplet.

Mixing processes have been widely studied in the microfluidics community, since biological and chemical assays require multiple mixing steps. In such processes viscous forces dominate any flow of the liquid due to the high surface-to-volume ratio of the microchannels within the droplet. This property is characterized by a non-dimensional number, the Reynolds number Re. In typical microfluidic systems, the Reynolds number is small, which implies a laminar flow inside the channel. The distribution of velocities in a laminar flow is deterministic and most notably characterized by non-intersecting stream lines. If two fluids are pumped through the same microfluidic channel, a well defined interface will be formed between them. When the flow is fully developed, no more convective transport of mass across the interface is observed. This property has been extensively exploited to conduct assays in microfluidic devices, but raises a substantial barrier to efficient mixing since turbulent flows are not possible in the case of a low Reynolds number. Reactions between very small volumes of liquids are also known to be slow and inefficient in current reactor methods for the same reason.

The present invention seeks to solve or at least ameliorate the above problems by providing a device and method for fast and efficient mixing, reaction, dissolution, dispersion, and/or emulsification of components which may comprise liquid/liquid, gas/liquid or solid/liquid components. According to the present invention there is provided a device for mixing or reacting two or more components within a droplet, the device comprising: a substrate upon which the droplet is placed in use; excitation means arranged, in use, to cause the droplet to resonate at one or more excitation frequencies; and a generator for generating the one or more excitation frequencies; wherein the one or more excitation frequencies comprise one or more eigenfrequencies of the droplet.

The present invention further provides a method of mixing or reacting two or more components within a droplet, the method comprising the steps of: positioning the droplet upon a substrate; and generating one or more excitation frequencies and resonating the droplet at the one or more excitation frequencies via an excitation means, wherein the one or more excitation frequencies comprise one or more eigenfrequencies of the droplet.

In accordance with a further aspect of the invention, there is provided a device for detecting one or more eigenfrequencies of a droplet, the device comprising: a substrate upon which the droplet is placed in use; excitation means arranged, in use, to cause the droplet to resonate at one or more excitation frequencies; a detector for detecting the one or more eigenfrequencies at which the droplet resonates.

The present invention further provides a method of detecting one or more eigenfrequencies of a droplet, the method comprising the steps of: positioning the droplet upon a substrate; generating one or more excitation frequencies and resonating the droplet at the one or more excitation frequencies via an excitation means; and detecting the one or more eigenfrequencies at which the droplet resonates.

In the present invention, the major interest in exciting surface eigenmodes is the internal flow it induces. The trajectories of fluid particles are given by the gradient of the velocity potential gradψ since the amplitude of oscillation is small. Mixing can be optimised by the present invention by using several eigenfrequencies at the same time, and furthermore by controlling singularities and surrounding streamlines which appear as a function of the eigenfrequencies, which can be manipulated to create a compartmentalised mixer or reactor.

Mixing processes involved in chemical and biological assays or microfluidic devices are often based on the concept of applying vibrations to a container filled with a liquid to be mixed, and the closed container itself must be physically shaken, or some other vibration applied thereto, in order to mix the liquids. Mixing in microfluidics can be induced by the topology of the channel (grooves etc.). In the present invention, however, the resonance of the liquid itself is manipulated in order to induce internal flow patterns inside the volume of liquid, which can be a partially wetting or non-wetting droplet, by exciting its eigenfrequencies, and thereby efficiently mixing, separating or reacting discrete liquids which form the drop. Mixing can be optimised by the present invention by using several eigenfrequencies at the same time. The present invention therefore provides a cheap, simple and efficient mixing device and method on a microscopic or macroscopic scale which is fully applicable to commercial applications such as stand alone mixers, chemical reactors, mixing in micro or macro arrays, microfluidic systems, and inkjet technology. The device and method of the present invention can operate within a wide range of temperatures, and are preferably configured to accommodate droplet volumes ranging between nl (nanolitres) and ml (millilitres).

The present invention also allows an unknown parameter of a droplet to be determined by resonating the droplet at certain excitation frequencies and detecting eigenfrequencies at which the droplet resonates. The unknown parameter can then be calculated through knowledge of other parameters and one or more detected eigenfrequencies.

Examples of the present invention are described below with reference to the accompanying drawings, in which:

Figure 1 shows a liquid mixer or reactor device according to the present invention;

Figure 2 illustrates the effects of mixing of two liquids according to the present invention by exciting them with square wave or sinusoidal excitation signals (shown by square and circular icons, respectively);

Figures 3a-d illustrate the compartmentalisation of a droplet when it is resonated at four different eigenfrequencies according to the present invention;

Figure 4 shows an example of the present invention as a continuous mixer/reactor; and Figure 5 shows a microfluidic device, where the device of the present invention is integrated into the mixing stage of the device.

Before describing specific examples of the present invention in detail, however, the concept underlying the present invention will be introduced briefly. The concept of the invention is based on the eigenmodes of vibration of a droplet sitting on a substrate, the droplet having a free surface which contacts the surrounding medium.

By oscillating the droplet at these "eigenfrequencies" surface forces generate streamlines within the droplet, creating controllable flows therein. Droplets of incompressible fluids tend to assume their shape from a compromise between the forces of surface tension (or capillary forces) and gravity. This compromise depends on the capillary length K ^'1=

which is obtained by the equality between

Laplace's pressure γ/κ ^~1 and the hydrostatic pressure pqκ^Λ for a liquid of density p. If the radius of the droplet R is higher than the capillary length, gravity dominates the capillary forces. However at the scale of microfluidic systems or for a small droplet volume, gravity can be neglected as the radius of the water droplet is smaller than the capillary length κ^~1.

At the limit of small amplitudes, the nature of capillary oscillations of a spherical or non-wetting droplet, with a radius R, is obtained by derivating Laplace's equation for a surface slightly different from a sphere. In this case, the droplet is not levitating, but is sitting on a substrate. In this case the external pressure P₀ is no longer a constant. By determining the curvature and the pressure difference between the external and internal pressure of the droplet, and knowing that the velocity potential must satisfy Laplace's equation Δψ = 0 with a boundary condition at n=R, the Laplace operator L²(ψ) can be employed in the equation,

to provide a boundary-value problem where the solutions are nonelementary functions, known as "special functions". In the case of a spherical droplet, where the pressure difference is due to the curvature of the droplet, we consider the Dirichlet problem for Laplace's equation in a unit sphere. Inside the droplet, for R < 1 , the potential velocity Δψ = 0 and at the surface of the sphere, for R =1 , the potential velocity ψ(r,θ,φ)=f(r,θ,φ). Consider a solution in the form of a stationary wave ψ = exp(-iωt)f(r,θ,φ). This solution can be represented by spherical harmonics Yi_1nPf(COS(Q)) exp(imφ) which are the restriction to the sphere of a homogeneous harmonic polynomial.

By seeking a particular solution to the problem in the form,

ψ = exp{-iωt)∑ A(l)r%_m{θ ,φ) ₍₂₎

and substituting this, together with the following equation for the external pressure,

into Eq. 1, the following dispersion relation, which gives the eigenfrequencies of capillary oscillations for a non-wetting droplet, is found,

with the surface tension y, the density p, the droplet radius R and the degree of Legendre function /. From this it appears that the dispersion relation depends only on the number of nodal lines /on the sphere.

The above model can be extended to the case of a partially wetting droplet with a contact angle of 0 <θ_m≤2π defined by the Young-Dupre relation. The boundary conditions are modified for this problem to reflect the need to loop the spherical harmonics. The dispersion relation found in this case is a function both of the degree l>2 and the order |m|, depending on wetting defects, and is given by,

Figure 1 shows a device according to the present invention which allows discrete liquids to be mixed or to react within a small liquid volume, such as a liquid droplet. A liquid droplet 1 of controlled volume is placed on a substrate 2, which in turn can be connected to an excitation device 3, such as a piezoelectric device (Ceramitone), pulsed laser, mechanical device, electric or magnetic field generator, acoustic wave generator or air flow generator (a small airgun, for example). The excitation device 3 is driven by a function generator 4 which is capable of delivering square, sinusoidal or triangular signals at fixed or swept frequencies. The frequency range is approximately 10Hz to 5 kHz and the vibration amplitude of the piezoelectric is approximately 0.1mm, at the maximum rms voltage (10V). However, higher frequency up to 20 kHz and higher amplitudes can be used in order to optimize the mixing/reaction process. The excitation device 3 is preferably placed on a heavy mass 5, in order to stabilize the device and isolate the vibration generated to the liquid. Alternatively, the excitation device need not be connected to the substrate, but can excite the droplet without any physical contact with the droplet or the substrate (for example, a pulsed laser can excite the droplet from above). The substrate 2 upon which the liquid droplet 1 is positioned can be reactive

(i.e. a substrate on which a reagent or a chemical/biological sensor is already bound) or non-reactive, and may have a flat or patterned substrate, for example on a nano/micrometric scale, small wells can be provided on the surface of the substrate 2 in order to confine arrays of droplets. Additionally, microchannels can be provided inside the substrate in order to dispense and/or absorb the liquid droplet(s). The contact angle of the liquid droplet on the substrate is dependent on the material thereof, and a substrate on which the contact angle with the liquid is high is preferably used; for example in the case of water on a PTFE film in air, this has a contact angle (θ) of approximately 105°, and generally presents few defects. Other materials such as silica wafer and glass substrate are also possible. Controlled defects on the surface can be created, if required, by adding scratch lines (which can be carefully controlled, for example via UV lithography or microcontact printing) or by juxtaposing two substrates of different wettability.

An important feature of the invention is that the droplet must be provided with a free surface which contacts the surrounding medium (this is preferably gaseous, but can comprise a non-miscibie liquid). The substrate can be an open or a closed container, but in the case of a closed container, the container cannot solely enclose the droplet or volume being mixed/reacted, but must also enclose a surrounding medium. The substrate 2 can be permanently attached to the excitation device 3, or may be temporarily fixed, via screws, clips, double-sided tape or glue, for example, (not shown) to the excitation device 3.

The excitation device 3, which generates an oscillating perturbation on the droplet's surface, can be, for example, a laser (radiative pressure, temperature gradient), a piezoelectric device, an acoustic wave or a vibrating membrane. The excitation device 3 generates periodical (square, sinusoidal or triangular) or non- periodical signals of variable amplitudes and frequencies, thereby causing the substrate and liquid droplet to oscillate. These droplet oscillations result in the creation of internal flows inside the droplet, which mainly depend on the excitation frequencies. These flows can be controlled and hence exploited for a mixing process or in order to provide a compartmentalised reactor. The ideal frequency for mixing or reacting will of course depend on factors such as the dimensions of the droplet and viscosity of the liquid. In the case where a compartmentalised reactor is produced, streamlines within the droplets can be controlled in order to produce several separate areas therein. In this case the excitation signal should be stable in frequency, however this precision is not necessary for non-optimised mixing. The amplitude of the excitation signal should also be variable as, when using reactive substrates, gentle mixing is imperative in order to avoid the destruction of anything bound to the surface of the substrate. The excitation device 3 can also employ a frequency sweep to cover a wide range of excitation frequencies when the parameters of the droplet, such as its size, density and viscosity, are unknown, or where further information such as the contact angle of the droplet on the substrate (which depends on factors such as the components of the droplet, the substrate and the surrounding medium) is unknown. The use of a frequency sweep that includes several (at least two) eigenfrequencies is an efficient mixing method which is easy to carry out. For example, when mixing a 20μl droplet of pure glycerol with a 0.4μl droplet of black ink, the application of a 5-second frequency sweep between 200 Hz and 450 Hz results in a mixing time (estimated from image analysis) of less than 20 seconds. An important parameter when mixing as described above is the amplitude of the free surface oscillations produced in the droplet. The relative mixing time does not depend greatly on viscosity when the oscillation amplitude of the free surface is fixed to be a constant, for example, when mixing the ink with each of a droplet pure water and a droplet of pure glycerol by applying the same surface oscillation amplitude, the same approximate mixing time is measured. This is also the case for droplets of intermediate concentrations, i.e. different ratios, of glycerol/water.

The oscillation amplitude of the free surface depends on two parameters: the contact angle - the surface area between the solid substrate and the droplet is preferably as small as possible, however this depends on the nature of the substrate surface; and the amplitude of the substrate oscillations generated by the excitation device 3 (for example, the piezoelectric device). This amplitude will define the resonating amplitude of the free surface, which also depends on the viscosity of the droplet. As viscosity dissipates the energy, it limits the amplitude of oscillations of the free surface. Hence, for example, in a first droplet with a higher viscosity than a second droplet, a more powerful excitation device 3 is required when mixing the first droplet in order to obtain the same free surface oscillation amplitude, and consequently the same mixing time, as that of the second droplet. Generally, when using an excitation device such as a piezoelectric device, more efficient mixing is achieved by using a more powerful piezoelectric device. One notable observation which limits the mixing efficiency is that if the free surface oscillation amplitude is sufficiently high, atomisation of the droplet may occur.

As described above, the mixing time does not depend dramatically on the viscosity. However, for a much more viscous liquid (such as honey, which is approximately 10 times more viscous than glycerol), the waves of the free surface must propagate to the inner volume of the droplet. Due to the existence of an attenuation length (related to the viscosity), the mixing in such a case requires comparatively smaller droplets to achieve the same efficiency.

The device of figure 1 can also include detection means in order to detect the eigenmode(s) produced by resonating the droplet at the appropriate eigenfrequency(ies), however detection means are not necessarily present, as visual observation may be sufficient if the droplet is large enough. Referring again to figure 1 , the droplet can be illuminated, by a light source 6, such as a laser or fibre optic, such that its shape is projected onto a screen for observation. Alternatively, a CCD camera or video camera 7 linked to a monitor 8 is employed, together with a calibration scale for measuring the size, volume and contact angle of the liquid droplet.

Preferably, detection of the eigenmodes is carried out by a CCD camera 7 which is placed above the droplet, and by combining two different optical detection methods: the first of these involves following the trace of a small reflection dot on the droplet; and the second uses a divergent beam shining over the top of the droplet in order that light reflected from the static nodal lines on the surface of the droplet can be detected. By combining these two simple detection methods, it is possible to detect accurately the i m modes and hence calculate the associated eigenfrequencies given by the appropriate dispersion relation. However, the detection of eigenfrequencies can be realized by using more sophisticated methods like interferometry and a laser. The images can be digitalized and analysed if required. The mixing time can also be calculated if necessary by measuring the pixel mean intensity integrated over the droplet. It is possible to excite highly viscous liquids with powerful excitation means.

As all eigenfrequencies are discrete modes, it is crucial that the function generator delivers sinusoidal, square or triangular signals at fixed and stabilised frequencies. Another way in which periodical or non-periodical signals can be generated is by using a sound card or National Instrument card with Labview programs. One advantage of using these type of cards is the ability to generate only one signal with several frequencies (a Fourier transform) and also to derive a program for passing from one signal to another one. When mixing with the above device, other possibilities can be exploited to maximise the process by inducing chaotic flows inside the droplet. This can be achieved by inserting fixed objects, such as rods inserted perpendicular to the surface, by adding soluble beads⁵ to the droplet or by structuring the substrate. The generation of internal flows using the eigenfrequencies of a droplet has been described above. A wide range of applications, especially in the field of mixing and reactor processes, can be exploited for micro-assays and microfluidics devices.

The droplet can be stimulated by sinusoidal and square signals. Figure 2 shows the result obtained with a droplet of water dyed with black ink under sinusoidal wave (shown by circles) and square wave (shown by squares) perturbations. With a square signal, it appears that the mixing process is more efficient. As it contains several harmonics, in which some of them are close to eigenfrequencies, a chaotic regime is created, whereas the flow induced by the sinusoidal signal contains only closed streamlines (loops). A signal composed of a plurality of exact eigenfrequencies provides a much more efficient mixing process, especially where viscous liquids, such as ethyleneglycol, must be mixed.

Various phenomena may be employed to improve the mixing time of the droplet. The "degeneracy modes" are frequencies for which several different eigenmodes exist. The eigenmodes can either alternate in time, or be excited in the same time. Another effect that can lead to degeneracy modes is frequency doubling when the amplitude of the oscillations is increased. This is a non-linear phenomenon that superposes the eigenmodes. Non-linear effects can also be observed when increasing the amplitude of oscillations, leading to a chaotic transition. In this regime, the mixing _. process is optimized since the internal flows are chaotic.

Another interesting effect provided by the present invention is the compartr^ήentalisation of the droplet, which is achieved by exciting the droplet with a signal which is a function of eigenfrequencies for pure modes. Examples of such compartmentalisation within the droplet are shown in figures 3a-d. The diffusion process occurs preferentially in each compartment and also across the domain interface, but in this latter case the diffusion process is slower. These localised flows may be visualised in a number of ways, for example by depositing a water droplet (of, for example, approximately 24 μl) on a teflon substrate, and spreading glass beads on the surface. A sinusoidal signal at an eigenfrequency is then sent to the piezoelectric device. The glass beads remain on the surface and allow visualisation of the surface flows. In another example, after deposition of a pH indicator droplet (again of approximately 24 μl) on a teflon substrate, a sinusoidal signal at an eigenfrequency is sent to the piezoelectric device to induce localized flows. Approximately 0.01 μl of HCI is then injected inside a compartment in order to change the colour of the pH indicator and to point out the localization of the bulk flow within the droplet. Moreover, by changing the eigenfrequency by another one, the orientation of the bulk and surface flows can be modified.

Another possible way in which to mix the contents of the droplet by exciting its eigenfrequencies is through droplet bouncing. By dropping the droplet onto a hot plate (for example at 200 degrees, the Leidenfrost effect) or a hydrophobic surface, the mixing inside the droplet is homogeneous just after the time of impact, as several eigenfrequencies are selected during the impact.

The device and method of the present invention can be employed on a macroscopic or microscopic scale, and are therefore of great advantage in the fields of chemical and biological assays. The device and method of the present invention can be employed on a macroscopic or a microscopic scale, and can be used on single droplets or on arrays of droplets. The droplets can be placed on flat surfaces, or contained with depressions in a surface, or within test-tubes or arrays thereof (provided that a free liquid surface is available for excitation, as described previously). For example, experiments have successfully used the device and method of the present invention to mix arrays of droplets contained within standard microtiter plates (as used for biological and chemical research). In this case, care must be taken to increase the mixing time appropriately with the depth of the liquid, in order to ensure adequate mixing penetrates through to the bottom of each test-tube (since mixing according to the invention takes place from the surface of the liquid). Devices and methods according to the invention can be configured to operate under pressure or in a moderate vacuum by providing a container which encloses the droplet (whilst maintaining a free surface thereof) in conjunction with a pump. The use of radiation, such as UV or IR, is envisaged, and the medium surrounding the droplet can be gaseous or a non-miscible liquid. Figure 4 shows an example of the present invention in which a continuous mixer/reactor is provided. One or more inlet channels 9 provide an initial droplet of liquid to which further materials can be added via further inlets, to a substrate 2, which can be enclosed. Each droplet is vibrated, for example via an excitation device 3 placed below the substrate, at one or more suitable eigenfrequencies until it is sufficiently mixed or reacted at position 11. The resultant droplet is then caused to flow (via, for example, a pump) through one or more respective outlet channels 10, and another initial droplet is dispensed onto the substrate via each inlet channel 9. It is important to note here that the resonance introduced is that of the liquid droplet itself, and not of a mechanical part of the device which is resonated (this latter idea is just another way of providing a physical shaker). In this way, a continuous, efficient and exact mixing or reacting regime is provided.

By using the effect of inducing compartments in the droplet, one or more reactants can be kept in separate areas; the droplet can therefore be considered as a microreactor in which the reagents are mixed during a reaction. This compartmentalisation allows different physical or chemical processes, such as the dissolution of a solid or a gas in the liquid droplet, an interfacial reaction at a droplet/air or droplet/fluid interface, an interfacial liquid-liquid reaction between compartments in the droplet, and processes such as emulsification to be created and observed. In the case of, for example, medical or pharmaceutical applications, the substrate can be an array which is analysed after evaporation or removal of all of the droplets. Alternatively, the array is analysed with the droplets still on the substrate. The invention may further be employed in microfluidic systems, for example in lab-on-chip devices such as those used for testing the DNA of biological-samples. Such a chip is generally composed of different stages, such as a sample loading stage, a drop metering stage, and a mixing stage, in which the device of the present invention is integrated, as shown in Figure 5 in relation to electro-wetting in a microfluidic device. In this case the drop is preferably automatically dispensed and pumped through the device, the mixer acting as a module of the complete device. By imposing controlled motion of the droplet, a chain reaction is created whereby the droplet is translated through successive compartmentalisation and reaction steps induced by a piezoelectric device 3 provided beneath electrodes 12, as described with respect to Figure 1. This mixing is an important issue in microfluidic device as the Reynolds number (which characterises the turbulence of a flow) is always very low due to the small scale of the device. A micro flow is generally non-turbulent, and diffusion is a dominant process for mixing as convection does not easily occur. The present invention allows a chaotic flow to be induced in a very small volume of liquid, hence dramatically improving the mixing time thereof.

The invention can also be employed to measure various parameters of a liquid droplet. The dispersion relation depends on five physical parameters: the droplet volume V, the density p, the viscosity/;, the surface tension y and the contact angle θ_m . One parameter can be determined if the other four are known or are measured with a separate method. For example, V, p, and μ are usually known parameters, and the contact angle θ_m can be measured directly, for example, with a camera and calibration scale on a picture of the droplet. The droplet can then be resonated at a variety of frequencies and, when eigenmodes of the droplet are detected, for example, using the detection means described in relation to figure 1 , the surface tension is calculated.

A further area where the present invention can be employed is in the field of inkjet technology. Current letters of colour mixing in this field consist of spreading three colours in a controlled way at the same point, thus defining the size of one pixel and the resolution of the printing. This method is employed due to the small volume of ink required, and it is not possible to obtain the desired colours by mixing the three droplets at this scale in such a short time. By employing the present invention, the colours can be mixed quickly and efficiently before the ejection of the ink, therefore improving the resolution of the print and reducing the overall amount of ink required.

The present invention provides a device and method for fast mixing, reaction, dissolution, dispersion, and/or emulsification of a fixed volume of liquid, for example a liquid droplet, based on the eigenmode frequencies of a droplet, which are directly related to internal flows created by vibrating the droplet at its eigenfrequencies. These forced flows are fully controllable and this property can therefore be exploited.

The device and method of the invention are non-invasive, cheap to manufacture and can be employed in a controlled atmosphere. The invention can be employed to greatly reduce the mixing time and increase the efficiency when mixing small amounts of liquid within a droplet, and this efficiency can be improved further by exciting the droplet with a square signal or with a sum of frequencies. By exciting the droplet with, for example, a sinusoidal signal, the droplet can be compartmentalised into several areas determined by internal flows induced in the droplet, and can thereby act as a liquid reactor.

Claims

1. A device for mixing or reacting two or more components within a droplet, the device comprising: a substrate upon which the droplet is placed in use; excitation means arranged, in use, to cause the droplet to resonate at one or more excitation frequencies; a generator for generating the one or more excitation frequencies; wherein the one or more excitation frequencies comprise one or more eigenfrequencies of the droplet.

2. The device of claim 1 , wherein the excitation means generates, in use, periodical or non-periodical signals, wherein these comprise square, sinusoidal or triangular excitation signals.

3. The device of claim 1 or 2, wherein the device is arranged such that, in use, the droplet is excited at a plurality of eigenfrequencies.

4. The device of any preceding claim, wherein the excitation means is one of a piezoelectric device, a pulsed laser, a mechanical device, an electric field, a magnetic field, acoustic waves or air flows.

5. The device of any preceding claim, wherein the device is arranged such that, when the droplet is resonated, it is compartmentalised into discrete areas, due to the frequencies at which it is resonated.

6. The device of any preceding claim, wherein the device forms a component of a continuous droplet mixing and/or reacting apparatus.

7. The device of any preceding claim, wherein the substrate is arranged to support, in use, an array of droplets.

8. A method of mixing or reacting two or more components within a droplet, the method comprising the steps of: positioning the droplet upon a substrate; and generating one or more excitation frequencies and resonating the droplet at the one or more excitation frequencies via an excitation means, wherein the one or more excitation frequencies comprise one or more eigenfrequencies of the droplet.

9. The method of claim 8, the method further comprising the step of generating periodical or non-periodical signals, wherein these comprise square, sinusoidal or triangular excitation signals.

10. The method of claim 9 or 10, the method further comprising the step of exciting the droplet at a plurality of eigenfrequencies.

11. The method of any of claims 8 to 10, wherein the excitation means is a piezoelectric device.

12. The method of any of claims 8 to 11 , further comprising the step of compartmentalising the or each droplet into discrete areas, due to the frequencies at which the or each droplet is resonated.

13. The method of any of claims 8 to 12, further comprising the step of continuously mixing or reacting droplets.

14. The method of any of claims 8 to 13, wherein the substrate supports an array of droplets.

15. A device for detecting one or more eigenfrequencies of a droplet, the device comprising: a substrate upon which the droplet is placed in use; excitation means arranged, in use, to cause the droplet to resonate at one or more excitation frequencies, and a detector for detecting the one or more eigenfrequencies at which the droplet resonates.

16. The device of claim 15, the device further comprising means for determining, on the basis of the eigenvalues, an unknown parameter of the droplet, wherein the unknown parameter is one of the volume, density, viscosity, surface tension and contact angle of the droplet.

17. The device of claims 15 or 16, wherein the excitation means generates, in use, periodical or non-periodical signals, wherein these comprise square, sinusoidal or triangular excitation signals.

18. The device of any of claimsi 5 to 17, wherein the device is arranged such that, in use, the droplet is excited at a plurality of excitation frequencies.

19. The device of any of claims 15 to 18, wherein the excitation means is a piezoelectric device.

20. The device of any of claims 15 to 19, wherein the substrate is arranged to support, in use, an array of droplets.

21. A method of detecting one or more eigenf requencies of a droplet, the method comprising the steps of: positioning the droplet upon a substrate; generating one or more excitation frequencies and resonating the droplet at the one or more excitation frequencies via an excitation means; and detecting the one or more eigenfrequencies at which the droplet resonates.

22. The method of claim 21 , the method further comprising the step of determining, on the basis of the eigenvalues, an unknown parameter of the droplet, wherein the unknown parameter is one of the volume, density, viscosity, surface tension and contact angle of the droplet.

23. The method of claim 21 or 22, the method further comprising the step of generating periodical or non-periodical signals, wherein these comprise square, sinusoidal or triangular excitation signals.

24. The method of any of claims 21 or 23, the method further comprising the step of exciting the droplet at a plurality of excitation frequencies.

25. The method of any of claims 21 to 24, wherein the excitation means is a piezoelectric device.

26. The method of any of claims 21 to 25, wherein the substrate supports an array of droplets.