Electrowetting

5 Tunable Focus Liquid Microlenses Using Electrowetting 5.5.1 Electrowetting 

This concept was recently revisited. In the early 1900s, Berge introduced the idea of using a thin insulating layer to separate the conductive liquid from the metallic electrode to eliminate the problem of electrolysis. This is the concept now known as electrowetting on a dielectric (EWOD) [15]. 

In the absence of external electric fields, the behavior of the droplets is determined by surface tension alone. The free energy f of a droplet is a function of the droplet shape and expressed as the sum of the areas of the interfaces between the three phases involved the solid substrate (s), the liquid droplet (1), and the ambient phase assumed to be a vapor (v) for simplicity, weighted by the respective interfacial energies for solid-vapor, for solid-liquid, and for liquid-vapor)  

is a Lagrangian variable present to enforce the constant volume constraint and equals the pressure drop Ap across the liquid-vapor interface. Minimization of variation aids in maintaining the well-known conditions that any equilibrium liquid morphology must satisfy. These conditions have been described as the Young-Laplace equations discussed in Chapter 2. We can further break the procedure into two equations. The first is the Laplace equation stating that Ap is a constant, independent of the position on the interface  

rj and r2 are the two principal radii of curvature of the surface and k is the constant mean curvature. For homogeneous substrates, this means that droplets will adopt the shape of a spherical cap in mechanical equilibrium. The second condition is given by Young s equation 

It has been observed that ions and dipoles redistribute in the liquid due to the application of electric potential. This redistribution can cause a change in the wetting properties of the drop. 

This phenomenon is known as electrowetting. A hydrophobic surface like Teflon can behave like a hydrophilic surface due to the application of electric potential. Electrowetting can be used for the creation, transportation, and merging of droplet for digital microfluidic systems. Electrowetting can also be used in micromixers. 

Equation (5.72) can also be written using the expression for capacitance as 

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When a droplet is deformed asymmetrically, the ratchet motions of the droplet can be induced as demonstrated on the vibrated gradient surface and on a saw-shaped electrode on which the wetting was changed by electrowetting [48]. 

Electro wet drum separators, 15 443 Electrowetting, in microfluidics, 26 962 Electrowinning, 9 637-642 14 760 16 154 from aqueous solutions, 9 637-639 ... 

M. Pollack, R.B. Fair, and A.D. Shenderov Electrowetting-Based Actuation of Liquid Droplets for Microfluidic Applications. Appl. Phys. Lett. 77(11), 1725... 

For many applications it is desirable to be able to adjust the wetting properties of a solid surface for aqueous solutions. One method is called electrowetting [303,304], In electrowetting an electric potential is applied between a metal surface and a liquid via an electrode (Fig. 7.24). The metal is coated with an insulating layer of thickness h. Fluoropolymer coatings turned out to be suitable materials though it is not fully understood why [305], The change in contact... 

Figure 7.24 Electrowetting of a liquid drop on a metal which is coated with a thin insulating...

Example 7.10. Prins et al. [306] used electrowetting to control fluid motion in microchannels. To do so, they coated aluminum electrodes first with a 12 pm thick layer of parylene and then with a 10 nm thick fluoropolymer film. The channels were 0.35 mm wide. Due to the hydrophobic polymer water does not flow into the capillaries. Only after applying voltages of typically 200 V did the capillaries fill with water. When switching the voltage off, the water flowed out of the capillaries again. 

External energy sources for active mixing are, for example, ultrasound [22], acoustic, bubble-induced vibrations [23,24], electrokinetic instabilities [25], periodic variation of flow rate [26-28], electrowetting induced merging of droplets [29], piezoelectric vibrating membranes [30], magneto-hydrodynamic action [31], small impellers [32], integrated micro valves/pumps [33] and many others, which are listed in detail in Section 1.2. 

Moving- and Oscillating-droplet Mixing by Electrowetting Most Relevant Citations... 

The movement of droplets is based on an electrostatic method which changes the interfacial tension of the droplets by voltage, which is known as electrowetting (see Figure 1.33) [97, 98]. The nature of the liquid to be moved has to be polarizable and/or conductive. Application of an electric field on only one side of the droplet creates an imbalance of interfacial tension which can drive bulk flow of the droplet. 

Figure 1.33 The electrowetting effect. A droplet of conducting liquid has a contact angle 0with a solid hydrophobic insulator (solid contour).

Figure 1.34 Schematic of the cross-section of the electrowetting chip.

Figure 1.36 Schematic of top and side views of a central part of the electrowetting-based mixer [97] (by courtesy of RSC).

In addition to using electrowetting (see Figure 1.42), electrophoretic and dielectro-phoretic forces can be used for moving droplets [99], Electrophoretic droplet movement depends on the application of large DC fields, which may pose problems for fluid systems such as suspensions. For dielectrophoretic operation, however, AC fields are sufficient, as for electrowetting. 

Electrowetting forces depend largely on the cleanness of the substrates, since even small traces of impurities may notably change the wetting behavior [99], This certainly can be controlled under laboratory conditions, but may experience limitations under dirty real-world applications. Further limitations on the droplet volume are given by the geometric constraints of the electrode chamber which needs to be wetted at the floor and ceiling. 

Pair, P., Pamuia, V. K., Pollack, M. G., Fair, R. B., Electrowetting-based droplet mixers for microfluidic systems, Lab Chip 2003, 1, 28-32. 

One of the requirements in MALDI-MS analysis is the use of a liquid matrix. The electrowetting-on-dielectric (EWOD) method has been used to move and mix droplets containing proteins and peptides with the liquid matrix, all of which were situated at specific locations on an array of electrodes. With this method, insulin (1.75 pM), insulin chain B (2 pM), cytochrome c (1.85 pM), and myoglobin (1.45 pM) have been analyzed [518]. 

Wheeler, A.R., Moon, H., Kim, C.-J., Loo, J.A., Garrell, R.L., Electrowetting-based microfluidics for analysis of peptides and proteins by matrix-assisted laser desorption/ionization mass spectrometry. Anal. Chem. 2004, 76, 4833M838. 

Pollack, M.G. Fair, R.B. Shenderov, A.D. Electrowetting-based actuation of liquid droplets for microfluidic applications. Appl. Phys. Lett. 2000, 77 (11), 1725. 

Lee, J. Kim, C.-J. Surface-tension-driven microactuation based on continuous electrowetting. J. Microelectromech. Syst. 2000, 9 (2), 171-180. 

For example, dielectrophoresis can be used in a system of two parallel electrodes [44]. In this system it is possible to control of generation of droplets with pulses of an alternating electric field [45]. Also electrowetting can be used to this end [47-49]. 

F. Malloggi, H. Gu, A.G. Banpurkar, S.A. Vanapalh, and F. Mugele, Electrowetting - A versatile tool for controlling microdrop generation, European Physical Journal E, 26, 91-96, (2008). 

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