Fundamentals of Electrowetting and Applications in Microsystems
Electrowetting has become widely used to control the wettability of solid surfaces in microsystems. In this chapter, we briefly introduce basic concepts of wetting and we discuss in detail the fundamental physics behind the electrowetting phenomenon. We c
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and Frieder Mugele
‡
Dpto. Electr´ onica y Electromagnetismo. Facultad de F´ısica. Universidad de Sevilla. Sevilla (Spain). ‡ Physics of Complex Fluids, MESA+ and IMPACT Institutes. Faculty of Science and Technology, University of Twente. Enschede (The Netherlands) Abstract Electrowetting has become widely used to control the wettability of solid surfaces in microsystems. In this chapter, we briefly introduce basic concepts of wetting and we discuss in detail the fundamental physics behind the electrowetting phenomenon. We compare the different theoretical approaches to the electrowetting equation, i.e. the thermodynamic derivation and the electromechanical interpretation. The effects of using AC signals are discussed and the limits of validity of the electrowetting equation for increasing voltage are presented (contact angle saturation and contact line instabilities). In the second part of this chapter, we review applications where electroweting has shown itself as a powerful tool, like electrowetting-based displays and lenses. Special attention is dedicated to the use of electrowetting in microfluidic devices.
1
Introduction
As dimensions of physical systems are smaller, the surface-to-volume ratio increases rapidly and surface forces become, relatively, more important. Thus, the surface tension (i.e. the tendency of a given interface to minimize its area) has an increasingly importance as we reduce the volume of liquid we are dealing with. For example, the shape of droplets resting on solid substrates depends on the wettability and topography of the latter. The droplet morphology adjusts itself to find the configuration of minimum energy. As shown in Gau et al. (1999), several metastable states can exist for complex topographies and the liquid can abruptly change for one morphology to another when some parameteres are varied (see Figure 1a)). Two inmiscible fluids coflowing in a microchannel also illustrate the importance of wettability in microsystems. Figure 1b) shows a phase diagram of the
A. Ramos (ed.), Electrokinetics and Electrohydrodynamics in Microsystems © CISM, Udine 2011
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Pablo García-Sánchez and Frieder Mugele
flow patterns observed by Dreyfus et al. (2003). They studied two distinct situations. When the outer fluid completely wets the walls, they found well structured flow patterns. However, if the outer fluid partially wets the walls, no stable flow patterns are identified.
Figure 1. Two examples on the importance of wall wettability in microfluidic systems. a) An array of hydrophilic stripes on a hydrophobic substrate. For a low amount of water the stripes are uniformly covered by the liquid, forming channels of constant cross section (Left). For higher volumes, the channels develope a single bulge as soon as the contact angle exceeds a certain characteristic value (Right), (reproduced with permission from Gau c 1999 AAAS). b) Flow patterns observed for two inmiscible et al. (1999), coflowing fluids in a microchannel. If the outer fluid completely wets the walls, well structured flow patterns are found
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