Electrowetting employs an applied electric field to move liquid drops and films. It is therefore a precise, rapid and accurate means for liquid actuation in microfluidic devices. Our work on electrowetting has largely been theoretical in nature, in which we attempt to elucidate the physical mechanisms governing the electrowetting behaviour of drops lying on solid substrates. We advocate the delineation of two distinct electrowetting phenomena, which we classify as static electrowetting and spontaneous electrowetting.

Static electrowetting describes the phenomena observed in electrowetting-on-dielectric (EWOD) and electrowetting-on-insulator-coated-electrodes (EICE) schemes, in which the macroscopic contact angle of the drop is altered by the application of the electric field, which is predominantly normal to the drop interface and becomes weakly singular in the contact line region whose length scale is of order the dielectric film thickness. Since the field becomes divergent only in this very small region, the behaviour can be described by a force balance at the contact line, which means that there is no dynamics associated with the system, i.e., it is in static equilibrium wherein the drop simply relaxes to its new equilibrium state with a contact angle described by the Lippmann condition once an electric field is applied.

On the other hand, spontaneous electrowetting involves the spontaneous formation of a thin 10 μm order electrowetting film that is pulled out from the macroscopic liquid drop, as depicted in the spatio-temporal film evolution profiles in the figure. This electrowetting film advances along a pair of parallel line electrodes much faster than the macroscopically spreading (due to capillarity effects) drop itself. We show that the formation of this electrowetting film is analogous to gravity-induced films since the Maxwell electric body force acting on the contact line behaves in a similar manner to the gravitational force. As such, the film spreads in a self-similar manner similar to gravity spreading films. This dynamic situation, in contrast to static electrowetting, arises because of a non-localised and non-singular electric field gradient, tangential to the interface, which extends into the bulk of the liquid drop, thereby creating bulk pressure gradients that give rise to spontaneous liquid flow.

1. 1.LY Yeo, H-C Chang. Static and Spontaneous Electrowetting. Mod Phys Lett B 19, 549 (2005) (PDF).
   

2. 2.LY Yeo, H-C Chang. Electrowetting Films on Parallel Line Electrodes. Phys Rev E 73, 011605 (2006) (PDF).
   

3. 3.LY Yeo, RV Craster, OK Matar. Drop Manipulation and Surgery Using Electric Fields. J Colloid Interface Sci 306, 368 (2007) (PDF).
   

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