Electrowetting-induced morphological transitions of fluid microstructures

We studied the morphology of liquid droplets on hydrophobic substrates with hydrophilic stripes. Instead of using chemical surface patterning we made use of the electrowetting effect (with striped electrodes). This offers the unique opportunity to externally control the contact angle by varying the applied voltage. Upon increasing the voltage, droplets become elongated along the direction of the stripe. Beyond a critical volume-dependent voltage, they undergo a first order morphological transition to an elongated cylindrical shape with the contact line pinned along the edge of the stripe. This observation both confirms recent theoretical and numerical work by [M. Brinkmann and R. Lipowsky, J. Appl. Phys. 92, 4296 (2002)] and it demonstrates the flexibility of electrowetting for wetting studies with heterogeneous substrates. Finally, we outline possible applications of morphological transitions for microfluidic devices. (C) 2004 American Institute of Physics.