1. Application of an Electric Field to Control Wetting of Thin Fluid Films
Joelle Frechette, Chemical and Biomolecular Engineering Department
Thin film electrowetting
Using the SFA, we have designed experiments to test the mechanism driving electrowetting (Fig. 3). We have modified the instrument to allow for external potential control of both interacting surfaces and used capillary condensation to generate nanoscale water droplets. Our experiments allowed us to probe contact angle changes within the first tens of nanometer of a drop, and are not limited by possible issues caused by contact angle hysteresis. Using this approach we have unequivocally demonstrated that the real contact angle does not change in electrowetting experiments. Our results show that there is no measurable change in the solid-liquid surface energy in EWOD and that the mechanism at play is electromechanical in nature (see Fig. 4).
Deterministic hydrodynamics
We conducted simple experiments using a large array of obstacles and a uniform driving force (gravity) to explore the deterministic nature of the deterministic hydrodynamics separation method. We investigated the motion of stainless steel balls falling through a periodic array of obstacles created with cylindrical LEGO® pegs on a LEGO® board and immersed in a tank filled with glycerol (Fig. 1). We showed that the motion of the spheres was irreversible and displayed directional locking (Fig. 2). We also demonstrated that the locking directions could be predicted with a single parameter that distinguishes between reversible and irreversible particle-obstacle collisions. These results stressed the need to incorporate irreversible interactions to predict the movement of a sphere passing through a periodic array. This work was done in collaboration with German Drazer. ΔV