Shape Oscillation and Internal Mixing in Sessile Liquid Drops Using Electrowetting-on-Dielectric (EWOD)

Internal mixing within a sessile liquid drop can be significantly enhanced by means of so-called electrowetting-on-dielectric (EWOD), using an alternating electric potential. This is done experimentally by monitoring the coalescence and mixing of dyed liquid drops that are brought together by electrowetting actuation. The process is monitored using high-speed imaging and the extent of mixing with AC forcing after coalescence is tracked. Here, EWOD refers to manipulation of contact angle of a liquid drop on a surface by application of an electrical potential between the drop and an electrode embedded in the surface, separated from the drop by a thin dielectric layer. To understand the internal mixing, which appears to be driven by shape oscillations of the sessile drop, we also conduct fundamental studies of the shape oscillations of drops that are attached to a planar solid surface. Through image processing, we show that depending on the frequency of the applied alternating field, various modes of oscillation, including non-axisymmetric ones, may be excited.