Electro-manipulation of droplets for microfluidic applications

There is a growing technology-driven interest in using external influences to move or shape small quantities of liquids, a process that is referred to as microfluidic actuation. The use of electrical, rather than mechanical, forces to achieve this actuation is convenient, because the resultant devices contain no moving parts. In this work we consider a sessile drop of an incompressible liquid with a high conductivity resting on the lower substrate inside a parallel-plate capacitor subjected to a relatively low frequency A.C. field. With the application of an electric field the drop deforms into a new static shape where the apex of the drop rises towards the upper electrode in order to balance the Maxwell electric stresses, surface tension and hydrostatic pressure on the interface. From experimental, numerical and asymptotic approaches we determine a predictive equation for the deformation as a function of initial contact angle and drop width, surface tension and applied voltage.