• double droplet systems;
  • microfluidics;
  • charged interfaces;
  • nonlinearity

A capillary switch is a system of two liquid drops, one sessile and the other pendant, obtained by overfilling a hole of radius R in a plate. When surface tension dominates gravity, the equilibrium shapes of the drops are spherical sections of equal radii. If the combined volume of the top VT and bottom VB drops exceeds inline image, the system has three equilibrium states of which two are stable. This bistability is exploited in applications by toggling the system between its two stable states. Here, we examine the effectiveness of using an electric field for toggling. Bifurcation diagrams are obtained that depict how the system's response varies with applied field strength E, and show loss of stability at turning points and the possibility of hysteresis. A phase diagram in inline image space is presented to readily infer when an electric field is an effective means for toggling. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1451–1459, 2014