A capillary switch is a bi-stable system of liquid/gas interfaces with a trigger to toggle back and forth between the two stable equilibrium states. We use an electro-osmotic pump as trigger. The pump, consisting of two electrodes and a porous substrate arranged between the droplets, moves volume between the droplets. This bistable system is called an electro-osmotic droplet switch. With the pump off, for low total volumes, the stable states are a pair of identical sub-hemispherical droplets or, for large enough total volumes, a large-small droplet configuration (two mirror-symmetric states). With the pump on, these stationary states are shifted and, if the pump strength is too great, there are no stationary states at all. In this article, we report the pump-on behavior as a modification of the pump-off behavior. To build the dynamic model of the system, we first develop a characterization of the electro-osmotic pump in the spirit of the Blake-Kozeny correlation for viscous flow through a packed bed. The control-volume model is based on center-of-mass motion. Model predictions compare favorably to observation.
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Barz et al. (2013) studied this question.