Liquid Selection for Electrohydrodynamic Capillary Thermal Switches*

The deformation of sessile droplets and capillary bridging in a parallel-plate capacitor under DC field has been the subject of several scientific studies. Coaxially located droplets on opposing electrodes experience an attraction in the presence of an electric field. Application of a suitably large field will lead to either the droplets forming a liquid bridge or oscillation between bridged and de-bridged (i.e. droplet) states. We explored the bridging behavior of a variety of liquids in air. Among the liquids that could form a stable field-induced bridge, only a limited set could reversibly make and break the capillary bridge by switching the electric field on and off. The ability to form a switchable liquid bridge is a function of both the liquid’s properties, including surface tension, electric conductivity, and dielectric constant, and external conditions such as electrode separation, droplet volume, and the substrate selection. Taking advantage of this phenomenon, through periodically forming and breaking the capillary bridge, we demonstrate the realization of a planar thermal switch that can control the flow of heat.