Controlled electro-coalescence/non-coalescence on lubricating fluid infused slippery surfaces

Abstract

Aqueous drops on silicone oil infused lubricating surfaces are cloaked with a thin layer of oil to minimize their surface energy. These oil cloaked aqueous drops exhibit pseudo-stable coalescence or spontaneous coalescence depending upon the lubricating oil thickness which controls the interaction point of the drops. For thick oil films, drops interact with each other through the surrounding oil menisci resulting in pseudo-stable non-coalescence due to a thin oil layer between the drops. This stabilizing oil layer drains itself out due to the Laplace pressure of the aqueous drops. An external electric field applied between the drops forces the oil to drain faster, due to additional electrostatic pressure, resulting in faster coalescence. This happens in two steps: bulk drainage of the oil and final rupture due to the electric field induced hydrodynamic instability of the thin oil layer. For thin lubricating films, the contact point of aqueous drops is in the vicinity of the water–air interface resulting in spontaneous coalescence. By applying an external electric field between the drops and substrate (to decrease the apparent contact angle by electrowetting), the interaction point is brought close to the oil meniscus resulting in pseudo-stable aqueous drops against coalescence.