Ratchet-driven fluid transport in bounded two-layer films of immiscible liquids

We present a two-layer thin film model allowing us to study the behavior of a general class of ‘wettability ratchets’ that can be employed to transport a continuous phase. Brownian ratchets, in contrast, are normally used to transport particles or molecules within a continuous carrier fluid without transporting the fluid itself. The wettability ratchet is based on a switchable, spatially asymmetric, periodic interaction of the free surface of the film and the walls. To illustrate the general concept, we focus on an electrical dewetting mechanism based on the effective force exercised by a static electric field on the liquid–liquid interface between two dielectric liquids. In particular, we analyse (i) an on–off ratchet with a constant lateral force resulting in a dewetting–spreading cycle, (ii) a ratchet switching between two shifted potentials that shows a transition between oscillating and sliding drops, and (iii) a flashing external force ratchet. For the three cases, the macroscopic transport is studied in its dependence on spatial and temporal characteristics of the ratchet, and physical properties and volume of the liquids.