Reversible and dynamic transitions between sticky and slippery states on porous surfaces with ultra-low backpressure

Modulation of droplet mobility on surfaces is crucial in numerous technological applications. Here we present an easy to implement methodology in controlling the mobility of water droplets by means of backpressure application. Tuning the backpressure the inherently sticky hydrophobic porous surface may be readily rendered slippery, reversibly and dynamically with low response time. Gas pockets at the liquid-solid interface are formed and sustained, thus leveraging continuous droplet de-pinning. Thus the surface exhibits slippery macroscopic behavior, without fully developed droplet levitation. Two porous ceramic surfaces are studied: one consisting of sintered primary micro particles exposing randomly distributed micro-posts and a second one shaped by extrusion, part of a honeycomb ceramic structure exhibiting randomly distributed micro-holes. Appropriate vapor deposition was used to render them hydrophobic, exhibiting inherently sticky characteristics. The adequate backpressure to deliver slippery characteristics is experimentally measured for various tilt angles. Ultra-low adequate backpressures of some tens of mbar are reported for the case of the porous surface with the micro-holes, thus providing a rather attractive tool for micro- as well as for large-scale applications. Considering capillary bridging on the in-plane force balance the experimental variations between the two surfaces are explained and correlated to the porous surface microstructure.