Reversible pressure-induced switching of droplet mobility after impingement on porous surface media

We present a generic method to reversibly switch the mobility of a droplet after impingement onto a hydrophobic porous surface, from a sticky to a slippery state and back. By means of a pneumatic system, the backpressure of the porous medium is adjusted to favor the pressure balance towards the desired state. By applying overpressure above a certain threshold, the antiwetting pressures increase over the wetting pressures, which eventually results to a slippery state, while vacuum enhances the wetting pressures, thus, rendering the droplet sticky. This pressure-induced switching method is very flexible and can be applied even after decay of the hydrophobic character of the porous medium, does not entail the use of a superhydrophobic or hierarchical surface, and can be applied for actuation and valving in microfluidics. (C) 2013 AIP Publishing LLC.