Drop Self-Propulsion on Low Friction Lubricant Surfaces

In recent years, lubricated surfaces have raised significant attention because of their interesting wetting properties. Amongst these surface types, Slippery Liquid-Infused Porous Surfaces (SLIPS) have many applications in heat exchange phenomena, droplet motion, microfluidics and other engineering or medical applications. Recent work has focused on the modelling of the behaviour of water droplets on SLIPS, considering the effect of the lubricant choice, the droplet equilibrium condition through the liquid Young’s law contact angle, surface hysteresis and evaporation phenomena. Very few works have focused on the possibility of stabilizing two different lubricants of the same surface for the creation of a composite SLIP surface. We present a simple method for creating composite SLIP surfaces, with two different lubricants stabilized on the surface. Composite SLIP surfaces opens up interesting possibilities related to having a surface with different wetting properties and virtually no hysteresis. We exemplify this with a study on the motion of a water droplet on a composite SLIP surface having a wedge-like pattern with one lubricant inside and one outside the wedge. We observe droplet motion driven by the difference in wettability of the two lubricants to water, with droplets travelling distances of the order of a centimetre. A theoretical model for the droplet position along the wedge depending on the wedge angle and the lubricant selection is proposed. This provides excellent fits to the experimental data for droplet position with time.