Inducing droplet motion on SLIPS (Slippery Liquid Infused Porous Surfaces)*

Slippery Liquid-Infused Porous Surfaces (SLIPS) are super-hydrophobic surfaces inspired by the nepenthes pitcher plant. These surfaces allow a very low contact angle hysteresis, and consequently, a high droplet mobility. Based on this property, we have developed SLIPS capable of inducing and controlling the motion of droplets. This has many practical applications in microfluidic, including: (i) sorting droplets depending on their size and/or fluid type, (ii) accurately positioning droplets and (iii) merging and splitting droplets.

The low contact angle hysteresis exhibited by a droplet on a SLIPS implies that a small force is sufficient to induce its motion. We propose here to generate such force with a small imbalance in the underlying surface geometry. To do so, flat surfaces are patterned with macro-structures with an inherent roughness gradient. These surfaces are then made into SLIPS using a coating of nanoparticles imbibed with oil. When a droplet is deposited on the resulting surface, the force generated by the roughness imbalance, allied to the high droplet mobility, induces a motion in a specific, controlled direction.