Evaporation of Colloidal Suspension Droplets over Oil-infused Slippery Surfaces

Evaporative drying of colloidal droplets with different concentrations (Ci) on liquid infused slippery substrate (LISS) surfaces fabricated over biomimetically patterned sticky hydrophobic surfaces is reported here. These slippery surfaces remain stable under the aqueous droplets. The effective thickness of the infused oil layer (hE) was varied and its effect on the evaporation dynamics, wetting ridge formation and the final deposition pattern were studied using an optical microscope as well as a contact angle goniometer. The results were contrasted to those obtained over bare (oil-free) flat and patterned sticky surfaces. Unlike strong pinning and consequent peripheral ring deposit invariably observed in evaporation of a colloidal droplet over a solid surface (i.e. bare flat Sylgard 184 without oil), over a LISS, the dynamics, including the extent of pinning and the final deposit morphology, were found to be strongly dependant on hE. Due to the presence of the underlying patterns, the extent of pinning increased with a reduction in hE. Prolonged evaporation over a LISS leads to a uniform deposition due to gravity-induced settling of the colloids followed by capillarity-mediated rearrangement of the particles. Colloidal assembly at the droplet free surface and the droplet-wetting ridge interface leads to a thinner encapsulating oil layer and faster drainage of the wetting ridge resulting in a higher evaporation rate before pinning of the suspension droplets over all LISS surfaces.