Dip-coating deposition on chemically patterned surfaces: a mechanistic analysis and comparison with topographically patterned surfaces.

Chemically patterned surfaces containing hydrophilic features on a hydrophobic background have been used by a number of groups to deposit arrays of particles/crystals/substances by dip-coating deposition. In this technique, a substrate is simply withdrawn from a solution (or dispersion) of the desired substance, the solution dewets from the hydrophobic region and wets the hydrophilic features, and the particles/crystals/substances deposit on the hydrophilic features after solvent evaporation. An apparently similar approach, recently described by several groups, involves dip-coating deposition of substances from solutions onto hydrophobic topographic features (arrays of posts on superhydrophobic surfaces) that are separated by air. We report results of dip-coating deposition using chemically patterned surfaces and compare them directly with results from post-containing superhydrophobic surfaces. This comparison involves the analysis of events at receding three-phase contact lines; these events differ significantly in the two approaches with the key difference being tensile (normal to the surface) versus sessile (parallel to the surface) capillary bridge failure. Tensile failure occurs with the post-containing superhydrophobic surfaces and sessile failure with chemically patterned surfaces. The solvent evaporation stages of the processes, that occur subsequent to the capillary bridge failure events, also vary significantly in the two approaches and depend on the receding contact angles of the hydrophobic post tops and the hydrophilic chemically patterned features. These differences, as the adjectives suggest, are pronounced. Controlling the evaporation rate (adjusting the vaporization/condensation equilibrium) by raising the partial pressure of the solvent is identified as a useful variable for chemically patterned surfaces.