Pillar versus dimple patterned surfaces for wettability and adhesion with varying scales.

Inspired by biological topographical surfaces, micropatterned elastomeric surfaces with square pillars and dimples of different geometry scales were fabricated. Their wettability and adhesion properties with various liquids were systematically investigated and compared with flat surfaces. Interesting results were obtained in the case of silicone oil (the toe-pad-like wetting case) in that the scale-dependent wettability and adhesion performed inversely for pillars and dimples. Micropillars significantly enhanced the surface wettability with a geometry scale dependence, whereas the dimples suppressed the wettability independent of the geometry scale. The adhesion force of the micropillars increased with an increase of the geometry scale. However, in the case of the micro-dimples, the adhesion force obviously decreased with an increase of the geometry scale. This behaviour was attributed to the fact that pillars are 'open' to oil but dimples are 'close' to oil, presenting different orientations to the solid-liquid interface.