Diverging Effects of Topographical Continuity on the Wettability of a Rough Surface

The effects of the continuity of the surface pattern on wetting enhancement was investigated using micropillar and microhole arrays on hydrophilic and hydrophobic materials. Isolated micropillar arrays and continuous microhole arrays were prepared by a microscale imprinting technique using positive and negative Si molds fabricated by a conventional photolithography technique. The contact angles (CAs) and contact angle hysteresis (CAH) of the prepared surfaces were measured as a function of the surface parameter ξ, defined as the ratio of the top surface area of the microstructure to the surface area of the flat unit cell. It was found that the CAs of the micropillar array monotonically increased as the surface ratio decreased, regardless of the native wettability of the solid. However, an abnormal and consistent decrease of the CAs for the microhole array was observed when ξ < 0.5. To investigate the mechanism of this abnormality in wetting enhancement, the energy barriers for normal direction wetting, the so-called wetting transition from Cassi-Baxter (CB) wetting to Wenzel wetting, and lateral direction wetting, that is, spreading, were investigated with consideration of the trapped air in the microhole. The analysis unveiled that the hydrophobicity of the hydrophilic surfaces are attributable to the liquid-air interface pinning at the discontinuous edge of the pillar, which results in CB wetting. The abnormal decrease in the CAs of the microhole-patterned surfaces with ξ < 0.5 has been attributed to the relatively low energy barrier for spreading influenced by the continuity of the three-phase contact line. Additionally, trapped air in the microhole also plays a role in the spreading of water droplets by hindering the wetting transition from CB wetting to Wenzel wetting.