Wetting of Rough Surfaces by a Low Surface Tension Liquid

Abstract

There exist textured surfaces that demonstrate large advancing contact angles when the expected behavior is complete wetting due to high Wenzel roughness. The roughness can represent an impediment to the motion of the contact line, leading to the possibility of contact line pinning and thus increased advancing contact angle. A set of fabricated textured surfaces with varying pillar diameters and pillar spacing were tested using hexadecane. Because of the low surface tension of hexadecane, the majority of the surfaces exhibited penetration of the liquid into the roughness, also known as the Wenzel state. Because of this penetration, the empirical pinning force framework we previously developed for wetting behavior on smooth surfaces and surfaces with texture where liquid does not penetrate the roughness was expanded to include the Wenzel state. For the surfaces that had Wenzel wetting, the receding contact angle tended to follow the predictions of the Wenzel equation, while the advancing contact angle tended to increase with increasing roughness when according to the Wenzel equation it would be expected to decrease. For surfaces where nonpenetrated Cassie wetting was observed, a constant high advancing contact angle and a receding contact angle that follows the trend predicted by the Cassie equation were observed.