Comparison of wetting models and geometrical analysis in describing the effect of cavity number, size, and position on apparent contact angle

Abstract

A critical subject of surface science is whether contact area or the triple-phase contact line (TPCL) directly affects the apparent contact angle (APCA). On this premise, effect of cavity size and position on APCA is studied. Cavities were created using a laser machining process. Optimal conditions for laser patterning were obtained via trial and error. It was impossible to deposit a droplet that could rest on all of the cavities for about 0.16 mm distance between the edge of the patterns and the drop’s perimeter. Neither Wenzel nor Cassie–Baxter models can fully explain the acquired data from static contact angle measurements. Revised Cassie–Baxter equation also fails, while a simple geometrical analysis based on Spherical Cap assumption shows a better agreement with the data as long as the cavities are sufficiently far from TPCL. Based on our results, we believe that theoretical analyses regarding wetting models are not in line with engineering aspects of wetting which should be focused on TPCL.

Keywords:

• Apparent contact angle
• laser patterning
• contact line
• wetting models