Contact Angle Hysteresis and Work of Adhesion of Oil Droplets on Nanosphere Stacking Layers

Surface repellency of liquid droplets with different surface tensions (23.4-73.2 mN/m) on nanostructured surfaces built of one- and two-tier silica sphere stacking surfaces were investigated, and contact angle hysteresis (CAH) and work of adhesion were analyzed. It was shown that the binary nano- and sub-micrometer-scaled roughened surfaces exhibited better repellency toward oil drops with surface tension of 30 mN/m, and the CAH behavior strongly depended on both liquid types and density of three-phase contact line. This improvement of hysteresis from the two-tier texture is attributed to the facts that (i) the arrangement of submicrometer spheres generates a primary roughness that allows air cushion in the texture and (ii) the deposition of nanospheres creates a large number of point contact fashion to repel the oil penetration, reducing the length of the liquid-solid contact line (i.e., Cassie state). The existing air layer tends to impart the fraction of vapor-solid contact, which decreases the kinetic barrier of drop movement. Incorporated with the Young-Dupre equation, it has shown a linear relationship between the difference between cosines of advancing and receding angles and the work of adhesion. Accordingly, it reveals that the adhesion of solid-oil contact interface is reduced due to the design of surface topography.