Sliding friction and contact angle hysteresis of droplets on microhole-structured surfaces.

Microstructured surfaces with continuous solid topography have many potential applications in biology and industry. To understand the liquid transport property of microstructured surfaces with continuous solid topography, we studied the sliding behavior of a droplet on microhole-structured surfaces. We found that the sliding friction of the droplet increased with increasing solid area fraction due to enlarged apparent contact area and enhanced contact angle hysteresis. By introducing a correction factor to the modified Cassie-Baxter relation, we proposed an improved theoretical model to better predict the apparent receding contact angle. Our experimental data also revealed that the geometric topology of surface microstructures could affect the sliding friction with microhole-decorated surfaces, exhibiting a larger resistance than that for micropillar-decorated surfaces. Assisted by optical microscopy, we attributed this topology effect to the continuity and the true total length of the three-phase contact line at the receding edge during the sliding. Our study provides new insights into the liquid sliding behavior on microstructured surfaces with different topologies, which may help better design functional surfaces with special liquid transport properties.