Drop Friction on Textured Lubricant-Coated Surfaces.

Understanding drop friction on textured surfaces has implications in microfluidics and lab-on-a-chip devices. In this work, we investigated the drop friction on lubricant-coated pillars by systematically varying pillar height and density. First, we measured the friction force on a moving drop using a cantilever force sensor that has ±0.1 μN sensitivity. This measurement shows that drop friction on tall dense pillars is comparable to drop friction on short pillars, a significant result that suggests the presence of a Landau-Levich-Derjaguin (LLD) film underneath the moving drop. Second, we validated the force measurement by estimating the lubricant layer thickness by using white-light interferometry. Third, we visualized the lubricant film underneath the moving drop using reflection interference contrast microscopy. The three independent diagnostic tools and measurement techniques complement each other and reaffirm that drops oleoplane on tall dense pillars, while they graze over the pillar tops in tall sparse pillars. The critical density that forces this transition to drop friction is ≈50%. Furthermore, the experimental results show that friction on microholes and micropillars is comparable when the solid fraction is the same. The results reported in this study contradict past studies that reported the absence of an oil layer on tall pillars. Besides improving current understanding, the insights gained from this work provide design guidelines for turning drop friction on-off on demand for microfluidics applications.