Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.
In this paper we report the effects of surface roughness on contact line friction coefficients of water droplets on micro- and nano-patterned surfaces. Both advancing and receding contact line friction coefficients have been measured, analyzed and compared on smooth, one-tier (with micropillars), and two-tier (with carbon nanotubes (CNTs) grown on micropillars) surfaces over a wide range of contact line velocities and droplet volumes. Our results indicate that superhydrophobic surfaces with CNTs on top of micropillars can significantly decrease both the advancing and receding contact line friction coefficients. In comparison, both the advancing and receding contact line friction coefficients on smooth surfaces were more than ten times larger than those on superhydrophobic surfaces. However, droplets on one-tier surfaces with only micropillars exhibit different dynamic behaviors on advancing and receding movements. We experimentally investigated the Wenzel-Cassie transition on micropillar structures and found that the receding motion on micropillars is dominated by the Wenzel behavior, which leads to higher receding contact line friction coefficients on one-tier surfaces. However, there is a high tendency for an advancing droplet to exhibit Cassie-type behaviors on one-tier surfaces. As a result, advancing contact line friction coefficient is considerably mitigated on micropillars. On two-tier superhydrophobic surfaces, it was the Cassie–Baxter behavior that dominates both the advancing and receding contact line motions giving rise to less friction coefficients. Furthermore, the effects of surface roughness on contact line hysteresis are discussed in this paper.