Dynamic characteristics of droplet impingement on carbon nanotube array surfaces with varying wettabilities

Abstract

This work experimentally investigated the water droplet impingement behaviors on carbon nanotube (CNT) array surfaces characterized by different wettabilities from hydrophilicity to superhydrophobicity. By varying the impact velocity, six typical dynamic behaviors of droplet impact were identified and a related droplet regime map was developed under different wettabilities. To further investigate the evolution behavior of droplet impact, the dynamic contact angle (DCA) and spreading factor were measured and analyzed. The damped oscillations of DCA were observed for wettable and mildly hydrophobic CNT array surfaces, while the approximate impact inertia independence was verified for high hydrophobicity or even superhydrophobicity. The contact line velocity in the vicinity of triple phase interface was found to be highly related to the DCA variation at the initial stage of spreading. The discrepancy of DCA on different wettable surfaces reduced significantly with the increase of Weber number. The reduction in post-impact CAs and resultant wettability enhancement after droplet impingement was mainly attributed to self-assembly structures caused by the capillary action during the spreading phase.