Dynamic water contact angle during initial phases of droplet impingement

We investigate the dynamics of water contact angle during droplet impact onto substrates with three different surface coatings (one hydrophilic and two hydrophobic). The experiments are carried out for three Weber numbers (We) of approximately 5, 7 and 10. The dynamic contact angle is determined by analysis of consecutive frames of video recordings taken at a frame rate of 50 000 fps. Measurements are carried out for the first advancing (spreading) and receding (retracting) phases of droplet impingement. We find that the evolution of contact angle during impingement is not directly related to contact angle values obtained from traditional static and quasi-static measurements (sessile drop, tilting plate, or Wilhelmy plate methods). We propose that the dynamics of contact angle resulting from fluid flow more accurately reflects surface physico-chemical properties than the values obtained from static and quasi-static measurements. (C) 2016 Elsevier B.V. All rights reserved.