Dynamic contact angle of dilute polymer solution drops impacting on a hydrophobic surface

The dynamic contact angle of dilute polymer solution drops impacting on a hydrophobic PTFE surface is studied experimentally by high-speed imaging, for polymer concentrations ranging between 0 ppm and 400 ppm, and impact Weber numbers between 15 and 115. Images with spatial resolution of 16.4 mu m/pixel were captured at 8000 fps to resolve the short time-scale dynamics. Results show that the receding contact angle of dilute polymer solution drops is significantly smaller than in case of drops of pure water, suggesting that the receding contact line encounter a higher resistance to its displacement. The minimum contact angle decreases with respect to both increasing polymer concentration and increasing Weber number. At lower Weber numbers, contact angle oscillations were observed, which can be related to the liquid flow to and from the rim during drop retraction. (C) 2015 Elsevier B.V. All rights reserved.