Effect of polymer additives on the apparent dynamic contact angle of impacting drops

The apparent dynamic contact angle of drops impacting on hydrophobic substrates is studied experimentally by high-speed imaging. Drops of pure water are compared with drops of a dilute polymer solution (200 ppm poly-ethylene oxide), to investigate the relationship between changes in the dynamic contact angle and the contact line retraction velocity after maximum spreading. It is shown that the dynamic contact angle observed in drops of polymer solution at the beginning of retraction is significantly smaller than that observed in drops of pure water impacting on the same surface with the same Weber number. These results are interpreted in terms of an effective resistive force on the contact line, arising as the drop edge sweeps the substrate, which is caused by polymer molecules stretching.