EFFECTS OF EVAPORATION ON CONTACT ANGLES ON POLYMER SURFACES

The evolution of contact angles of water on three polymer surfaces has been studied both with and without an environment saturated in water vapour. It was observed that in a saturated atmosphere, evolution was slow and involved a slight reduction in drop height and contact angle at constant contact diameter. This effect is attributed to a slight diffusion of water into the polymer. In a dry atmosphere, evolution was far more rapid and involved three stages. Initially, the contact diameter remained constant whilst both drop height and contact angle decreased. When a small value of contact angle had been reached, both the drop height and diameter decreased concomitantly, thus maintaining an essentially constant value of contact angle. This stage did not occur on a rough substrate. Finally, height, diameter and contact angle all decreased somewhat sporadically as the drop volume tended to zero. This behaviour was probably related to anchoring effects of the triple line on heterogeneities. A simple model explaining the essentially linear decrease in drop height with time during the first stage is proposed.