Significant Change in the Roll-Off Angle of a Liquid Droplet on a Polymer Surface at the Glass Transition Temperature

Detecting the glass transition temperature (<i>T</i>_g) of polymers is of great importance as the physical properties of polymers change dramatically at this temperature. In this study, we demonstrate that the <i>T</i>_g of a polymer surface can be determined from the roll-off angle of a liquid droplet. While the glass transition has a limited effect on the static contact angle, the roll-off angle exhibits a pronounced change at <i>T</i>_g. Above <i>T</i>_g, when the polymer surface becomes soft, the vertical force from the surface tension of the liquid droplet deforms the substrate. This deformation leads to an increased roll-off angle. We compared the behavior of spin-coated polymer surfaces with that of polymer brushes. In the case of polymer brushes, surface deformation is limited due to their constraints on the substrate. We further demonstrate that this phenomenon can be exploited to intentionally roll off a liquid droplet from a substrate upon cooling.