Contact Angle Hysteresis from a thin film suspended on a liquid surface*

We introduce a new probe of contact angle hysteresis in a one-dimensional geometry, similar to a Wilhemy plate. Instead of a rigid plate, though, we study wetting hysteresis by dipping and withdrawing an ultrathin polymer film on the surface of a liquid. This method alleviates some difficulties associated with the Wilhelmy method, such as buoyancy corrections and asymmetric wetting of the two faces of the plate. To validate our method, we show that contact angles extracted from measurements of the force on the film are consistent with values obtained by directly imaging the vicinity of the contact line at the bendo-capillary scale, where gravity has negligible effect, as well as at the capillary scale, where gravity and surface tension forces are in balance. The microscopic contact angle is only revealed at the bendo-capillary scale, but this angle fully determines the apparent angle at the capillary scale. Thus the method with the ultrathin film allows us to measure contact angle as a function of velocity history, and in particular, to extract the quasistatic advancing, receding, and equilibrium contact angles for a polymer-water system.