Evaporation Kinematics of Polystyrene Bead Suspensions

A study of the evaporation of polystyrene sphere−H 2 O suspensions placed on nonwetting surfaces was conducted using a video camera and computer-imaging interface. The height, diameter, contact angle, and mass were measured as functions of time for a range of sessile drop sizes, polystyrene sphere diameters, and initial suspension concentrations. For initial concentrations of polystyrene spheres greater than approximately 8%, the drop diameter changed by less than 5%, and universal trends, independent of the sphere and droplet size, in the time dependence of the height, mass, and contact angle were observed. Those situations for which the polystyrene sphere concentration was less than approximately 8% showed a larger variation of diameter with time. For the case where the diameter remained constant, a theoretical model, which is analogous to the Landau theory of phase transitions, successfully predicts the experimental height and mass data over the entire evaporation range.