M.Sc Thesis

M.Sc StudentRotem Glick-Carmi
SubjectTime-Dependent Surface Tension and Wettability of Drops due
to Adsorption and Dissolution
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Pokroy Boaz



Both surface tension and wetting characteristics are cardinal in nature as well as in various industrial processes. Moreover, the understanding of how surface tension can evolve with time and its effect on wettability may contribute considerably to the understanding of different related biological mechanisms as well as for the optimization of industrial processes.

Young was the first to describe the relation between the contact angle and the surface tension, more than two centuries ago. However, most real surfaces cannot be characterized by Young's contact angle which refers to ideally flat and chemically homogeneous surfaces. Two realistic wetting models were described by Wenzel and then by Cassie and Baxter. Wenzel described homogenous wetting, where a complete liquid penetration into the roughness grooves occurs. The Cassie-Baxter model describes the heterogeneous wetting regime, where another phase is trapped underneath the liquid within the roughness grooves. There is a threshold value of the wetting angle between these two models for the superhydrophobic regime (apparent CA higher than 150˚). In many cases, when the static contact angle is smaller than this critical value, the two superhydrophobic states might coexist. Such cases raise the question of the transitions between these states. The surface tension of the liquid has a strong effect on the different states and specifically on the transition between them.

One of the potential manners by which one can control the surface tension of droplets is by the adsorption process of surfactants onto their surfaces.

The results presented in this work reveal a time-dependent surface tension evolution of water droplets both in liquid and in gaseous phases due to the adsorption and dissolution processes accompanied by contact angle alterations. More specifically, the surface tension was altered by the adsorption of stearic acid and perfluoro-octanol and the dissolution of ethanol and acetone. When such an experiment was performed on a textured surface a Cassie-to-Wenzel transition of water droplets in a gaseous phase was clearly observed.  In addition, an unexpected trend of the contact angle change was found as a result of reversible surface tension alternation due to surfactant addition and elimination.