On the anomalous behavior of surfactant contaminated drops

In a previous paper (S. Padmanabhan and A. Bose, J. Colloid Interface Sci., 123, 494, 1988), experimental dimensionless sessile drop volumes for solutions of strong surfactant (large variation of surface tension with concentration) at the point of incipient motion on an inclined substrate were found to be several times the analytical prediction by Dussan V. (J. Fluid Mech., 151, 1, 1985). These experiments were conducted by placing a drop on a horizontal substrate, then tilting further to the critical configuration. The equilibrium surface tension was used to evaluate the nondimensional drop volume at critically. In experiments reported here, drops were placed on substrates pretilted to different initial angles, with subsequent tilting to criticality. The key results are that as pretilt angle is increased, the experimental drop volumes converge to their corresponding analytical predictions, while the critical inclination angle decreases. Additionally, if the equilibrium surface tension is used for computing the capillary force, the capillary force equals the gravitational force only for the most pretilted substrate. At each pretilt angle, an average adjusted surface tension allows experimentally and analytically predicted drop volumes to match. If this surface tension is used to compute the capillary force, the capillary force equals the gravitational force at criticality for each angle of pretilt. These results suggest that morphological changes that occur when the drop goes from its initial to its final state cause a nonequilibrium surface tension to develop on the liquid-vapor interface. Adsorption-desorption barriers or diffusional resistances to transport of surfactants prevent equilibration within the time scale of the experiments. Furthermore, these morphological changes are opposed by the hydrodynamic rigidity of the surfactant-laden interface. The role of these barriers as well as that of surfactant elasticity is progressively reduced with increasing substrate pretilt. These experiments validate the predictive power of the analysis by Dussan V. even for contaminated liquids and show that a similar analysis could be used in other geometries as well. However, to avoid artifacts, the proper procedure for obtaining critical inclination angles is to increasingly pretilt the substrate until the drop slides spontaneously and pick the final configuration for subsequent analysis. © 1990