Sessile-Water-Droplet Contact Angle Dependence on Adsorption at the Solid-Liquid Interface

We investigate both analytically and experimentally the possible role of line tension in determining the Contact angle of sessile water droplets on a polished Cu substrate. In a closed system with constraints that make the Helmholtz Function the thermodynamic potential, the Curvature of the three-phase line and the height of all axisymmetric droplet on its center line could be measured. The adsorption on each of the Surfaces used to construct the experimental chamber was taken into account, and the value of the total number of water moles in the system was determined from the minimum in the Helmholtz function. The number of water moles was then changed to a new value and the system allowed to come to equilibrium again. The contact angle in the second state Could be both measured and predicted with the adsorption in the solid-liquid and solid-vapor interfaces fully taken into account but with line tension completely neglected. The predicted values of the contact angle compared closely with those Measured, indicating line tension played no role in determining the contact angle of mm-sized water droplets on a polished Cu Surface, and that the dependence of the contact angle on the Curvature of the three-phase line could be predicted by including adsorption. The contact angle values ranged from 38.3 to 76.5 degrees, indicating that the contact angle cannot be viewed as a material property of a fluid-solid combination, but Must be viewed as a thermodynamic property. The Surface tension of the solid-vapor interface was approximately constant and equal to the surface tension of the adsorbing fluid; thus, the Young equation could be simplified. The Surface tension of the solid-liquid interface was changed by more than a factor of 3.3 in the experiments.