Sessile-Water-Droplet Contact Angle: the Effect of Adsorption

A method has been recently proposed for determining the surface tension of solid-vapor interfaces. The proposed method was used in conjunction with Gibbsian thermodynamics to investigate both analytically and experimentally the possible role of line tension in determining the contact angle of sessile-water-droplets. After forming a sessile-water-droplet in a closed system, its contact angle was determined by measuring the curvature of three-phase contact line and the height of the axisymmetric droplet on its centerline. The total number of the moles in the closed system was determined from the minimum in the Helmholtz function. The total number of moles in the system was then changed to a new value and the system allowed to come to equilibrium again. The contact angle in the new equilibrium condition could be measured and predicted by taking the adsorption at the solid-liquid and solid-vapor interfaces into account but with line tension completely neglected. The predicted values of contact angle are in closed agreement with those measured indicating line tension plays no role in determining the contact angle of mm-sized water droplets on a polished Cu surface. The surface tension of the solid-vapor interface was approximately constant and equal to the surface tension of adsorbing fluid; that is, the Young equation could be simplified.