ABSTRACT A mass-area method is proposed to overcome problems in the measurement of the equilibrium contact angles for rough and hydrophilic surfaces. A goniometer usually measures the contact angle at the top plane of a rough surface, not the contact line of the solid-liquid interface. The present method estimates the contact angle indirectly from the volume of the liquid and the size of the contact area, assuming a spherical cap and consistent with a minimization of the free energy. The present method shows a roughly linear relationship with measurements by a goniometer for smooth surfaces of various solid materials with various liquids, but the goniometer measurements are smaller. An example test and the error of the present measurement method are presented and discussed
