The Role of Surface Rigidity On Drop Retention Forces

We used the novel Centrifugal Adhesion Balance (CAB) to measure the lateral adhesion forces, f_||, at the solid-liquid drop interface in such a way that it is decoupled from the normal force, f_normal. We studied the variation of f_|| with f_normal and observed that f_|| increases with f_normal for positive f_normal but decreases with f_normal for negative f_normal, and reaches a minimum around f_normal=0. The variation of f_|| with f_normal has been theoretically explained in terms of molecular re-orientation which results in stiffening of the solid surface at the three phase contact line. We use our data to calculate the stiffness associated with this molecular re-orientation. The trend of the theoretical prediction fitted nicely with the experimental data.