The film “blow-off” experiments

Abstract

In the film “blow-off” experiments, a thin-liquid film on a solid substrate is blown off with a steady current of air. Conventional analysis in fluid mechanics shows that the blown-off profile is linear. Experiments, however, show a small departure from linearity in the immediate vicinity of the contact line. Besides this contact line anomaly, the contact line itself was observed to move against the direction of the blown air; both observations are inconsistent with the conventional theory. Since the liquid films are thin, the intermolecular forces of attraction between the liquid and the solid substrate are important. As a consequence of these forces, the film maintains equilibrium contact angle at the contact line. Away from the contact line, the film maintains a constant but time-dependent slope. These two slopes can be joined only through a curvature, giving rise to the contact line anomaly. The gradient of this curvature gives rise to a force which moves the contact line. The equations of motion and continuity were solved under relevant initial and boundary conditions for the case when the equilibrium contact angle is small but nonzero. Unlike the zero contact angle case, these contact lines can both advance and retract.