Retraction Phenomena of Surfactant Solution Drops upon Impact on a Solid Substrate of Low Surface Energy
Résumé
The impact of surfactant solutions drop on a low-surface-energy solid substrate is investigated using a high-speed photographic technique (one picture every 100 µs) which allows simultaneous top and side views. The influence of physicochemical properties is analyzed by varying the adsorption kinetics of the surfactants and the initial diameter and velocity of the drop before impact. Generally, the drop spreads and retracts under the action of inertia and capillarity, respectively. During spreading, the drop shape changes from a "truncated sphere" to a "flat pancake" and the drop surface is increased such that it is no longer at thermodynamic equilibrium. The relevant surface property is therefore the dynamic surface tension which is evaluated at the maximum diameter γd max , using the maximum bubble pressure apparatus. The dynamic surface tension has a critical influence on the drop behavior at the maximum diameter dmax and during the subsequent retraction. A simple relation combining γd max and the dynamic contact angle at dmax is derived to predict dmax. The results of this prediction agree well with the experimental measurements. Since γd max is large compared with the critical surface tension of the solid surface, a retraction of the drop is induced. The physical origin of this retraction is the apparent dynamic spreading coefficient Smax whose absolute value is correlated with the extent of the retraction. Two types of retraction are observed: a fast, destabilizing one which is described as an inertial peripheral dewetting and a slow, stabilizing one which relaxes exponentially. An empirical criterion is given on the basis of the difference between the thickness of the flattened drop at the maximum diameter and the critical thickness of metastability of a film in partial wetting conditions. It is demonstrated experimentally that this retraction proceeds on a clean solid surface and that the dewetted area is not modified by any surfactant adsorption which could have occurred during the contact time.