Coalescence of contaminated water drops at an oil/water interface: Influence of micro-particles

Abstract

The effect of micro-particles and interface aging on coalescence of millimetre-sized water drops with an oil/water interface is studied over long times. The system is not pure and interface contamination grows with time, resulting in a slow but continuous decrease of interfacial tension over time (from 35 to 10 mN/m), which is measured in situ using an original technique. Without added micro-particles, coalescence times are randomly distributed and uncorrelated to drop diameter or interfacial tension. In presence of 10 μm size hollow glass particles at the oil/water interface, coalescence times become more reproducible and show a clear dependence upon drop diameter and interface aging. Results are consistent with a classical drainage model assuming that the critical thickness at which interstitial film ruptures scales as the micro-particle diameter, a result that tends to validate the bridging scenario. Interestingly, the film retraction speed during the coalescence process does not follow theoretical predictions in a planar geometry. High-speed imaging of the retracting film reveals that the hole rim is bending upward while retracting, resulting in a strong slowdown of retraction speed. This is caused by the difference of interfacial tension between oil/drop freshly formed interfaces and the aged oil/water interface.