Spreading and wetting phenomena in four-phase (liquid/liquid/substrate/vapor) configurations. I. Experimental studies of static profiles

Abstract

The spreading and wetting of an oil drop on a solid substrate partially submerged in a thin film of water was measured and recorded photographically. The configuration that a four-phase (oil/water/substrate/air) model adopts is determined mainly by the thickness of the water film. Decreasing this initially causes no change in the oil-drop diameter while the oil/water/substrate contact angle is increasing from its equilibrium to its advancing value; once this is reached, the oil drop spreads rapidly until, at a critical water-film thickness, it disrupts or separates from the water phase, to form an oil drop surrounded by bare substrate. However, if this decrease is reversed short of the critical value for disruption, the oil-drop diameter does not change while the oil/water/substrate contact angle is decreasing from its advancing to its receding value; once this is reached, the oil drop recedes rapidly until, at another critical water-film thickness, it splits to form a fully submerged drop on the substrate and an oil lens floating on the surface. If the decrease in water film thickness is reversed short of the critical value for splitting, the wetting hysteresis cycle can be repeated. The factors affecting spreading and wetting in these configurations have been studied and it has been demonstrated that extensive properties (e.g., nature and volume of oil, thickness of water film) as well as the usual intensive properties (e.g., surface and interfacial tensions, nature of substrate surface) have to be taken into account to fully describe these states. Spreading and wetting phenomena were very reproducible and four-phase states were found not only to exist over a wide range of conditions for the same substrates but particular states could be approached from several directions with complete reversibility. Some consideration has been given to five-phase configurations where a heterogeneous solid substrate provides a complex matrix of areas either preferentially wetted by water or oil. These systems can be described by extending the theories developed for four-phase (homogeneous substrate) systems.