Liquid droplets and solid particles at surfactant solution interfaces

Abstract

We consider the behaviour of liquid droplets at the liquid/vapour interface. We discuss the feasibility of drop entry into the interface in terms of the various interfacial tension in equilibrated systems, noting the thermodynamic constraints on equilibrium tensions; we also note the important possibility of the formation of metastable thin films between drop and interface. Dispersed oil droplets are often effective antifoam agents, and the discussion is extended to a consideration of the rupture of thin liquid films (and foams) by dispersed liquid droplets. Attachment of solid particles to liquid interfaces is determined by the solid wettability, i.e. the contact angle of the liquid interface with the particle. As with liquid droplets, solid particles can bridge and rupture thin liquid films. We analyse this process for symmetrical (gas/liquid/gas and liquid/liquid/liquid) films, which is central to the effects of solid particles on the stability of foams and emulsions. Some preliminary work on the bridging of asymmetrical (liquid/liquid/gas) films is also briefly discussed. A recent method for the direct determination of the contact angle of a liquid interface with small (diameter ⩽3 µm say) monodisperse spherical particles is explained and it is shown how this method can be extended to the measurement of line tensions of solid/liquid/gas contact lines. We point to important effects on particle wettability at interfaces (wetting transitions) which could result from the existence of high line tensions.