Wettability of spherical particles at liquid surfaces

Small solid particles when present in thin liquid films are often able to change the stability of the films and hence, for example, influence the stability of foams and emulsions in which they are present. To be effective, a particle must be capable of entering a liquid surface. After entry the behaviour of the particle then depends on the extent to which it is wetted by the liquid(s). Wetting of particles is conveniently discussed in terms of the contact angle which the liquid surface makes with the particle at the contact line; here we are concerned with spherical solid particles. Both the ability of a particle to enter a liquid surface, and the contact angle established after entry are in principle dependent on the existence and magnitude of the line tension operating in the three-phase contact line. When a significant line tension exists (and very high values of the order of 10(-6)-10(-5) N have recently been reported), the contact angle also depends on particle radius. If the line tension is high enough, a particle can be prevented from entering the interface either because a significant activation energy exists for entry or because the particle would be thermodynamically unstable in the interface. In the present report we describe a simple method for the observation of the entry (or otherwise) of small spherical particles, radius 10 mu m and upwards, into the liquid/vapour interface and for the determination of the contact angle as a function of particle size. It is concluded from the present work in conjunction with a previous analysis of line tension effects (R. Aveyard and J. H. Clint, J. Chem. Sec., Faraday Trans., 1996, 92, 85) that high line tensions (of the order of 10(-6) N and higher) do not operate in the systems studied, which involve dodecane in contact with fluorinated glass surfaces.