Attachment of spherical particles to the surface of a pendant drop and the tension of the wetting perimeter

Abstract

The attachment of small glass spheres (<50 µm diameter) to the air–water interface is studied by micro-photography on a pendant drop in the presence of a cationic surfactant. The angle of wetting on the attached particles can be estimated from the photographs. Increasing the surfactant concentration increases this angle as well as the ratio of attached to unattached particles in the surface. There is a distribution of wetting angle for the attached particles which is independent of particle size. Under conditions where the number of attached particles is roughly equal to the number of unattached particles the size distributions for both sets of particles are estimated and the average limiting size for attachment calculated. The formation of a three phase contact is opposed by a line tension associated with the wetting perimeter and a force balance on the particle gives an expression for the limiting size of attached particle from which a value of the line tension can be derived. A value of 10^–5 dyn (10^–11 N) compares very favourably with values obtained from related experiments on foam-flotation.