CONTACT ANGLES IN RELATION TO THE EFFECTS OF SOLIDS ON FILM AND FOAM STABILITY

It is well known that solid hydrophobic particles can reduce foaming, and this fact is used in formulating industrial antifoam preparations. The work to be described has centred on the acquisition of unambiguous experimental data on well-defined systems in order to probe the role of the contact angle, theta, between particles and surfactant solutions in foam and single film breaking. It is expected that spherical particles and cylindrical rods will rupture soap films by a bridging-dewetting mechanism for theta values of 90-degrees or more. Spherical glass beads (diameters about 45mum) and cylindrical rods have been silanised to varying degrees to give a range of wettabilities. For theta greater-than-or-equal-to 90-degrees rods rapidly rupture single soap films into which they are incorporated. However, spherical particles maximally stabilise foams for theta = 90-degrees. Angles in excess of about 92-degrees are needed for effective foam reduction. The stabilising effect is attributed to collection of particles in the Plateau border regions of the foams and the concomitant reduction in the rate of film drainage.