I•ECHA•ISMS OF DETERGEiGY 367 i.e. wetting is perfect. Only one effect has so far been recorded x9 whereby foam exerts a specific action. Physically, foam consists of a large number of lamellae or sheets of liquid phase which bind the individual bubbles. The regions where such lamellae intersect are known as plateau borders (Fig. 11). In the centre of the lamellae the surfaces are effectively parallel and the internal pressure is essentially the same as in the adjacent air cell. At the intersection the surface is cylindrically curved and consequently a reduction in pressure occurs, equal to y/r where y is the surface tension (a) Oil patch before contact with foam. (b) After 15 sec. contact. • ,'.. : .2 • •: --'- ..... •.... (c) After 45 sec. (d) After 3 min. Figure 1oe Action of fine foam {alkyl sulphate) on oil patch ( X 8). and r the radius. (At the tetrahedral points where the plateau borders intersect a spherically curved surface arises with double the effect.) This pressure differential between the lamellae and the borders assists drainage of the foam. When a mass of foam comes into contact with an oily smear (Fig. 1oe), the oil is drawn into the plateau borders, and if sufficient oil is

•68 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS present, a lacy 3-dimensional network of oil spreads out far beyond the original bounds of the smear. The efficiency of the process varies with the pressure differential, which varies inversely with the moisture content of ß •-- ,1 • • -'-.ß-• • -: .• ß .• • . • ...•.•... .... ::,: . ...... Figure 13 Action of alkyl sulphate foam on vertical oil smear. the foam. This is clearly demonstrated in Fig. 15, in which the higher, and therefore drier, regions of a foam extend the oil smear more effectively. Where foam moves over an oily surface a succession of pressure waves is Figure 1• (a) Eefore exposure. (b) Alkyl sulphate foam after 2 min. (c) CeIyl trimethyl ammonium bromide foam after 2 min.