MECHANISMS OF DETERGENCY 363 in Fig. 10 is further complicated by the deposition of acid soap complex due to the low pH of the sweat salts contained in the wool. In this sequence, no rolling up can be observed but rather a slow penetration, emulsification Figure 9 Osmotically inflated oil (liq. paraffin-oleic acid mixture) on wool fibre in 0-02% sodium oleate ((300). (This is in effect a coating of a water-in-oil emulsion.) and dispersion of the soiling matter. The addition of alkali will neutralise the free fatty acid component to these complex envelopes and reduce their extent, but the fatty alcohol and sterol complexes will not be affected. MECHANICAL ACTION To most people the word "washing" conjures up a picture involving some form of motion. It is not difficult to visualise the need for agitation, in order to assist diffusion processes, e.g. the migration of detergent to the soiled surface and reverse migration of dirt away from the surface into the bulk solution. In particular, the viscous drag of a flow of liquid past an oil drop can cause detachment before the contact angle reaches 180 ø and likewise can assist the breakdown and dissolution of liquid crystalline complex gel. In most cases, particulate or pigment dirt appears to be bound to the substrate by wetting forces, e.g. by grease films. The removal of such dirt is then governed by the removal of the fatty binder. In some cases, however, the pigment is attached direct to the surface by mechanical imprisonment (micro-occlusion) or by the normal Van der Waals London forces. In these cases, a direct mechanical force is necessary to move the soiling matter. High velocity water jets minimise the thickness of the "stagnant" layer at the surface, but due to viscous forces the velocity of the detergent at the actual surface always approaches zero. Thus it is found very difficult to achieve the same degree of cleaning by violent agitation of a fluid bath, as can be achieved by a direct wipe over with a cloth. In the latter case the two surfaces are in direct contact, and soiling particles are physically pushed from the surface.

•4 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (b) (c) (d) , _F•gu•e 10 Removal and emulsification of a 3: 1 mixture of Oildag and oleic acid from a n•turally saline wool fibre, in 0-2% sodium oleate { x300). (a) 1 rain (b) 2 rain (c) 10 min (d) 30 min