Studies of the factors controlling

the action of hair spraysI: The spreading of hair spray resin solutions on hair R. W. RANCE* Synopsis--The importance of SPREADING in the action of HAIR SPRAYS is discussed from the points of view of the WETTABILITY of HAIR FIBRES by hair spray solutions and the rate of spreading of such solutions in bundles of hair fibres. The wettability depends on the magnitude of the surface tension of the solution compared with the critical SURFACE TENSION of the hair fibres, and measurements of both of these quantities showed that for the commonly employed hair spray solvents complete wettability (zero CONTACT ANGLE) invariably occurs, giving rise to spontaneous spreading on the fibre surface. The rate of spreading is shown to depend mainly on the viscosity of the solution, the rate of evaporation of the solvent and the rate of increase in the solution viscosity with concentration due to evaporation. The Washburn equation describing the rate of capillary penetration of liquids into porous systems is shown to be insufficient when dealing with volatile fluids. A new equation which is a modification of the Washburn equation, allowing for the effects of solvent evaporation and increasing solution viscosity with evaporation, is described and shown to be in good agreement with experimental data for various resin solutions preading in a bundle of human hairs