J. Soc. Cosmet. Chem. 2• 545-561 (1974) ¸ 1974 Society of Cosmetic Chemists of Great Britain Studies of the factors controlling the action of hair sprays-I i i.' The influence of particle velocity and diameter on the capture of particles by arrays of hair fibres R. W. RANCE*t Synopsis--The VELOCITY of an AEROSOL HAIR SPRAY has been determined by measuring the gas velocity within the spray with a PITOT-STATIC TUBE. The velocity rises to a maximum at the centre of the spray cone and falls rapidly with increasing distance from the spray orifice. For a given distance from the orifice the velocity also falls with decreasing pressure of the aerosol pack. Measurements of CAPTURE and PENETRATION of hair spray droplets into a model ARRAY OF HAIR FIBRES backed by a solid plate representing the scalp have shown that coarse sprays give better penetration than fine sprays. This is in contrast to the behaviour predicted by classical aerosol capture theory and reasons for the observed behaviour are dis- cussed. INTRODUCTION This paper is one of a series which describes studies of the factors influencing the action of hair sprays. Previous papers in this series (1, 2) have been concerned with the events occurring after the hairspray droplets * Unilever Research, Isleworth Laboratory, 455 London Road, Isleworth, Middlesex. ? Present address: Dow Chemical Co. Ltd, European Agricultural Research and Development Centre, King's Lynn, Norfolk. 545

546 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS have been deposited on the hair, and studies of the spreading of hairspray resin solutions on hair (1), and of the adhesion of hairspray resins to hair fibres have been reported (2). In another publication (3) the author has described measurements of the particle size distribution of hair sprays. The present study is concerned with measurements of the velocity of particles within the sprays, and with the capture of the particles by arrays of hair fibres. It is shown that the capture and penetration of the particles into an array of fibres depends on both the size of the particles and their velocity. Particle capture by fibre assemblies is generally treated by considering a model system in which a partide-laden gas stream is drawn through a filter composed of the fibres. In experimental studies it is convenient to control as many parameters as possible and monodisperse aerosols, travelling at well-defined air-stream velocities, are captured in a model filter in which fibres of uniform diameter are arranged with a regular interfibre spacing. In the present study we have not restricted ourselves to such a model system and have studied the capture of actual hair spray particles produced by pressurized aerosol packs. The particles were captured by grids com- posed of hair fibres arranged in a roughly parallel configuration but without any regular interfibre spacing. In order to characterize the sprays we have measured the mass median diameter of the particles, and assessed the velocity of the particles by measuring the velocity of the partide-laden gas stream with a Pitot-static tube. THE THEORY OF PARTICLE CAPTURE BY FIBRES Recently Light (4) has reviewed the forces which determine the move- ment of an aerosol particle subsequent to the initial velocity imparted by its source. The combined effect of forces such as gravity, drag, inertia, diffusion and electrostatic charges determines the path followed by a particle. For a comprehensive discussion of these forces the reader is referred to this article and we shall only consider here those forces which influence the deposition of hair spray droplets on hair fibres. As pointed out by Light, the particle size is perhaps the most important variable which is within the control of the formulator and for deposition on small surfaces, such as human hair, the particle diameter should be of the same order of magnitude as the surface dimension. The particles in hair sprays are generally quite large. The mass median diameters are commonly in the range 60 to about 300 •tm, and there is rarely greater than about 10•o by weight of the spray with diameters less