560 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS since the gas within the array will probably be stagnant. The higher the inertia of the particles the further will they be able to penetrate. This effect will also be shown on the head. Many of the smaller particles will be deflected away by the gas flow around the head and only the larger particles, or the smaller particles at the very centre of the spray, will be deposited on the hair. The second effect which could produce greater penetration of the larger particles is incomplete capture of the particles by the hair fibres. The aerosol capture theory assumes that those particles which contact a fibre are com- pletely captured, but it is very likely that large high velocity particles might shatter on impact with the fibres, producing several smaller droplets which penetrate further into the array. Only a fraction of the initial droplet is retained at the first impact. This effect would become of greater significance when the particles approach or become larger than the diameter of the fibres, a condition which exists for many of the sprays studied. CONCLUSIONS Measurement of the velocity of aerosol sprays using a Pitot-static tube to measure the velocity of the gas stream rather than the actual particle velocity have shown that there is a velocity distribution across the spray cone. The velocity rises to a maximum at the centre of the cone and this maximum falls off with increasing distance from the actuator, and with decreasing pressure of the aerosol pack for a given distance from the actuator. The capture of hair spray droplets by arrays of fibres backed by a solid plate representing the scalp does not agree with the behaviour predicted from classical aerosol capture theory, that is that the fine sprays containing small droplets would be more penetrating than coarse sprays. In practice it has been found that coarse sprays are more capable of achieving penetration into the fibre array. It has been found that the penetration increases with increasing value of the product d•V where d is the mass median diameter of the aerosol spray, and V is the maximum velocity of the spray at a distance of 150 mm from the actuator. This was the experimental distance used between the actuator and the fibre array and corresponds approximately to the spraying distance used by the consumer. The observed capture behaviour can be explained in terms of the greater inertia of the larger particles which is necessary to carry the particles into the array of fibres. Normal aerosol capture experiments use a filter

FACTORS CONTROLLING THE ACTION OF HAIR SPRAYS--Ill 561 which is open at both ends so that the gas stream and the particles can flow right through. Larger particles are then more efficiently captured by a com- bination of the inertial and direct interception mechanisms. When the array of fibres is backed by a solid plate most of the gas stream will be deflected around the front of the array. Small, low inertia particles will tend to follow the gas flow lines and any particles which do enter the array will travel only a small distance before losing their remaining inertia since the air within the array will be largely stagnant. Large particles will leave the gas flow lines much more easily to enter the fibre array and will then travel further because of their greater inertia. A second factor which could help to achieve greater penetration with larger particles is splitting of droplets on impact with hair fibres. This splitting is liable to be greater the greater the particle inertia. The droplet fragments so produced are then capable of further penetration into the array. (Received: 2nd May 1974) REFERENCES (1) Rance, R. W. Studies of the factors controlling the action of hair sprays--I: The spreading of hair spray resin solutions on hair. J. Soc. Cosmet. Chem. 24 501 (1973). (2) Rance, R. W. Studies of the factors controlling the action of hair sprays--II: The adhesion of hair spray resins to hair fibres. J. Soc. Cosmet. Chem. 25 297 (1974). (3) Rance, R. W. Particle size distribution measurement of hair sprays using an image-splitting particle size analyser. J. Soc. Cosmet. Chem. 23 177 (1972). (4) Light, W. The movement of aerosol particles. J. Soc. Cosmet. Chem. 23 657 (1972).