FACTORS CONTROLLING THE ACTION OF HAIR SPRAYS--Ill 557 ,o F Figure 6. Pressure Actuator (kN m -s) 1. Mechanical break-up 165 2. Mechanical break-up 262 3. Precision 2-piece 165 4. Mechanical break-up 303 5. Aerosol Research PKN 38 165 6. Mechanical break-up 359 I oo i I 2 3 4 5 6 F•ller number Penetration of hair spray droplets into model fibre array, placed 15 cm from the actuator. Pressure Actuator (kN m -2) 7. Precision 2-piece 214 8. Precision 2-piece 283 9. Aerosol Research PKN 38 234 10. Precision 2-piece 372 11. Aerosol Research PKN 38 303 12. Aerosol Research PKN 38 372

558 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table IV. Penetration of sprays into model filter placed 150 mm from actuator Ranking Actuator/ d V d s V order pressure (kN m -s) (l•m) (cm s -•) (cm 3 s-x) 1 MBU/165 1000 200 2.00 2 MBU/262 500 400 1.00 3 P2P/165 270 200 0.146 4 MBU/303 121 450 0.066 5 PKN-38/165 75 400 0.023 6 MBU/359 81 600 0.039 7 P2P/214 225 400 0.203 8 PKN-38/283 160 200 0.051 9 P2P/234 100 450 0.045 10 P2P/372 60 600 0.022 11 PKN-38/303 80 450 0.029 12 PKN-38/372 63 600 0.024 To investigate the dependence of penetration on the inertia of the particles more fully, it is necessary to consider both the velocity and the dimensions of the particles. If a particle is projected into still air with an initial velocity V cm s -1, and if the particle motion subsequently obeys Stokes law, then the distance travelled by the particle before coming to rest is known as the 'stop distance' and is given by: 'stop distance' = 307 d 2 p V (7) where d is the particle diameter in cm and p is the particle density in g cm -a. The quantity d2V is a measure of the inertia of the particle. Since the velocity and size of the particles within each spray both follow a distribution it is not a simple matter to calculate an inertia value which can be rigidly applied to each spray. Perhaps the best that can be hoped for is to use some average value of particle diameter and velocity for each spray. This may be done conveniently with respect to the particle size by calculating the mass median diameters of the sprays from particle size distribution measurements as shown previously (3). It is much more difficult to obtain an average velocity for each spray and the best that could be obtained in the present study was to measure the maximum velocity of the spray at a distance of 150 mm from the actuator. The values of the mass median diameter and maximum velocity for each spray are listed in Table IV together with the calculated values of d•V. It can be seen that the general