2001 ANNUAL SCIENTIFIC MEETING 137 REAL TIME OBSERVATION OF INTERACTIONS OF HAIR SPRAYS WITH HUMAN HAIR Yash Kamath, Ph.D and Xuemin (2hen, Ph.D. TRI/Princeton, Princeton, NJ 08542 Introduction Hair sprays occupy an important position in the world of styling products. These consist of a polymer formulated in a carrier with appropriate solvents, which can be converted into an aerosol. The particles of the spray when delivered to a hair assembly will form a uniform film on the surface of fibers which becomes unstable with time to form small droplets. These droplets will move along the fiber and coalesce to form larger drops. These droplets ultimately form deposits after the evaporation of the solvent. The nature and the location of these deposits play an important role in the efficacy of the spray as a styling medium. The nature of the final deposits can be altered by changing the properties of the liquid such as surface tension and viscosity and the solvent composition. This paper is aimed at understanding the fundamental aspects involved in the interaction of sprays with hair fibers. We have investigated the behavior of a holding spray and a shine spray. An attempt has been made to explain the observed behavior on the basis of the physicochemical principles involved in the interaction of thin liquid films on a solid surface. Experimental Materials: In this work we have used a commercial "holding spray" and a "shine spray" on European dark brown hair. Method: We have developed a PC-based imaging system to record the real time interaction of a spray on hair fibers (Figl). The system consists of a high speed digital CCD camera with a zoom lens, a power supply, an image acquisition board, a pentium II computer, a light source and a fiber mounting assembly. Board Hmr•pr•y .,,.:,.:'."? • ,, .,...:,.::'5'5!':'i:!,? •ix•E•a•,•,,• •Imr•ib•(,) 0•.•Lo• a Fib•Mount•g •s•bly Pentium III I Computer i Fig. 1 Schematic diagram of the PC-based imaging system.

138 JOURNAL OF COSMETIC SCIENCE The camera has a 256x256 pixel resolution (3.51,tm/pixel) and is capable of acquiring images at a maximum rate of 200/s. A computer program [TRI/DCAM (Labview)] has been developed in house to operate the camera, to display images on the computer screen in real time, and to acquire and store images on the computer hard drive. The program adds a time stamp to each image. The images can be processed and converted into videos after the experiment. In a typical experiment the hair fiber is stretched on the pulleys by loading 10g weights on both ends. The system is activated and the spray is deposited on the fiber. The images are collected and analyzed after the experiment. Results and Discussion Stability of Liquid Films on Fiber surfaces: Figure 2 shows the sequential events which occur when a film of hexadecane (surface tension 27mN/m and viscosity 3cp) is deposited on a nylon fiber. The waves are set up whose amplitude increases with time until the film breaks up into small droplets. There are three factors that need to be considered in analyzing the stability of the film on the fiber surface. They are: 1) The positive Laplace pressure of the film with a convex curvature. 2) The disjoining pressure of the film which has a tendency to break the film 3) The adhesion between the fiber surface and the liquid film which fights the break up of the film. Fig. 2 Liquid film on a synthetic fiber that undergoes Raleigh instability and breaks up into droplets. Neimark and Kornev [1] analyzed these factors mathematically to define the stability region of the film. This is shown in Fig. 3, which shows that the stability increases with an increase in fiber diameter and decreases with film thickness. Critical Film Thickness 450 2•o 20o 0 0 10o 20o 30o 400 500 600 70o 8(:X:] 900 10o0 fiber radius, microns Fig. 3 Estimate of critical film thickness of thermodynamically stable films as a function of fiber radius