Investigation of penetration abilities of various oils into human hair fibers.

In this work we have explored capillary adhesion between hair fibers treated with different types of oils. With coconut, olive, and sunflower oils the capillary adhesion was found to decrease with time, but not with mineral oil. Application of heat reduced the capillary adhesion further for coconut and sunflower oils. Again, this was not observed with mineral oil. Based on an earlier study, where coconut oil was found to penetrate hair while mineral oil was unable to do so, it was hypothesized that the reduction in capillary adhesion resulted from the penetration of oil into the fiber, leaving a thinner oil film on the surface. Such a reduction in capillary adhesion can be explained by changes in Laplace pressure and in the areas of liquid bridges formed between the fibers. The thinning of oil films on the surface of hair has been confirmed independently by goniophotometric measurements on single hair fibers treated with coconut, sunflower, and mineral oils. Thick films of oil (thicker than approximately 0.5 microm) are known to mask the scale structure. As the oil is absorbed into the hair, the film thins with time and application of heat, and the scale structure reappears. This change can be conveniently determined by measuring the scale angle, using the well established goniophotometric protocol. The agreement between the two methods supports the concept that the reduction in capillary adhesion between hair fibers is most likely due to thinning of oil films by absorption of oil into the hair.