Modeling the time-dependent water wave stability of human hair.

The viscoelastic bending recovery of human hair is described by a hydro-rheologically complex, two-phase model, where the humidity dependence of the pertinent parameters as well as the effects of physical aging are known. Model calculations are conducted to assess the consequences of the time- and humidity-dependent bending recovery of human hair for the formation and the stability of the water wave. It is shown that a hair fiber that has been set in bending will achieve at 65% RH a recovery of about 50% after about ten times its storage time prior to release, if it is a non-aging material. However, aging drastically slows the recovery process so that it approaches an apparent "equilibrium," final recovery value of about 60%. The values of final recovery decrease linearly with water content, vanishing as expected at maximum water content, where the hair fiber is above its glass transition. The calculations further show that damage to the elastic modulus, attributed to the intermediate filaments, is expected to reduce recovery and thus enhance fiber set. The calculations demonstrate that it is in fact the phenomenon of physical aging that makes water waving a feasible and practically successful process for hair styling.