Shear-Induced Birefringence in Critical Mixtures

A large birefringence δn has been discovered in the critical binary fluids nitrobenzene---n-hexane and isobutyric-acid-water submitted to a shear flow (rate S). In agreement with the Onuki and Kawasaki theory, two typical regions are in evidence: (i) strong shear, where δn~S1/3 and is temperature independent; (ii) weak shear, where δn~S and varies strongly with temperature. Moreover, this effect confirms the contribution of fluctuations to the critical anomaly of the refractive index in fluids at equilibrium, and could become a new tool for studying critical phenomena.