Shear-Induced Birefringence in Critical Mixtures

A large birefringence $\delta n$ 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 $\delta n\sim S^{\frac{1}{3}}$ and is temperature independent; (ii) weak shear, where $\delta n\sim S$ 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.