Shear-induced effects on critical concentration fluctuations

The effect of a shear flow on the critical behavior of a binary fluid (cyclohexane-aniline) has been investigated by light-scattering techniques. Turbidity and scattered-light-intensity measurements have been made, and the influence of all the independent parameters of the phenomenon were considered, i.e., temperature $T$, shear rate $S$, transfer wave vector $\left|\overrightarrow{\mathrm{q}}\right|$, and its projection $q_X$ along the flow direction $X$. The data support the Onuki-Kawasaki (OK) description, namely, (i) an effect exists only in the region $S\tau >1\mathrm{}$, with $\tau$ the lifetime of concentration fluctuation. (ii) The critical temperature $T_c$ is lowered by the shear, so that $T_c=T_c\mathrm{}\left(S\right)=T_c\mathrm{}\left(0\right)-T_0{S}^{{\sigma }_0}$, with ${\sigma }_0=\frac{1}{3\nu }$ and $\nu =0.630\mathrm{}$ the standard exponent of the correlation length in fluids. We find experimentally $T_0=(1.8\mathrm{}\pm 0.2)\times {10}^{-4\mathrm{}}$ and ${\sigma }_0=0.53\mathrm{}\pm 0.03$, to be compared with the OK values 1.28 × ${10}^{-4\mathrm{}}$ and 0.529. (iii) The susceptibility ${\chi }_{\overrightarrow{\mathrm{q}}}$ shows an anisotropy versus $X$, and follows a mean-field behavior versus temperature. The OK dependence $\chi _{\overrightarrow{q}}^{}{}_{}{}^{-1\mathrm{}}\mathrm{v}\simeq A{S}^{{\sigma }_1}{[T-T_c\mathrm{}(S\left)\right]}^{\gamma }+B{S}^{{\sigma }_2}{q}_X^{\omega }+q^2$ fits the data well, with ${\sigma }_1\simeq 0.14\pm 0.05 [\mathrm{OK}:\frac{(2\mathrm{}\nu -1)}{3\nu }=0.137]$, $\gamma \simeq 1.00\pm 0.06 (\mathrm{OK}:\gamma =1)$, $\omega \simeq 0.40\pm 0.05 (\mathrm{OK}:\frac{2}{5})$, except for