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 |→q|, and its projection qX along the flow direction X. The data support the Onuki-Kawasaki (OK) description, namely, (i) an effect exists only in the region Sτ>1, with τ the lifetime of concentration fluctuation. (ii) The critical temperature Tc is lowered by the shear, so that Tc=Tc(S)=Tc(0)-T₀S^σ₀, with σ₀=13ν and ν=0.630 the standard exponent of the correlation length in fluids. We find experimentally T₀=(1.8±0.2)×10^-4 and σ₀=0.53±0.03, to be compared with the OK values 1.28 ×{} 10^-4 and 0.529. (iii) The susceptibility χ_→q shows an anisotropy versus X, and follows a mean-field behavior versus temperature. The OK dependence ${χ}_{{{→}}{q}}^{}{}_{}{}^{{-}1}v{}A{S}^{{{σ}}₁}{[T{-}{T}c(S)]}^{{γ}}+B{S}^{{{σ}}₂}{q}X^{{ω}}+{q}²$ fits the data well, with ${{σ}}₁{}0.14±{}0.05 [OK:{(2{ν}{-}1)}{3{ν}}=0.137]$, ${γ}{}1.00±{}0.06 (OK:{γ}=1)$, ${ω}{}0.40±{}0.05 (OK:2/5)$, except for ${{σ}}₂$, whose experimental value ${{σ}}₂{}1±{}0.15$ is about twice the OK value 8/15. Discrepancies of about a factor 2 are also found for the amplitudes $A$ and $B$.