Electrical-conductivity fluctuations in binary liquid mixtures near the critical point: The effects of fluid flow

We have measured dc electrical conductivity fluctuations in isobutyric-acid—water (I-W) and 2,6-lutidine-water (L-W) mixtures near the critical temperature $T_c$. The noise was measured in a capillary separating two reservoires initially filled to slightly different heights. Power spectra $S_V\mathrm{}\left(f\right)\mathrm{}$ encompassed the frequency ($f$) range $0.01\le f\le 100$ Hz and the temperature ($T$) range $1\le |T-T_c|\le 1100$ mK. Very close to $T_c$ the spectra for L-W and I-W (at early times after mixing the sample) are consistent with the assumption that the noise is generated by flow. The temperature dependence of the noise can be represented by a power law $S_V\mathrm{}(f,T)\mathrm{}\propto {|T-T_c|}^{-\gamma }$, with $\gamma =1.0\mathrm{}\pm 0.1$. This value of $\gamma$, being smaller than the Ising value of 1.25, suggests that the composition fluctuations are suppressed by the shear flow in the capillary. We cannot explain the appreciable noise which persists after the flow has ceased. An ac method of determining $S_V\mathrm{}\left(f\right)\mathrm{}$ yields results identical with those of the dc method. Finally, we demonstrate, through a direct test, that the noise in a binary mixture near $T_c$ has a negligible nonlinear component.