Critical Dynamics of the Binary System Nitroethane/3-Methylpentane: Relaxation Rate and Scaling Function

Thermo-osmosis of a near-critical binary fluid mixture: A general formulation and universal flow direction

We consider a binary fluid mixture, which lies in the one-phase region near the demixing critical point, and study its transport through a capillary tube linking two large reservoirs. We assume that short-range interactions cause preferential adsorption of one component onto the tube's wall. The adsorption layer can become much thicker than the molecular size, which enables us to apply hydrodynamics based on a coarse-grained free-energy functional. For transport processes induced by gradients of the pressure, composition, and temperature along a cylindrical tube, we obtain the formulas of the Onsager coefficients to extend our previous results on isothermal transport, assuming the critical composition in the middle of each reservoir in the reference equilibrium state. Among the processes, we focus on thermo-osmosis-mass flow due to a temperature gradient. We explicitly derive a formula for the thermal force density, which is nonvanishing in the adsorption layer and causes thermo-osmosis. This formula for a near-critical binary fluid mixture is an extension of the conventional formula for a one-component fluid, expressed in terms of local excess enthalpy. We predict that the direction of thermo-osmotic flow of a mixture near the upper (lower) consolute point is the same as (opposite to) that of the temperature gradient, irrespective of which component is adsorbed on the wall. Our procedure would also be applied to dynamics of a soft material, whose mesoscopic inhomogeneity can be described by a coarse-grained free-energy functional.

Dynamic Scaling and Slowing Down in Chemical Reactions of the Critical Triethylamine−Water System

Between 100 kHz and 1 MHz, special ultrasonic attenuation measurements of the triethylamine-water mixture of critical composition have been performed for the determination of the Bhattacharjee-Ferrell scaling function. The experimental data are evaluated considering two noncritical Debye-type relaxation terms as revealed by broadband ultrasonic spectra. Shear viscosity and dynamic light scattering data from the literature are re-evaluated to yield the relaxation rate of order parameter fluctuations of the critical system as a function of temperature. The power law behavior found for the relaxtion rate fits to the scaling function in the ultrasonic spectra. The relaxation times of the noncritical Debye terms display a non-Arrhenius temperature dependence, pointing at effects of slowing in the chemical reactions associated with the relaxations.