Isothermal transport of a near-critical binary fluid mixture through a capillary tube with the preferential adsorption

TL;DR

This paper investigates the isothermal transport of a near-critical binary fluid mixture through a capillary tube, focusing on preferential adsorption effects, diffusioosmosis, and how conductance varies with temperature, using numerical hydrodynamics based on free-energy functional theory.

Contribution

It introduces a numerical hydrodynamics approach based on a renormalized free-energy functional to analyze cross effects like diffusioosmosis near the critical point.

Findings

Flow rich in the preferred component when driven by pressure difference.

Diffusioosmosis occurs when flow is driven by mass-fraction difference.

Conductance in diffusioosmosis varies non-monotonically with temperature.

Abstract

We study isothermal transport of a nonelectrolyte binary fluid mixture, which lies in the homogeneous phase near the demixing critical point, through a capillary tube connecting two reservoirs. Usually, one component is preferentially adsorbed onto the tube wall, and the adsorption becomes significant owing to large osmotic susceptibility. The mixture flowing out of the tube is rich in the preferred component when flow is driven by the pressure difference between the reservoirs. When flow is driven by the mass-fraction difference, the total mass flow occurs in the presence of the preferential adsorption, which means that diffusioosmosis emerges. These phenomena can be regarded as cross effects linked by the reciprocal relation. We also study these phenomena numerically by using the hydrodynamics based on the coarse-grained free-energy functional, which was previously obtained in terms of the renormalized local functional theory. It is shown in particular that the conductance, or the total mass flow rate under a given mass-fraction difference, in diffusioosmosis can change non-monotonically with the temperature.