Solvent mediated forces in critical fluids

TL;DR

This paper investigates solvent-mediated forces between walls in super-critical fluids using advanced density functional theory, revealing a transition from oscillatory to long-range attractive interactions near the critical point, with implications for colloidal aggregation.

Contribution

It introduces a new formulation combining Weighted Density Approximation and Hierarchical Reference Theory to accurately capture critical fluctuations in fluid-wall interactions.

Findings

Oscillatory forces diminish near criticality

Long-range attractive tail emerges close to the critical point

Results align with critical Casimir effect predictions

Abstract

The effective interaction between two planar walls immersed in a fluid is investigated by use of Density Functional Theory in the super-critical region of the phase diagram. A hard core Yukawa model of fluid is studied with special attention to the critical region. To achieve this goal a new formulation of the Weighted Density Approximation coupled with the Hierarchical Reference Theory, able to deal with critical long wavelength fluctuations, is put forward and compared with other approaches. The effective interaction between the walls is seen to change character on lowering the temperature: The strong oscillations induced by layering of the molecules, typical of the depletion mechanism in hard core systems, are gradually smoothed and, close to the critical point, a long range attractive tail emerges leading to a scaling form which agrees with the expectations based on the critical Casimir effect. Strong corrections to scaling are seen to affect the results up to very small reduced temperatures. By use of Derjaguin approximation, this investigation has natural implications for the aggregation of colloidal particles in critical solvents.