Layering Transitions and Solvation Forces in an Asymmetrically Confined Fluid

TL;DR

This study uses classical density functional theory to explore how asymmetric confinement between a solvophilic and a solvophobic wall affects layering transitions and solvation forces in a simple fluid, revealing distinct behaviors depending on wall interactions.

Contribution

It provides a detailed analysis of layering transitions and solvation forces in asymmetrically confined fluids, highlighting differences between fully dry and partially dry solvophobic walls using DFT.

Findings

Layering transitions occur with a repulsive solvation force in the dry case.

Solvation force oscillates around zero when no layering transitions occur.

Discontinuous jumps in excess grand potential at layering transitions.

Abstract

We consider a simple fluid confined between two parallel walls (substrates), separated by a distance L. The walls exert competing surface fields so that one wall is attractive and may be completely wet by liquid (it is solvophilic) while the other is solvophobic. Such asymmetric confinement is sometimes termed a `Janus Interface'. The second wall is: (i) purely repulsive and therefore completely dry (contact angle 180 degrees) or (ii) weakly attractive and partially dry (the contact angle is typically in the range 160-170 degrees). At low temperatures, but above the bulk triple point, we find using classical density functional theory (DFT) that the fluid is highly structured in the liquid part of the density profile. In case (i) a sequence of layering transitions occurs: as L is increased at fixed chemical potential (mu) close to bulk gas--liquid coexistence, new layers of liquid-like density develop discontinuously. In contrast to confinement between identical walls, the solvation force is repulsive for all wall separations and jumps discontinuously at each layering transition and the excess grand potential exhibits many metastable minima as a function of the adsorption. For a fixed temperature T=0.56Tc, where Tc is the bulk critical temperature, we determine the transition lines in the L, mu plane. In case (ii) we do not find layering transitions and the solvation force oscillates about zero. We discuss how our mean-field DFT results might be altered by including effects of fluctuations and comment on how the phenomenology we have revealed might be relevant for experimental and simulation studies of water confined between hydrophilic and hydrophobic substrates, emphasizing it is important to distinguish between cases (i) and (ii).