Wetting and nonwetting near a tricritical point

TL;DR

This paper challenges the traditional view of critical-point wetting by demonstrating, through a mean-field theory, that a nonwetting gap exists near a tricritical point, with complete wetting only very close to it.

Contribution

It provides the first theoretical demonstration of a nonwetting gap near a tricritical point in three-phase equilibria, contradicting the common belief in universal critical-point wetting.

Findings

Complete wetting occurs only very close to the TCP.

Nonwetting persists away from the TCP, with no wetting transition.

Reentrant wetting can occur far from the TCP with a different phase.

Abstract

The dihedral contact angles between interfaces in three-fluid-phase equilibria must be continuous functions of the bulk thermodynamic fields. This general argument, which we propose, predicts a nonwetting gap in the phase diagram, challenging the common belief in "critical-point wetting", even for short-range forces. A demonstration is provided by exact solution of a mean-field two-density functional theory for three-phase equilibria near a tricritical point (TCP). Complete wetting is found in a tiny vicinity of the TCP. Away from it, nonwetting prevails and no wetting transition takes place, not even when a critical endpoint is approached. Far from the TCP, reentrant wetting may occur, with a different wetting phase. These findings shed light on hitherto unexplained experiments on ternary water-oil-nonionic amphiphile mixtures in which nonwetting continues to exist as one approaches either one of the two critical endpoints.