Theoretical study of line and boundary tension in adsorbed colloid-polymer mixtures

TL;DR

This paper presents a theoretical analysis of interface and boundary tensions in adsorbed colloid-polymer mixtures using a free-energy functional approach, revealing how these tensions vary near wetting transitions.

Contribution

It introduces a method to construct complete interface potentials from single-crossing profiles and computes line and boundary tensions in different wetting regimes.

Findings

Line tensions can be positive or negative, decreasing with contact angle.

Line tension near first-order wetting is around 10^-14 to 10^-12 N.

Boundary tension increases from zero at prewetting critical point to line tension value.

Abstract

An extended theoretical study of interface potentials in adsorbed colloid-polymer mixtures is performed. To describe the colloid-polymer mixture near a hard wall, a simple Cahn-Nakanishi-Fisher free-energy functional is used. The bulk phase behavior and the substrate-adsorbate interaction are modelled by the free-volume theory for ideal polymers with polymer-to-colloid size ratios q=0.6 and q=1. The interface potentials are constructed with help from a Fisher-Jin crossing constraint. By manipulating the crossing density, a complete interface potential can be obtained from natural, single-crossing, profiles. The line tension in the partial wetting regime and the boundary tension along prewetting are computed from the interface potentials. The line tensions are of either sign, and descending with increasing contact angle. The line tension takes a positive value of 10^-14 - 10^-12 N near a first-order wetting transition, passes through zero and decreases to minus 10^-14 - 10^-12 N away from the first-order transition. The calculations of the boundary tension along prewetting yield values increasing from zero at the prewetting critical point up to the value of the line tension at first-order wetting.