Capillary Waves in a Colloid-Polymer Interface

TL;DR

This study uses large-scale Monte Carlo simulations to analyze capillary wave fluctuations at a colloid-polymer interface, confirming theoretical predictions at small wavevectors and revealing deviations at larger scales.

Contribution

It provides detailed quantitative analysis of interfacial fluctuations in a colloid-polymer mixture, validating capillary wave theory and exploring wavevector-dependent interfacial tension.

Findings

Capillary wave theory is quantitatively verified at small wavevectors.

Pronounced deviations from theory appear at larger wavevectors.

Interfacial tension decreases monotonically with increasing wavevector.

Abstract

The structure and the statistical fluctuations of interfaces between coexisting phases in the Asakura-Oosawa (AO) model for a colloid--polymer mixture are analyzed by extensive Monte Carlo simulations. We make use of a recently developed grand canonical cluster move with an additional constraint stabilizing the existence of two interfaces in the (rectangular) box that is simulated. Choosing very large systems, of size LxLxD with L=60 and D=120, measured in units of the colloid radius, the spectrum of capillary wave-type interfacial excitations is analyzed in detail. The local position of the interface is defined in terms of a (local) Gibbs surface concept. For small wavevectors capillary wave theory is verified quantitatively, while for larger wavevectors pronounced deviations show up. For wavevectors that correspond to the typical distance between colloids in the colloid-rich phase, the interfacial fluctuations exhibit the same structure as observed in the bulk structure factor. When one analyzes the data in terms of the concept of a wavevector-dependent interfacial tension, a monotonous decrease of this quantity with increasing wavevector is found. Limitations of our analysis are critically discussed.