Instability of a fluid–fluid interface in driven colloidal mixtures

Abstract

A Rayleigh–Taylor-like interface instability is studied in a compressible Brownian Yukawafluid mixture on the ‘molecular’ scales of length and time of the individual particles. As amodel, a two-dimensional phase-separated symmetric binary mixture of colloidal particlesof type A and B with a fluid–fluid interface separating an A-rich phase from a B-rich phaseis investigated, by means of Brownian computer simulations, when brought intonon-equilibrium via a constant external driving field which acts differently on the differentparticles and perpendicular to the interface. Two different scenarios are observed whichoccur either for high or for low interfacial free energies as compared to the drivingforce. In the first scenario for high interfacial tension, the critical wavelengthλc of the unstable interface modes is in good agreement with the classical Rayleigh–Taylor formulaprovided that dynamically rescaled values for the interfacial tension are used. The wavelengthλc increases with time, representing an effect of self-healing of the interface due to a localdensity increase near the interface. The Rayleigh–Taylor formula is confirmed even ifλc is of the order of a molecular correlation length. In the second scenario for very largedriving forces as compared to the interfacial line tensions, on the other hand, the particlespenetrate the interface easily due to the driving field and form microscopic lanes with awidth different from the predictions of the classical Rayleigh–Taylor formula. The resultsare of relevance for phase-separating colloidal mixtures in a gravitational or electric field.