Instability of a fluid-fluid interface in driven colloidal mixtures

A Rayleigh-Taylor-like interface instability is studied in a compressible Brownian Yukawa fluid mixture on the 'molecular' scales of length and time of the individual particles. As a model, a two-dimensional phase-separated symmetric binary mixture of colloidal particles of type A and B with a fluid-fluid interface separating an A-rich phase from a B-rich phase is investigated, by means of Brownian computer simulations, when brought into non-equilibrium via a constant external driving field which acts differently on the different particles and perpendicular to the interface. Two different scenarios are observed which occur either for high or for low interfacial free energies as compared to the driving force. In the first scenario for high interfacial tension, the critical wavelength λc of the unstable interface modes is in good agreement with the classical Rayleigh-Taylor formula provided that dynamically rescaled values for the interfacial tension are used. The wavelength λc increases with time, representing an effect of self-healing of the interface due to a local density increase near the interface. The Rayleigh-Taylor formula is confirmed even if λc is of the order of a molecular correlation length. In the second scenario for very large driving forces as compared to the interfacial line tensions, on the other hand, the particles penetrate the interface easily due to the driving field and form microscopic lanes with a width different from the predictions of the classical Rayleigh-Taylor formula. The results are of relevance for phase-separating colloidal mixtures in a gravitational or electric field