Diffusion in a nonequilibrium binary mixture of hard spheres swelling at different rates.

The nonequilibrium dynamics of a probe in a driven binary mixture of effective hard-sphere particles has been measured computationally in molecular dynamics simulations so as to obtain a better understanding of the energy and spatial correlations that persist through the coupling between the binary components. The driving of the particles is manifested through a change of the effective volume (or equivalently, diameter of the hard spheres) and each component is assumed to have a different time-dependent profile. Such a driving is possible in a suspension of one-component colloidal mesogens, for example, in which the particle volume has been seen to change with pH or temperature changes in the solution. It can also be realized by growing nanoparticles during a nucleation process. The full particle dynamics has been projected onto Langevin-type models of the probe motion by representing the environment using two different reservoirs and distinct bath-probe coupling coefficients with different nonstationary properties. The bath particles corresponding to each reservoir swell with time at various rates, nonsynchronously changing their volume fractions. Under the assumption of a weak bath-bath interactions, the coupling coefficients between the probe and two baths are expressed via those in the case of a simple-consisting of one bath-environment. The general form of the resulting irreversible Langevin equation is in agreement with the MD simulations of a hard sphere probe particle diffusing in the nonstationary binary mixture.