  The Soret effect is the tendency of fluid mixtures to exhibit concentration
gradients in the presence of a temperature gradient. Using molecular-dynamics
simulation of two-component Lennard-Jones liquids, it is demonstrated that
spatially-sinusoidal heat pulses generate both temperature and pressure
gradients. Over short timescales, the dominant effect is the generation of
compressional waves which dissipate over time as the system approaches
mechanical equilibrium. The approach to mechanical equilibrium is also
characterized by a decrease in particle density in the high-temperature region,
and an increase in particle density in the low-temperature region. It is
demonstrated that concentration gradients develop rapidly during the
propagation of compressional waves through the liquid. Over longer timescales,
heat conduction occurs to return the system to thermal equilibrium, with the
particle current acting to restore a more uniform particle density. It is shown
that the Soret effect arises due to the fact that the two components of the
fluid exhibit a different response to pressure gradients. First, the so-called
isotope effect occurs because light atoms tend to respond more rapidly to the
evolving conditions. In this case, there appears to be a connection to previous
observations of fast sound in binary fluids. Second, it is shown that the
partial pressures of the two components in equilibrium, and more directly the
relative magnitudes of their derivatives with respect to temperature and
density, determine which species accumulates in the high- and low-temperature
regions. In the conditions simulated here, the dependence of the partial
pressure on density gradients is larger than the the dependence on temperature
gradients.
