  Recent molecular dynamics simulations show that thermal gradients can induce
electric fields in water that are comparable in magnitude to electric fields
seen in ionic thin films and biomembranes. This surprising non-equilibrium
phenomenon of thermomolecular orientation is also observed more generally in
simulations of polar and non-polar size-asymmetric dumbbell fluids. However, a
microscopic theory linking thermomolecular orientation and polarization to
molecular properties is yet unknown. Here, we formulate an analytically
solvable microscopic model of size-asymmetric dumbbell molecules in a
temperature gradient using a mean-field, local equilibrium approach. Our theory
reveals the relationship between the extent of thermomolecular orientation and
polarization, and molecular volume, size anisotropy and dipole moment.
Predictions of the theory agree quantitatively with molecular dynamics
simulations. Crucially, our framework shows how thermomolecular orientation can
be controlled and maximized by tuning microscopic molecular properties.
