  A potential strategy for controlling stratification in a drying suspension of
bidisperse particles is studied using molecular dynamics simulations. When the
suspension is maintained at a constant temperature during fast drying, it can
exhibit "small-on-top" stratification with an accumulation (depletion) of
smaller (larger) particles in the top region of the drying film, consistent
with the prediction of current theories based on diffusiophoresis. However,
when only the region near the substrate is thermalized at a constant
temperature, a negative temperature gradient develops in the suspension because
of evaporative cooling at the liquid-vapor interface. Since the associated
thermophoresis is stronger for larger nanoparticles, a higher fraction of
larger nanoparticles migrate to the top of the drying film at fast evaporation
rates. As a result, stratification is converted to "large-on-top". Very strong
"small-on-top" stratification can be produced with a positive thermal gradient
in the drying suspension. Here we explore a way to produce a positive thermal
gradient by thermalizing the vapor at a temperature higher than that of the
solvent. Possible experimental approaches to realize various thermal gradients
in a suspension undergoing solvent evaporation, and thus to produce different
stratification states in the drying film, are suggested.
