Control Stratification in Drying Particle Suspensions via Temperature Gradients

The drying of polydisperse particle suspensions has recently attracted great attention as the particles may stratify according to sizes after drying, which may lead to new fabrication methods of multilayered films. We use molecular dynamics simulations to study a drying suspension of bidisperse nanoparticles and demonstrate a strategy of controlling stratification. When the suspension is kept isothermal during fast drying, it can exhibit “small-on-top” stratification with the smaller particles accumulated in the top region of the drying film. However, when only the region near the substrate is thermalized, a negative thermal gradient develops in the suspension because of evaporative cooling. Since the associated thermophoresis is stronger for larger nanoparticles, relatively more larger nanoparticles migrate into the top region of the drying film. The net result is either diminished “small-on-top” or converted “large-on-top”. By imposing a positive thermal gradient in the drying suspension via thermalizing the vapor at a higher temperature than the solvent, we observe very strong “small-on-top” stratification. Possible experimental approaches to realize various thermal gradients are suggested.