The role of hydrodynamic interactions in models and simulations of drying*

Nonequilibrium molecular modeling is valuable for predicting the microstructure of drying films. For example, simulations were recently key to uncovering the “small-on-top” stratification of binary colloidal mixtures. In the absence of solvent effects, drying can be well-described using implicit models without hydrodynamic interactions; however, these models tend to overestimate the solute migration speeds and stratification observed in experiments. Here, I will highlight two examples from our recent work that clearly demonstrate the important role of hydrodynamic interactions in nonequilibrium models of drying. I will first discuss the evaporation-induced crystallization of a colloidal dispersion, where we found that the crystal nucleation and growth depended sensitively on the hydrodynamic interactions because of the colloid distribution in the drying film. I will then discuss the stratification of a mixture of two differently sized polymers; the mixture stratified without hydrodynamic interactions, as predicted, but did not stratify when they were included. Our work shows that although implicit-solvent models neglecting hydrodynamic interactions are computationally efficient, the corresponding sacrifice in accuracy may not be acceptable for faithfully predicting structure in drying films.