The role of hydrodynamic interactions in models and simulations of drying

Nonequilibrium molecular modeling has proven valuable for predicting morphology evolution during drying; for example, simulations were recently key to uncovering the “small-on-top” stratification of colloidal mixtures. In the absence of solvent effects, such morphologies can be rationalized and predicted using implicit-solvent models; however, these models tend to overestimate the extent of stratification observed in experiments. Using a mixture of two differently sized polymers as a case study, I will systematically demonstrate that this overestimation is due to neglect of hydrodynamic interactions in prior models and simulations. I will compare hybrid simulations accounting for hydrodynamic interactions between polymers through the multiparticle collision dynamics technique (and in which the dried morphology is homogeneous) with free-draining Langevin dynamics simulations neglecting the same (and in which the dried morphology is stratified). The simulation methodology unambiguously attributes the drastically different morphologies to the treatment of the solvent. The role of the hydrodynamic interactions is further quantified and rationalized using multicomponent diffusion (Onsager) coefficients. This work indicates that it is important to include hydrodynamic interactions in faithful models of evaporation-induced stratification.