Stratification of drying particle suspensions: Comparison of implicit and explicit solvent simulations

Novel stratification phenomena have recently been discovered in drying suspensions of polydisperse mixtures of particles. In this process, molecular dynamics (MD) modeling has played an important role. One challenge faced by the MD approach is how to treat the solvent in a computational model. We use large scale MD simulations to study drying suspensions of a binary mixture of large and small particles. The solvent is first modeled explicitly and then mapped to a uniform viscous medium by matching the diffusion coefficients of the particles while keeping all the interactions unchanged. “Small-on-top'” stratification of the particles, with an enrichment of the smaller ones at the liquid/vapor interface during drying, is observed in both models under the same drying conditions. Using the implicit solvent model, we study the effect of the initial film thickness and show that the degree of stratification is controlled by the Péclet number defined with the initial film thickness as the characteristic length scale. When the Péclet numbers of large and small particles are much larger than 1, the degree of “small-on-top” stratification is first enhanced and then diminished as the Péclet numbers are increased.