Controlling Stratification of Colloids Using a Mixed Binary Solvent*

Stratification in drying suspensions of polydisperse colloids has recently attracted attention as it may lead to a facile method of fabricating layered coatings and thin films. To make it technologically viable, it is important to control the structures that result from the drying process. With large scale molecular dynamics simulations, we show that stratification of suspended colloids can be controlled by using a mixed binary solvent with each component possessing a different relative volatility. When the solvent evaporates, the two components evaporate at different rates and exhibit opposite concentration gradients in the solution phase, with the less volatile component enriched at the evaporating surface. For a binary colloidal mixture suspended in the mixed solvent with each colloidal species having opposite preferences to the two solvent components, the two colloids naturally stratify with the one on top that is more strongly coupled to the less volatile solvent component, even if the two colloidal species have the same size and interparticle interaction. Our results thus reveal a useful method of controlling stratification that can be easily implemented and scaled up.