Stratification in binary colloidal polymer films: experiment and simulations

When films are deposited from mixtures of colloidal particles of two different sizes, a diverse range of functional structures can result. One structure of particular interest is a stratified film in which the top surface layer has a composition different than in the interior. Here, we explore the conditions under which a stratified layer of small particles develops spontaneously in a colloidal film that is cast from a binary mixture of small and large polymer particles that are suspended in water. A recent model, which considers the cross-interaction between the large and small particles (Zhou et al., Phys. Rev. Lett., 2017, 118, 108002), predicts that stratification will develop from dilute binary mixtures when the particle size ratio (alpha), initial volume fraction of small particles (phi(S)), and Peclet number are high. In experiments and Langevin dynamics simulations, we systematically vary alpha and phi(S) in both dilute and concentrated suspensions. We find that stratified films develop when phi(S) is increased, which is in agreement with the model. In dilute suspensions, there is reasonable agreement between the experiments and the Zhou et al. model. In concentrated suspensions, stratification occurs in experiments only for the higher size ratio alpha = 7. Simulations using a high Peclet number, additionally find stratification with alpha = 2, when phi(S) is high enough. Our results provide a quantitative understanding of the conditions under which stratified colloidal films assemble. Our research has relevance for the design of coatings with targeted optical and mechanical properties at their surface.