Evaporation-induced Cluster Formation in High-Concentration and Low-Tg Colloidal Suspensions

Cluster formation in colloidal suspensions is often observed during drying, and understanding the origins and physical properties of the clusters is important to optimizing coating formulation. This talk reports a study of cluster formation induced by drying latex with initial volume fraction of 50% in a microfluidic channel, in a manner that allows the dynamic structure evolution to be observed by light microscopy as a function of evaporation rates, salt concentrations and particle concentrations. For our experiments, we synthesized latex particles with average diameter of 200 nm, and a glass transition temperature (Tg) that could be robustly tuned from 0 °C to 80 °C. We find that fluid flow induced by evaporation drives an initially equilibrated, well-mixed sample into a phase-separated state, raising questions of how flow forces can overcome electrostatic repulsion. Since particles deform and adhere when heated above Tg, we varied the ambient temperature to investigate whether adhesion also played a role in cluster formation. Comparing with experimental data, we are developing theoretical models to study the competition between flow force which induces clusters and electrostatic repulsion which stabilizes the suspensions.