Effects of phase Behavior on the drying of colloidal suspensions

We study the effects of phase behavior on the evaporative drying of droplets of a suspension of hard-sphere colloidal particles and nonadsorbing polymer. The presence of the polymer induces a depletion attraction between the colloidal particles. As drying (evaporation of solvent) progresses, the concentrations of both colloid and polymer increase, so that the droplet's average composition traces out a "drying line" across the composition diagram. We find that drying behavior can be broadly classified according to the initial composition of the suspension, into three regions on a "drying behavior diagram". We relate these three regions to the bulk phase diagram of the system and show how drying behavior and final residue properties such as homogeneity can be understood by considering the orientation of the "drying line" with respect to the equilibrium and nonequilibrium boundaries in the bulk phase diagram. Our findings have relevance for predicting the sensitivity of droplet and film residue properties to initial suspension composition, in the case of technologically important systems such as detergents, coatings, ceramics, and paints, as well as being an interesting example of the response of a model soft matter system continuously driven across its phase diagram toward a shifting "target" equilibrium.