Some Things Never Change: A simple model explaining drying droplet depositions of colloidal particles

Drying colloidal suspension droplets are known to leave a ring-shaped deposit of particles, an effect seen commonly in stains left by drops of coffee. This ring is formed by an evaporative-driven flow transporting particles radially towards the droplet’s edge. Over the last 20 years, investigations were motivated by the goal of controlling this deposit pattern, focusing on spherical particles in low concentrations. Recent work investigates the role of particle geometry and higher concentrations on the nature of the dried deposit, but not in a systematic way. Here, we experimentally explore the effect particle shape has on ring formation using chemically identical colloidal particles of varying shape. We synthesize silica rods at aspect ratios ranging from 1-20, and study how shape and particle concentration influence the final deposit pattern. Using surface profilometry, we demonstrate that the ring pattern persists for all aspect ratios and concentrations. We find that ring width is independent of particle shape and follows a square root dependence on concentration. The dependence is well-captured by a simple geometric model, which characterizes the pattern at vanishingly small concentrations. Surprisingly, the data reveals that this model describes the pattern exceptionally well, even at concentrations approaching 40% by volume. Our work suggests that previous findings of shape-induced ring suppression are better explained by modification of absorption properties, and not particle shape.