Deposition of Colloidal Drops Containing Ellipsoidal Particles: Competition between Capillary and Hydrodynamic Forces

Ellipsoidal particles have previously been shown to suppress the coffee-ring effect in millimeter-sized colloidal droplets. Compared to their spherical counterparts, ellipsoidal particles experience stronger adsorption energy to the drop surface where the anisotropy-induced deformation of the liquid-air interface leads to much greater capillary attractions between particles. Using inkjet-printed colloidal drops of varying drop size, particle concentration, and particle aspect ratio, the present work demonstrates how the suppression of the coffee ring is not only a function of particle anisotropy but rather a competition between the propensity for particles to assemble at the drop surface via capillary interactions and the evaporation-driven particle motion to the contact line. For ellipsoidal particles on the drop surface, the capillary force (F_γ) increases with the particle concentration and aspect ratio, and the hydrodynamic force (F_μ) increases with the particle aspect ratio but decreases with drop size. When F_γ/F_μ > 1, the surface ellipsoids form a coherent network inhibiting their migration to the drop contact line, and the coffee-ring effect is suppressed, whereas when F_γ/F_μ < 1, the ellipsoids move to the contact line, resulting in coffee-ring deposition.