Persistence of Coffee-Ring Deposits in Concentrated Suspensions of Anisotropic Colloids

When droplets of colloidal suspensions evaporate, the suspended colloids are transported to the edge of the drop by an outward radial flow. The resulting dried deposit has a high concentration of colloids along it's perimeter, and a low concentration near the center. This effect has been dubbed the coffee-ring effect [Deegan et al., Nature, 1997, 389, 827], and is undesirable in applications such as inkjet printing, where a smooth deposition would lead to more efficient coatings. Previous attempts at suppressing this effect using particle anisotropy have paradoxically shown that anisotropic particles can (1) form even deposits [Yunker et al., Nature, 2011, 476, 308] and (2) enhance the coffee ring effect [Dugyala et al., J. Phys. Chem. B, 2014, 119, 3860]. Here we use surface profilometry data to characterize the width of the dried deposits containing rod-shaped silica colloids of aspect ratios ranging from 1 to 20. We observe deposits over a broad range of concentrations (up to 0.35 volume fraction), and find that the width of the ring is independent of the particle anisotropy, even at previously unexplored high volume fractions. We also observe that polystyrene spheres form more even deposits than silica spheres when evaporated at high temperatures. This result serves to unify the literature by identifying a key parameter in controlling the deposit shape: the difference between the sedimentation velocity of colloids in solution and the velocity of the air-water interface.