The role of evaporation and particle self-assembly on deposition morphologies of inkjet-printed drops
Authors:   , Kim Dong-Ook , Sun Ying 
Published: July 2021

Inkjet printing is a powerful tool in various applications from printable electronics to biomaterials due to its low-cost, compatibility with a wide range of ink materials, and accurate deposition of droplets in the micron diameter range. In this work, the dynamics of the contact line and deposition of inkjet-printed drops are investigated to develop the fundamental understandings of the transport phenomena and improve inkjet printing techniques by controlling final deposition morphologies. 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, we demonstrate how the suppression of the coffee-ring is not only a function of the 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. When the capillary force / the hydrodynamic force >1, ellipsoids on the drop surface form a coherent network inhibiting their migration to the drop contact line and the coffee-ring effect is suppressed, whereas when the capillary force / the hydrodynamic force <1, the ellipsoids move to the contact line resulting in coffee-ring deposition. Moreover, contact line dynamics is crucial in determining the deposition patterns of evaporating colloidal droplets. Using high-speed interferometry, we directly observe the stick-slip motion of the contact line in situ and are able to resolve the instantaneous shape of the inkjet-printed, evaporating pico-liter drops containing nanoparticles of varying wettability. The results show that the stick-slip motion of the contact line is a strong function of the particle wettability. While the stick-slip motion is observed for nanoparticles that are less hydrophilic (i.e., particle contact angle [theta] ~ 74° at the water-air interface) which results in a multi-ring deposition, a continuous receding of the contact line is observed for more hydrophilic nanoparticles (i.e., [theta] ~ 34°) which leaves a single-ring pattern. A three-fold increase in the number of particles required to pinning is predicted from our model when the particle wettability increases from the wetting angle of [theta] ~ 74° to [theta] ~ 34°. This finding explains why particles with greater wettability form a single-ring pattern and those with lower wettability form a multi-ring pattern. Finally, due to the instantaneous change in viscosity and surface tension of an evaporating biopolymer ink, solvent evaporation and polymer deposition dynamics of biopolymer drops are complex and not well understood. Using high-speed interferometry, we directly observe in real time the instantaneous drop shape of inkjet-printed pico-liter gelatin carboxyl drops containing glycerol and water. Stagnation zone due to accumulation of gelatin and glycerol driven by the evaporation-driven outward flow and surface tension-driven Marangoni flow dictates the final deposition of bi-solvent polymer droplets from a spherical cap to a volcano shape during evaporation. The results reveal new insight into the complex role surfactant plays during polymer drop evaporation and deposition processes.