Suppression of the coffee-ring effect using a contact-angle radial-band-patterned substrate: A lattice Boltzmann study

The coffee-ring effect is a phenomenon in which particles are deposited on a contact line during the evaporation of a particle-laden sessile droplet. In this study, a patterned contact-angle substrate was designed using lattice Boltzmann simulation to suppress the coffee-ring effect. The simulation model was a combination of multiphase flow, heat transfer, and nanoparticle dynamics models. The simulation results repeatedly showed the contact line of the evaporating droplet on the patterned substrate during pinning and depinning. The contact line dynamics also affected the particle distribution. A multi-ring pattern was observed on the patterned substrate compared to the particle distribution on a substrate with a constant contact angle. This was a result of the repeated pinning and depinning of the contact line, and it was compared using the particle uniformity index presented in this study. This revealed a particle distribution that was 4.24-fold more uniform than the coffee-ring pattern. In subsequent pattern optimization, the particle uniformities of 51 cases with different geometries were measured. The pinning time in each patterned area of the case with the most uniform particle distribution was measured. Other factors, in addition to the pinning time, affected particle deposition, and the internal flow of the droplet was further analyzed. Within the droplet, the Marangoni flow generally decreased with continuous oscillations, whereas the capillary flow increased sharply in the region with a receding angle of 45 degrees. This sharp increase in the capillary flow caused the particles trapped in the Marangoni flow to escape and increased the deposition rate.