Neutralizing coffee-ring effect using gradual structures for uniform particle distribution

Abstract

The coffee-ring effect is a process that takes place when a tiny drop of liquid-containing particles disappears, resulting in a distinct deposit around the edge of the spill resembling a ring. One of the primary causes of this effect is the uneven distribution of particles due to dissimilar evaporation rates on the droplet surface and contact line pinning effects. In this research, a microwell with a gradual structure has been developed to regularize the slippery Wenzel wettability of the surface and control the particle distribution. The mobility and pinning of droplet contact lines during the evaporation process are greatly influenced by this structure. The uniform curvature of the sidewall structure is predisposed to contact line depinning and thus helps the sliding of droplets to further neutralize the coffee-ring effect. Contact line pinning is predominantly observed at the corners of the structure, which are encircled by the ends and bottoms of the microwell's sidewalls. The curvature changes at this point, enhancing van der Waals' forces and resulting in a uniform monolayer of sediment being deposited at the microwell's bottom. The restraining efficacy of microwells of gradual structure on coffee rings with polystyrene spheres was confirmed, and the impact of microwell diameter, particle diameter, and loading efficacy on particle deposition distribution was investigated. The results show that the gradient configuration permits a single layer and even distribution of particles, enabling the sediment's dimensions to be anticipated and evaluated.