coffeering

Coffeering, or the coffee-ring effect, is a phenomenon where an evaporating drop containing suspended particles leaves a ring-shaped stain around its perimeter as the liquid dries.

This occurs when the droplet's edge remains pinned to the surface; evaporation is faster at the edge, creating a capillary flow from the center outward to replenish liquid at the contact line. The outward flow transports particles toward the edge, resulting in a higher concentration of solids at the rim and a visible ring after drying.

The effect depends on particle size and concentration, solvent properties, substrate roughness, contact angle, and drying conditions such as humidity and temperature. Strong pinning and slow internal diffusion favor ring formation; if Marangoni flows induced by surface tension gradients are significant, the ring can be suppressed or modified.

Named for its association with coffee stains, the phenomenon was analyzed in detail in the late 1990s by Deegan and colleagues, who identified capillary flow as a primary cause. It has since been studied across the physics of soft matter, colloids, and industrial printing.

In printing and coating, the ring can be either undesirable or exploited for self-assembly of particles. Approaches to control or suppress coffee-ring deposition include adding surfactants, using mixed solvents with different evaporation rates, promoting inward Marangoni flows, heating the substrate, or engineering substrate texture.