Coffee Stains

In physics, a coffee ring is a pattern of particles on a surface left by a suspension puddle after the liquid phase evaporates. The phenomenon is named for the characteristic ring-like deposit along the perimeter of a spill of coffee. It is also commonly seen after spilling red wine. The mechanism behind the formation of these and similar rings is known as the coffee ring effect or in some instances, the coffee stain effect, or simply ring stain. It originates from the capillary flow directed from the interior to the edge of the puddle as it evaporates.