Recent Examples on the WebThe coffee ring effect occurs when a single liquid evaporates and the solids that had been dissolved in the liquid, like coffee grounds, form a telltale ring. Jennifer Ouellette, Ars Technica, 2 Dec. 2020 The shape of the droplets is also a factor in the coffee ring effect. Jennifer Ouellette, Ars Technica, 12 Aug. 2020 The top is scarred with coffee rings, the penny jar never full.oregonlive.com, 16 June 2019