Department of Physics Papers

Document Type

Journal Article

Date of this Version

8-2013

Publication Source

Physics Today

Volume

66

Issue

8

Start Page

60

Last Page

61

DOI

10.1063/PT.3.2093

Abstract

As many a coffee drinker knows, a drying drop of coffee typically leaves behind a ring-shaped stain of small grounds. Though the phenomenon is common, the mechanisms that drive it are rich with physics. As first elucidated by Robert Deegan and colleagues in 1997, the coffee ring results from radially outward fluid flows induced by so-called contact line pinning: The outer edge of a spilled coffee droplet grabs onto rough spots on the solid surface and becomes pinned in place. The evaporating drop thus retains its pinned diameter and flattens while it dries. That flattening, in turn, is accompanied by fluid flowing from the middle of the drop toward its edge to replenish evaporating water. Suspended particles—the coffee grounds—are carried to the edge of the drop by that flow. Once there, they pile up, one at a time, into a tightly jammed packing and produce the coffee ring. Deegan and company studied the ring growth empirically by following the individual frames in a video of plastic colloidal spheres suspended in an evaporating droplet.

Copyright/Permission Statement

This article was originally published by the American Institute of Physics. You van view the original article at: http://physicstoday.scitation.org/doi/10.1063/PT.3.2093

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Date Posted:10 October 2017

This document has been peer reviewed.