Will Coffee Rings Find a Place in the Laboratory?

Microfluidics is a hot topic and one we have previously written about in this column. We have covered everything from microchips to rethinking the glucometer. Recently, we rediscovered a 2011 report (1) that takes the ideas behind microfluidics—manually or passively manipulating a fluid, particularly to identify what is in it—and suggests applying new knowledge we have about the ever-familiar “coffee ring effect” to clinical needs. The researchers' disarmingly simple idea is what we tackle here.“In conventional microfluidics, a bulky, external pressure source is often required to push the fluid through the microscale channels to control fluid motion. To separate particles in a fluid, microfabricated structures within the channels are needed,” says Tak Sing Wong, a postdoctoral fellow at the Wyss Institute for Biologically Inspired Engineering at Harvard University.“We thought to ourselves whether some of the process could be simplified,” says Wong, who was a postdoctoral scholar at UCLA in the laboratory of Dr. Chih-Ming Ho when the work was published. He chose to focus on ways to separate micro- or nanoparticles within a fluid that do not require either an external pump or microfabricated structures.A discovery by Thomas Witten and colleagues about capillary flow in coffee rings, first discussed in 1997 in Nature (2), made engineers Wong and Ho wonder if the simple hydrodynamic forces at work in an evaporating droplet might be harnessed.The coffee ring effect, as many know, refers to the phenomenon in which a drop of particulate-filled …