Coffee-Ring Effect Separates Particles

That old coffee ring on your desk may be an eyesore but some researchers see in it an analytical technique. Scientists have developed a chromatography method on glass slides that uses the same physics as the coffee stain: It separates nanometer- and micrometer-scale particles by size as a droplet dries (Anal. Chem., DOI 10.1021/ac102963x). The inexpensive technique could find use in disease diagnostics.

When a droplet of a colloidal suspension, such as coffee, dries on a surface, the evaporating liquid pushes the particles to the edge, concentrating them and leaving a ring at the droplet's rim. To see if the phenomenon could not only concentrate particles but also separate them, Tak-Sing Wong, Chih-Ming Ho, and colleagues at Harvard University, the University of California, Los Angeles, and Stanford University delved into the theory of how particles move as a droplet evaporates. They surmised that as the droplet grew thinner towards its edge, the particles would stop moving at a point where their diameter matched the droplet's height.

The investigators made an aqueous suspension of fluorescent beads with diameters of 40 nm, 1 μm, and 2 μm. They let a 0.5-μL droplet of the suspension dry at room temperature. Three distinct rings appeared, with the smallest beads in the outermost ring and the largest ones in the innermost ring. The investigators next showed that the technique could separate proteins, bacteria, and mammalian cells in an aqueous mixture.

Because it doesn't require an external power source, the method could enable people to easily carry out separations of biological molecules from mixtures even where electricity is scarce, Wong says. Separation is a critical step in disease diagnostics to help identify the culprit of an illness.