The field of biosensors may be getting a boost from an unlikely source: coffee rings. UCLA researchers are studying the "coffee ring" phenomenon - the observation that many liquids, when spilled, evaporate to leave a darker ring around the perimeter that contains a much higher concentration of particles than the center. A better understanding of how these rings behave at the micro- and nano-scale would help provide guidance for the development of more advanced biosensors.

To determine the smallest droplet size that would still show a coffee ring after evaporation, the research team manufactured a special surface coated in a checkerboard pattern that featured alternating hydrophilic (water-loving) and hydrophobic (water-repelling) material. The group then placed latex particles in water; the particles were similar in size to disease-marker proteins that biosensors would look for. They found that at a droplet diameter of approximately 10 micrometers, the water evaporated before the particles had enough time to move to the perimeter.

"Knowing the minimum size of this so-called coffee ring will guide us in making the smallest biosensors possible," said Tak-Sing Wong, one of the researchers and a postdoctoral scholar in UCLA Engineering's department of mechanical and aerospace engineering. "This means that we can pack thousands, even millions, of small micro-biosensors onto a lab-on-a-chip, allowing one to perform a large number of medical diagnostics on a single chip."

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