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UCLA Engineering researchers identify the world’s smallest coffee ring

Findings have implications for new types of disease-detecting biosensors that take advantage of a liquid’s evaporation

By Matthew Chin

The field of biosensing has recently found an unlikely partner in the quest for increased sensitivity: Coffee rings.  The next time you spill your coffee on a table, look at the spot left after the liquid has evaporated. You’ll notice it has a darker ring around its perimeter that contains a much higher concentration of particles than the center.  This is known as the coffee ring phenomenon, and occurs with many other liquids after they have evaporated.

coffee_ring_before_after

Because this "coffee ring" phenomenon occurs with many liquids after they have evaporated, scientists have suggested that such rings can be used for examining blood or other fluids for disease markers by using biosensing devices. But a better understanding of how these rings behave at the micro- and nano-scale would probably be needed for practical bionsensors.

"Understanding micro- and nano-particle transportation within evaporating liquid droplets has great potential in several technological applications, including nanostructure self-assembly, lithography patterning, particle coating, and biomolecule concentration and separation,” said Chih-Ming Ho, UCLA’s Ben Rich-Lockheed Martin Professor and director of the Center for Cell Control . “However, before we can engineer biosensing devices to do these applications, we need to know the definitive limits of this phenomenon. So our research turned to physical chemistry to find the lowest limits of coffee ring formation.”

A research group led by Ho, a member of National Academy of Engineering, at the UCLA Henry Samueli School of Engineering and Applied Science has found the definitive microscopic minimal threshold of coffee ring formation, which now can set standards for biosensor devices for multiple disease detection, as well as other uses. The research has been published in the Journal of Physical Chemistry B on April 29 and is currently available online.

“If we consider human blood, or saliva, it has a lot of micro- and nano-scale molecules or particles that carry important health information,” said Tak-Sing Wong, one of the researchers and a post-doctoral scholar in the Department of Mechanical and Aerospace Engineering. “If you put this blood or saliva on a surface, and then it dries, these particles will be collected in a very small region in the ring. By doing so, we can quantify these biomarkers by various sensing techniques, even if they are very small and in a small amount in the droplets.”

As water evaporates from a droplet, particles that are suspended inside the liquid move to the droplet’s edges. Once all the water has evaporated, the particles are then concentrated in a ring around the stain that is left behind.  However, if a droplet is small enough, then the water will evaporate faster than the particles move. Rather than a ring, there will be a relatively uniform concentration in the stain, as the particles did not have enough time to move to the edges while still in the liquid.

“It is the competition between the timescale of the evaporation of the droplet, and the timescale of the movement of the particles that dictates coffee ring formation,” said Xiaoying Shen, the paper’s lead author and a senior microelectronics major at Peking University in China, who worked on these experiments while at the UCLA Cross Disciplinary Scholars in Science and Technology (CSST) program last summer.

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 alternated with a hydrophilic, or water-loving material, and a hydrophobic, or water-repelling material.

The group then placed latex particles, ranging in size from 100 nanometers to 20 nanometers, in water. The particles were similar in size to disease-marker proteins that biosensors would look for.

The group washed the new surface with the particle-infused water. The remaining water lined up as droplets on the hydrophilic spots, much like checkers on a checkerboard. The group repeated the experiments with smaller grid patterns, until the coffee-ring phenomenon was no longer evident. For the 100-nanometer sized particles, this occurred at a droplet diameter of approximately 10 micrometers, or about 10 times smaller than the width of human hair. At this point the water evaporated before the particles had enough time to move to the perimeter.

coffeering2
Coffee ring formation at decreasing droplet size. When using 100 nanometer-sized particles, the smallest coffee ring is about 10 micrometers in diameter, which is about one-tenth the diameter of a human hair.


“Knowing the minimum size, of this so-called coffee ring, will guide us in making the smallest biosensors possible,” Wong said. “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. This may also open the doors to potentially detecting multiple diseases in one sitting.”

“There’s another important advantage, this whole process is very natural, it’s just evaporation,” Wong added. “We don’t need to use additional devices, such as an electrical power source, or other sophisticated instruments to move the particles. Evaporation provides a very simple way of concentrating particles, and has potential in medical diagnosis. For example, researchers at Vanderbilt University were recently awarded a Gates Foundation Research Fund for proposing the use of the coffee ring phenomenon for malaria detection in developing countries.

The researchers are currently optimizing the ring formation parameters, and will then explore the application of this approach towards biosensing technologies that are being developed in Ho’s laboratory.

The research was supported by the Center for Cell Control (CCC), through the National Institutes of Health Roadmap for Nanomedicine and the Center for Scalable and Integrated Nanomanufacturing (SINAM), through the National Science Foundation. Shen received financial support from UCLA’s Cross Disciplinary Scholars in Science and Technology (CSST).