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 concentrated in a ring around the stain that is left behind. However, if a droplet is small enough, 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 have not had 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 featured alternating hydrophilic, or water-loving, material and 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 chec
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University of California - Los Angeles