Experimental Soft Condensed Matter Group Harvard University, Prof. D. A. Weitz Cool Picture of the Moment

Reverse coffee-ring effect: Sequential optical micrographs were superimposed to indicate the motion of microspheres. We observed a striking inward migration of 20-μm-diameter polystyrene microspheres suspended in a drying water droplet on a solid substrate, demonstrating the reverse coffee-ring effect, whereas the ring-like stains appear at the perimeter by the coffee-ring effect.

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