When a drop of liquid containing round particles evaporates, it leaves behind a ring-like stain called the coffee ring effect.
As liquid evaporates, fluid flows outward from the middle of the droplet to its edges, carrying particles to the edges. Round particles at the edge pack closely, producing the coffee ring effect.
But if you change the shape of the particles, the coffee stain behavior changes too. Elongated particles do not exhibit the coffee ring effect; rather, they are deposited across the entire area of the drop.
Instead of flowing to and piling up near the edges, the elongated particles deform the droplet surface, which in turn causes them to clump all over the droplet surface.
Clumps are loosely-packed and eventually spread towards the middle of the drop.
Adding surfactant--soap--also changes how the particles interact with the surface.
In this case, the elongated particles plus surfactant behave more like round particles and pack in a ring at the edge of the drop.
Credit: Kurtis Sensenig, University of Pennsylvania
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