Applied Physics

Not So Faulty Towers

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Science  25 Nov 2011:
Vol. 334, Issue 6059, pp. 1033-1035
DOI: 10.1126/science.334.6059.1033-d
CREDIT: CHOPIN AND KUDROLLI PHYS. REV. LETT. 107, 208304 (2011)

The shape of soap bubbles, the patterns in a coffee ring stain, and the stability of a sand pile all reflect the interplay and balance between material properties and inertial, surface, and viscous forces. Similar considerations arise in inkjet printing, a technique that has gone beyond creating documents and photos to patterning biological cells and electronic circuits. Chopin and Kudrolli consider a related scenario, namely the dripping of a dense granular suspension onto an absorbent surface, where the rapid removal of the liquid by the substrate causes jamming of the granular particles that leads to the formation of stable structures. By manipulating the droplet rate, free-fall time, and density of the suspension, they are able to create a range of delicate structures. For example, in a simple tower built from linearly stacked droplets, the final shape of each segment depends on how the substrate (filter paper or a porous bed of glass beads) varies the drainage rate of the fluid. As the flux rate increases, towers become wider and smoother; individual segments lose their corrugated nature and begin to fuse into each other as neighboring droplets can interact before jamming occurs. Perhaps most unusual are the towers formed from low-impact droplets at high packing fraction, which can be stacked in a zig-zag fashion.

Phys. Rev. Lett. 107, 208304 (2011).

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