Split personality dense suspensions

Split personality of dense suspensions

10:01 AM, 10th April 2012
Split personality of dense suspensions
Water containing zirconium dioxide particles measuring 850 microns in diameter detaches from a nozzle. The suspension neck maintains a symmetric profile until the neck gradually narrows to a width of only one particle, when the liquid surrounding the particles ruptures.

ILLINOIS, US: Stir lots of small particles into water, and the resulting thick mixture appears highly viscous. When this dense suspension slips through a nozzle and forms a droplet, however, its behaviour momentarily reveals a decidedly non-viscous side. Scientists at University of Chicago recorded this surprising behaviour in laboratory experiments using high-speed photography, which can capture action taking place in one hundred-thousandths of a second or less.

Marc Miskin, Student, UChicago; Heinrich Jaeger, Student, UChicago; William J Friedman, Professor, UChicago and Alicia Townsend Friedman, Professor, UChicago, expected that the dense suspensions in their experiments would behave strictly like viscous liquids, which tend to flow less freely than non-viscous liquids. Viscosity certainly does matter as the particle-laden liquid begins to exit the nozzle, but not at the moment where the drop’s thinning neck breaks in two.

New behaviour appears to arise from feedback between the tendencies of the liquid and what the particles within the liquid can allow. “While the liquid deforms and becomes thinner and thinner at a certain spot, the particles also have to move with that liquid. They are trapped inside the liquid,” Jaeger explained. As deformation continues, the particles get in each other’s way.

“Oil, honey, also would form a long thread, and this thread would become thinner and break in a way characteristic of a viscous liquid. The particles in a dense suspension conspire to interact with the liquid in a way that, when it’s all said and done, a neck forms that shows signs of a split personality: it thins in a non-viscous fashion, like water, all the while exhibiting a shape more resembling that of its viscous cousins,” said Jaeger.

It took Miskin and Jaeger six months to become convinced that the viscosity of the suspending liquid was a minor player in their experiments. “It is a somewhat heretical view that this viscosity should not matter,” said Jaeger. In their experiments, Miskin and Jaeger compared a variety of pure liquids to mixtures in which particles occupy more than half the volume. Few studies have examined droplet formation in dense suspensions. As scientists noted, such work could greatly impact applications such as inkjet printing, combustion of slurries involving coal in oil, and the drop-by-drop deposition of cells in DNA microarrays.

The UChicago study showed that particles cause deformations and often protrude through the liquid, rendering any such description incomplete until fundamental questions about the interface between a liquid mixture and its surroundings are properly addressed.

© University of Chicago News

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