PerspectiveMaterials Science

Through Thick and Thin

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Science  02 Sep 2011:
Vol. 333, Issue 6047, pp. 1230-1231
DOI: 10.1126/science.1211155

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The ratio of shear stress to shear rate in a flowing fluid defines its viscosity, or resistance to shear. For a Newtonian fluid like water, the viscosity is constant. Such simple behavior can change drastically, however, when small particles are suspended in the liquid. In some instances, the viscosity decreases with increasing shear rate and the fluid is said to exhibit shear thinning. For applications such as paints, this is desirable because it keeps suspended pigments on the painted surface at rest but lets them flow easily when brushed. There is the opposite possibility of shear thickening whereby the viscosity increases with shear rate. For some suspensions, such as cornstarch in water, this effect can be so dramatic that a person can run across the surface of a pool filled with the suspension, but sinks when standing still. Such non-Newtonian flow behaviors are thought to be caused by changes in the particle arrangements under shear. To investigate this, Cheng et al. (1), on page 1276 of this issue, report direct measurements of particle arrangements while moving between regimes of shear thinning and thickening.