Materials Science

A Stressful Situation

See allHide authors and affiliations

Science  10 Dec 2010:
Vol. 330, Issue 6010, pp. 1456
DOI: 10.1126/science.330.6010.1456-b

We typically think of fluids as materials that flow when stressed. Water and honey, for instance, can both be poured, even though the flows occur on different time scales because of differences in viscosity. However, some fluids, such as mayonnaise or tomato paste, show yield properties that are typically associated with a solid. When undisturbed, they withstand the force of gravity and do not flow out of a container, and when gently pushed and released they elastically return to their original shape. Reliable measurements of the yield stress are difficult to obtain and can vary considerably from one technique to the next. Fall et al. measured the rheological behavior of emulsions of surfactant-stabilized oil droplets in water. For this simple fluid, the yield stresses measured through increasing or decreasing shear stress sweeps, i.e., in going from/to a solid or static state to/from a liquid or dynamic one, were the same. However, most real yield-stress fluids are thixotropic: Induced flow temporarily changes the internal structure of the fluid, so that the viscosity also depends on the shear history. The authors achieved this property by adding clay particles to their emulsions, which linked together the oil droplets. In this composition, the transition is complicated by the constant rejuvenation and breakdown of the aggregates under changing flow conditions. However, the authors found that by measuring the lowest stress under which steady-state flow occurs, they could define a dynamic yield stress, which also provides valid measurements for simple fluids.

Phys. Rev. Lett. 105, 225502 (2010).

Navigate This Article