| Wed March 9th 2011 16:00 ZH286 | Seminar | Strain correlations that govern material arrest and failure Peter Schall |
Details:Soft materials such as suspensions and pastes show remarkable mechanical properties: they can be rigid when confined at high density (sand castle), but can also flow easily when small stresses are applied (hour glass). This transition from rigidity to flow is central to phenomena in geology and biological systems, and for many consumer products in food and cosmetic industry. We investigate the role of correlations in this transition. By using transparent colloidal and granular model systems we obtain direct microscopic insight into flow and arrest: In three dimensions and real time, we track the motion of the individual particles, and we visualize correlations in the affine and non-affine particle displacements. At the transition from rigidity to flow, shear is neither macroscopically uniform nor strongly localized: We observe system-spanning strain correlations that reveal a novel form of mechanical criticality at the transition from rigidity to flow. These correlations are isotropic when thermal rearrangements dominate at small applied stress. However, when the applied stress increases, the scaling of correlations becomes anisotropic. These new, anisotropic correlations cause the flow to localize, and ultimately lead to failure of the material. While our model systems allow direct imaging of these critical strain fluctuations, we expect very similar mechanisms to hold for other soft materials as well as atomic and molecular glasses. | ||
