2010-10-13
10:15 at HCI J 574

New phenomena in the flow of concentrated colloids

Rut Besseling

School of Physics and Astronomy, The University of Edinburgh, UK

Concentrated hard sphere colloids are among the simplest of soft matter systems and a popular model system to study the dynamics of the glass transition. Yet the flow behaviour of ‘glassy’ suspensions, especially their nonlinear rheology is poorly understood. In this talk I will discuss both shear banding as well as shear melting behaviour on the particle level, which we have studied via fast confocal microscopy and simultaneous rheology. For volume fractions above ~60%, we observe a type of shear banding so far unobserved in soft glasses, with velocity profiles which become increasingly nonlinear below a characteristic rate and strongly localized near yielding. I will discuss a new theory which attributes this behaviour to small concentration gradients arising from a fundamental flow instability due to flow-concentration coupling. The model accounts for all aspects of the observed phenomenology and may have strong implications for non-uniform flow in other glassy materials [1]. On the particle level, we have studied the shear induced speeding up of structural relaxation in the system, which shows a nontrivial powerlaw dependence on the flow rate [2]. This behaviour is inconsistent with flow induced relaxation expected for an ‘ideal’ glass, and instead provides support for a recently proposed Eyring-type model. [1] R. Besseling, et al. cond-mat. arXiv:1009.1579 [2] R. Besseling, et al. Phys. Rev. Lett. 99, 028301 (2007).