Soft Condensed Matter, Biological Physics Experiment & Statistical Physics Seminars
Monday 27 Jun 11 - 1:00pm
Yielding and flow of colloidal glasses and gels
Nikos Koumakis (Foundation for Research & Technology - Hellas (FORTH))
Steady and oscillatory rheology was utilized to study the mechanical response of colloidal glasses and gels with particular emphasis in their yielding behavior. We used a suspension of hard sphere colloidal particles with short-range depletion attractions induced by the addition of non-adsorbing linear polymer. While high volume fraction hard sphere glasses exhibit a single yield point due to cage breaking, attraction dominated glasses show a two-step yielding reflecting bond and cage breaking, respectively. Here we investigate the yielding behavior of frustrated colloidpolymer systems of various particle volume fractions, phi, spanning the region from an attractive glass (phi=0.6) to a low volume fraction (phi=0.1) attractive gel. It is found that as volume fractions are decreased, the yield strains increase, reflecting the increasing heterogeneous void structures within the system. Applying Brownian dynamics simulations on these systems, we examine the structural properties under steady shear. The non-linear stress response in a concentrated gel is be affected by the structural changes during shear. The structural heterogeneities under shear at steady state, are found to be reflected by a non-dimensional number, (Pe_dep), which is the ratio of depletion attraction forces to the applied shear forces.