2008-04-16
10:15 at HCI J 574

Hard-Mode Elasticity of Dense Colloidal Aggregates and Its Application to Problems in the Rheology of Colloidal Dispersions

Alessio Zaccone

Institute for Chemical and Bioengineering Department of Chemistry and Applied Biosciences, ETH Zurich

Aggregates of small (colloidal) particles in fluids are relevant to many industrial problems (e.g., solid-liquid processing) as well as to environmental systems (e.g., mudstone accretion in sedimentary geology, dynamics of sol particles in the atmosphere etc.). The time-evolution of these systems is controlled by the kinetics of aggregation and breakage which are function of different physical and chemical parameters such as the surface properties, the morphology and the mutual interactions of the particles as well as the properties of the flow. I will focus on the hitherto unsolved problem of breakage of aggregates of colloidal particles in shear and turbulent flows. Varying the shear-rate of the flow, more than 20 years ago it has been observed for the first time that the steady-state size of the aggregates scales with shear-rate as a power-law with exponent about -0.5. Despite theoretical and experimental efforts to explain this observation (that has been confirmed by many experiments and simulations), no satisfactory physical explanation for this scaling is available. In the case of turbulent flow, combining the most accurate theory of turbulence in terms of its multifractal spectrum within a breakage-equation model with a Cauchy-Born (free-energy expansion), hard-mode model for the elasticity of the disordered shear-generated aggregates (and accounting for their dense glassy-like structure in a statistical mechanical fashion), we obtained an approximate theory which predicts very accurately the scaling between the steady-state size of the aggregates and the shear-rate.