João Maia

Case Western Reserve University, Department of Macromolecular Science and Engineering, Cleveland, OH 44106, USA

Computational rheology of colloidal systems under shear flows: from soft to hard suspensions and on to gels



Over the past few decades and due to the continuous advances in the computational capabilities, computer simulation techniques have tremendously contributed to our understanding of polymeric materials at the molecular level. Nevertheless, the time/length scales associated with macroscopic phenomena observed in polymers are not accessible by these methods. Hence, the majority of reports have focused on the quasi-equilibrium behavior of polymer melts, blends and composites. In this work, we show how Dissipative Particle Dynamics, a mesoscale simulation method that enables study of much larger time/length scales through coarse-graining, in order to study the dynamics of colloidal systems, both in suspension and gel states under shear flow conditions.
This modified DPD scheme includes the explicit expression for short-range lubrication interactions, which depends on the rigidity of the particles, as well as a frictional contact model. This allows us to recover for the first time the correct flow dynamics of colloidal suspensions, namely the second shear-thinning regime following the shear-thickening region commonly observed in relatively soft colloids, the physically correct first and second normal-stress differences and the Discontinuous Shear-Thickening in highly packed, hard particles suspensions. The microstructural analysis of these systems enables us to bridge the underlying physics of these suspensions to their macroscopic rheological responses.

Another general category of colloidal systems that has attracted a lot of attention from the scientific community is that of colloidal gels. However, the interplay between different types of interactions from quantum scale, Van der Waals interaction, to short range interactions, depletion interaction, and long range interactions such as electrostatic double layer makes these systems challenging to simulate. Common computational methods, such as Molecular Dynamic and Brownian Dynamics fail to capture multi-body hydrodynamic interactions (BD) or are limited short time/length scales (MD). We again used a DPD scheme with a modified depletion potential to address the gel formation process in bimodal, soft-hard colloidal mixtures. By looking into the arising topological frustrations (caging) in a range of weak and strong attractive colloidal suspensions at the semi-dilute region, we validate the model proposed by Zaccarelli and Poon and show that the bonding from attractive potentials and topological frustrations are intertwined even at semi-dilute colloidal gels. Also, due to the possibility to separate short and long ranged-hydrodynamic forces, we study the effect of each of these forces on the final morphology and dynamics of the arrested system.

Date: 26/6/2015
Time:16:00 (coffee & cookies will be served at 15:45)
Place:FORTH Seminar Room 2