Por João Maia (Case Western Reserve University, Department of Macromolecular Science and Engineering, United States).

Abstract: For decades it was widely accepted that the shear-thickening of a suspension is due to short-range hydrodynamics (so-called lubrication) forces between colloidal particles, which consequently give rise to formation of hydro clusters that resist against the flow. However, computational efforts based on lubrication theory have not been able to explain discontinuous shear-thickening (DST) in suspensions. Recently, some reports have incorporated both frictional and frictionless contact potentials and their dissipative role in colloidal interactions and have successfully reproduced higher viscosity ratios including DST. Nevertheless, the predominance of tangential frictional forces in this regime gives rise to positive first normal stress differences (N1), which is in contradiction with recent experimental developments. Therefore, there is a clear need for a unified description of the relationship between structure formation and rheology of dense suspensions. 

In order to achieve the objective above, we have modified the Dissipative Particle Dynamics (DPD) particulate mesoscale structure modeling formalism to include both hydrodynamic and frictional contacts and show that in the semi-dense regime the interactions in colloidal suspensions are dominated by hydrodynamics while fraction of frictional bonds remains negligible and consequently the size of frictional clusters remain small. For these systems, the normal stresses remain negative and large. For dense suspensions, frictional forces are necessary to capture discontinuous shear-thickening; however, the microstructure and rheology are sensitive to the level of roughness of colloidal particles. Furthermore, we show that the frictional bonds in the dense and semi-dense regime follow the same statistics as random networks introduced by Erdős-Rényi, where the presence of frictional bonds in dense suspensions promotes formation of Giant percolated frictional cluster. In the second part of this presentation, we extend the study to the rheological behavior and microstructure development of the same soft to rigid semi-dense and dense colloidal suspensions in Pouseille flow in mildly and strongly confined geometries and analyze how they are changed from unconfined conditions.