Arman Boromand

Microstructure and rheology of soft to rigid shear-thickening colloidal suspensions

The shear rate-dependent rheological properties of soft to rigid colloidal suspensions are studied using computational models. We show that a contact force defined based on an elasto-hydrodynamic deformation theory captures an important rheological behavior of colloidal suspensions: While near hard-sphere particles exhibit a strong and continuous shear thickening the evolves to a constant viscosity state, soft suspensions undergo a second shear-thinning regime at high Peclet numbers when the hydrodynamic stresses become larger than the modulus of the colloidal particles. We measure N1 and N2 to be large and negative in the shear-thickening regime; however, for soft spheres at the onset of second shear-thinning N2 reduces in magnitude and eventually becomes positive. We show that for near hard-sphere suspensions, colloidal pressure, shear stress, and normal stress difference coefficients tend to diverge near the maximum packing fraction while P>σ>N1>N2.

Generalized mapping of multi-body dissipative particle dynamics onto fluid compressibility and the Flory-Huggins theory

In this work, a generalized relation between the fluid compressibility, the Flory-Huggins interaction parameter (χ), and the simulation parameters in multi-body dissipative particle dynamics (MDPD) is established. This required revisiting the MDPD equation of state previously reported in the literature and developing general relationships between the parameters used in the MDPD model. We derive a relationship to the Flory-Huggins χ parameter for incompressible fluids similar to the work previously done in dissipative particle dynamics by Groot and Warren. The accuracy of this relationship is evaluated using phase separation in small molecules and the solubility of polymers in dilute solvent solutions via monitoring the scaling of the radius of gyration (Rg) for different solvent qualities. Finally, the dynamics of the MDPD fluid is studied with respect to the diffusion coefficient and the zero shear viscosity.

Viscosity measurement techniques in Dissipative Particle Dynamics

Abstract In this study two main groups of viscosity measurement techniques are used to measure the viscosity of a simple fluid using Dissipative Particle Dynamics, DPD. In the first method, a microscopic definition of the pressure tensor is used in equilibrium and out of equilibrium to measure the zero-shear viscosity and shear viscosity, respectively. In the second method, a periodic Poiseuille flow and start-up transient shear flow is used and the shear viscosity is obtained from the velocity profiles by a numerical fitting procedure. Using the standard Lees–Edward boundary condition for DPD will result in incorrect velocity profiles at high values of the dissipative parameter. Although this issue was partially addressed in Chatterjee (2007), in this work we present further modifications (Lagrangian approach) to the original LE boundary condition (Eulerian approach) that will fix the deviation from the desired shear rate at high values of the dissipative parameter and decrease the noise to signal ratios in stress measurement while increases the accessible low shear rate window. Also, the thermostat effect of the dissipative and random forces is coupled to the dynamic response of the system and affects the transport properties like the viscosity and diffusion coefficient. We investigated thoroughly the dependency of viscosity measured by both Eulerian and Lagrangian methodologies, as well as numerical fitting procedures and found that all the methods are in quantitative agreement.

A generalized frictional and hydrodynamic model of the dynamics and structure of dense colloidal suspensions

Controlling the structure and the rheological properties of colloidal suspension is essential in numerous applications to control the phenomenon known as shear-thickening. Here, we report on the nontrivial interplay between hydrodynamic and frictional interactions using mesoscopic characterization of semidense, φ = 0.48, and dense, φ = 0.58, colloidal suspensions. Monitoring computationally both rheology and microstructure of these complex fluids under an external deformation, we show that in the semidense regime the interactions in colloidal suspensions are dominated by hydrodynamics while the 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 (DST); however, the microstructure and rheology are sensitive to the level of roughness of colloidal particles. Furthermore, we show that the fr...