Rheological behavior of colloidal suspensions : the effects of hydrodynamic interactions

• Main Author: Foss, David R.
• Format: Others
• Language: en
• Published: 1999
• Online Access: https://thesis.library.caltech.edu/628/1/Foss_dr_1999.pdf; Foss, David R. (1999) Rheological behavior of colloidal suspensions : the effects of hydrodynamic interactions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/g6s5-x267. https://resolver.caltech.edu/CaltechETD:etd-02132007-143514 <https://resolver.caltech.edu/CaltechETD:etd-02132007-143514>

The rheological behavior of hard-sphere colloidal suspensions in simple shear flow is examined theoretically and by dynamic simulation. The Stokesian Dynamics and Brownian Dynamics simulation techniques are used to study suspensions with and without many-body hydrodynamic interactions, respectively. Suspensions near equilibrium, where Brownian motion dominates, and at high shear rates, where hydrodynamic forces dominate, are examined. Steady-state simulations are performed using both simulation algorithms. The Brownian Dynamics system is found to be shear-thinning at low shear rates and undergoes a phase transition at high shear rates to a phase of hexagonally-packed strings aligned in the flow direction. Inclusion of hydrodynamic interactions eliminates the phase transition at high shear rates. Instead, the suspension is found to shear thicken due to a boundary layer of high particle probability that forms near contact where convection balances Brownian diffusion. This increases the role of strong hydrodynamic lubrication forces. Shear thinning and thickening data from many different volume fractions are collapsed using scaling theories. A previous steady-state theoretical analysis of the boundary layer at high shear rates (Brady & Morris 1997) is extended to include unsteady flow conditions. Theoretical results are found to be in agreement with start-up and flow-cessation simulations. A relation between start-up flow at low shear rates and the relaxation of equilibrium fluctuations is made. Equilibrium fluctuations are characterized using the shear-stress autocorrelation function and Green-Kubo formulae. Behavior of this function at short times is related to the behavior in an oscillatory shear flow at high frequencies that is also well-described by a boundary layer where unsteady convection balances Brownian diffusion. A new method for determining the components of the long-time self-diffusion tensor is proposed. Self-diffusion is found to be a decreasing function of volume fraction near equilibrium and an increasing function of volume fraction at high shear rates. Data is in agreement with previous theory and experiment. Due to the buildup of particle probability along the compressional axis relative to the extensional axis in simple shear flow, there is a nonzero off-diagonal component to the long-time self-diffusion tensor which is proportional to the shear rate. This component is positive near equilibrium and negative at high shear rates