Phung, Thanh Ngoc (1993) Behavior of concentrated colloidal suspensions by Stokesian dynamics simulation. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-02052008-140111
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The Stokesian dynamics simulation method is applied to study the behavior of concentrated suspensions of hydrodynamically interacting colloidal particles in a shear flow. The aim of this study is the prediction of suspension macroscopic properties from the microstructure - the temporal and spatial distribution of suspended particles. The macroscopic properties includes the shear viscosity, normal stress differences, short- and long-time self-diffusivities. Suspension macroscopic properties and the microstructure are modeled as functions of two parameters: particle volume fraction, [...], and the Peclet number, Pe, which measures the relative importance of the imposed shear and Brownian forces. Stokesian dynamics accurately accounts for both the hydrodynamic and Brownian forces of a colloidal dispersion. The method, which is very general and computationally efficient, imposes no restriction on the particle displacements and allows simulation of flowing suspension with particle volume fractions from infinite dilution to dense packing and a continuous range of the Peclet number from pure Brownian motion (Pe 0) to pure hydrodynamics (Pe —> [...]). The method is first employed for the pure Brownian suspensions (Pe=0) at a volume fraction [...]=0.45. The accuracy of Stokesian dynamics is demonstrated by an excellent comparison of the radial pair-distribution function obtained from dynamic simulation which captures the same isotropic hard-sphere distribution computed by the random Monte-Carlo method. The simulation method is then applied to study the dynamics of sheared SCC, BCC, and FCC periodic lattices of non-colloidal spheres (Pe [...]) with particle volume fraction ranging from dilution to maximum close packing. Results of the resistivity and the shear viscosity of sheared periodic lattices are successfully determined as a function of the volume fraction. The Stokesian dynamics simulation method is finally applied to the dynamic simulation of unbounded concentrated suspensions of force- and torque-free colloidal particles. The particle volume fractions are varied from 0.316 to 0.6 and the Peclet numbers are ranged from the strong Brownian limit (Pe=0.01) to the hydrodynamic dominated regime (Pe=[...]). Comparisons of simulation results for the steady shear viscosities, self-diffusivities, and the structure factors with experiments are remarkably good. For the first time, the flow of particles are probed with detail to illustrate the shearing deformation to suspension microstructure. This information provides a physical understanding of the fundamental mechanisms causing interesting shear thinning and shear thickening behavior and its important relation to the shear-induced microstructure. The simulation results reveal three distinct behaviors of hard-sphere suspensions in the regions of strong Brownian motion, balance of Brownian and hydrodynamic interactions, and hydrodynamic domination. In the region of strong Brownian motion with small Peclet numbers (Pe < 1), the suspension shear thins due to a decrease of Brownian contribution to particle stress. The isotropic microstructure is slightly deformed, but the particles are very well dispersed. More importantly, simulation results do not reveal ordered microstructure in the shear thinning region. For the special plateau region with Pe [...] 10, the suspension no longer shear thins and the shear viscosity is minimized. The balance of hydrodynamic and Brownian forces induce a strongly ordered flowing suspension with hexagonally packed strings of particles flowing with the bulk flow. The string formation is due to the Brownian forces which act as short-range springlike repulsive and random forces to counter the shearing deformation to the suspension by the imposed shear; the string formation does not relate to the shear thinning. In the region of hydrodynamic domination (Pe > [...]), the suspension shear thickens due to formation of large, elongated clusters of particles. In this region, the hydrodynamics contribute all particle stress as the direct Brownian contribution has essentially vanished, but weak Brownian forces are seen to perturb and induce a local anisotropic microstructure. The complete relation of the steady shear viscosity to particle volume fraction and the Peclet number for concentrated hard-sphere suspensions is also given.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Division:||Chemistry and Chemical Engineering|
|Major Option:||Chemical Engineering|
|Thesis Availability:||Public (worldwide access)|
|Defense Date:||10 November 1992|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||20 Feb 2008|
|Last Modified:||26 Dec 2012 02:30|
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