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The rheological behavior of concentrated colloidal dispersions

Brady, John F. (1993) The rheological behavior of concentrated colloidal dispersions. Journal of Chemical Physics, 99 (1). pp. 567-581. ISSN 0021-9606. http://resolver.caltech.edu/CaltechAUTHORS:BRAjcp93b

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Abstract

A simple model for the rheological behavior of concentrated colloidal dispersions is developed. For a suspension of Brownian hard spheres there are two contributions to the macroscopic stress: a hydrodynamic and a Brownian stress. For small departures from equilibrium, the hydrodynamic contribution is purely dissipative and gives the high-frequency dynamic viscosity. The Brownian contribution has both dissipative and elastic parts and is responsible for the viscoelastic behavior of colloidal dispersions. An evolution equation for the pair-distribution function is developed and from it a simple scaling relation is derived for the viscoelastic response. The Brownian stress is shown to be proportional to the equilibrium radial-distribution function at contact, g(2;phi), divided by the short-time self-diffusivity, D0(s)(phi), both evaluated at the volume fraction phi of interest. This scaling predicts that the Brownian stress diverges at random close packing, phi(m), with an exponent of -2, that is, eta0' approximately eta(1 - phi/phi(m))-2, where eta0' is the steady shear viscosity of the dispersion and eta is the viscosity of the suspending fluid. Both the scaling law and the predicted magnitude are in excellent accord with experiment. For viscoelastic response, the theory predicts that the proper time scale is a2/D0s, where a is the particle radius, and, when appropriately scaled, the form of the viscoelastic response is a universal function for all volume fractions, again in agreement with experiment. In the presence of interparticle forces there is an additional contribution to the stress analogous to the Brownian stress. When the length scale characterizing the interparticle forces is comparable to the particle size, the theory predicts that there is only a quantitative contribution from the interparticle forces to the stress; the qualitative behavior, particularly the singular scaling of the viscosity and the form of the viscoelastic response, remains unchanged from the Brownian case. For strongly repulsive interparticle forces characterized by a length scale b (much greater than a), however, the theory predicts that the viscosity diverges at the random close packing volume fraction, phi(bm), based on the length scale b, with a weaker exponent of -1. The viscoelastic response now occurs on the time scale b2/D0s(phi), but is of the same form as for Brownian dispersions.


Item Type:Article
Additional Information:Copyright © 1993 American Institute of Physics. Received 3 February 1993; accepted 16 March 1993. This work was supported in part by Grant No. CTS-9020646 from the National Science Foundation.
Subject Keywords:RHEOLOGY; CONCENTRATION RATIO; COLLOIDS; DISPERSIONS; MATHEMATICAL MODELS; BROWNIAN MOVEMENT; HARD–SPHERE MODEL; STRESSES; EQUILIBRIUM; HYDRODYNAMICS; ENERGY LOSSES; VISCOSITY; FREQUENCY DEPENDENCE; VISCOELASTICITY; DISTRIBUTION FUNCTIONS; EQUATIONS; SCALING LAWS
Record Number:CaltechAUTHORS:BRAjcp93b
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:BRAjcp93b
Alternative URL:http://dx.doi.org/10.1063/1.465782
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:5284
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:09 Oct 2006
Last Modified:26 Dec 2012 09:04

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