Strong Critical Enhancement of the Shear Viscosity of Colloidal Systems

Abstract

The critical divergence of the shear viscosity of a colloid polymer mixture is experimentally found to be as strong as the divergence of the correlation length. This is in contrast with the divergence of the viscosity in atomic and molecular sytems, which is known to be very weak. The different behavior of the shear viscosity of Brownian systems and molecular systems is believed to be due to the long ranged character of the hydrodynamic interactions between the Brownian particles, a type of interaction absent in molecular systems. [S0031-9007(96)01946-1] PACS numbers: 82.70.Dd, 64.60.HtCritical enhancement of the shear viscosity of atomic and molecular systems is so weak that it is practically impossible to measure anomalous effects of more than about 30% relative to the background viscosity. Proper extrapolation of the background viscosity into the critical region has therefore been of major importance, and often obscured the determination of an accurate value for the very small critical exponent. The critical exponent is now believed to be equal to 0.035 6 0.003 [1]. This value agrees with some of the theories that have been developed in the past to describe anomalous behavior beyond the mean-field region [1-4]. Mean-field anomalous behavior of the shear viscosity of atomic and molecular systems is probably nonexistent.In a recent publication, one of the present authors (J. K. G. D.) predicted quite a different critical behavior of the shear viscosity for colloidal systems [5]. In addition to direct interactions, also present in molecular systems, there is another type of interaction particular to colloidal systems. These additional interactions are mediated via the fluid in which the Brownian particles are dispersed. These so-called hydrodynamic interactions are the result of scattering of the shear flow field by the Brownian particles, which scattered flow field affects other Brownian particles in their motion. Such interactions are long ranged and are responsible for the much stronger divergence of the shear viscosity as compared to molecular systems.The theoretical prediction for the critical divergence of the shear viscosity h for Brownian systems is [5] hwhere h B is the viscosity in the absence of long ranged critical correlations, the so-called background viscosity, and h A is the additive anomalous contribution, which is proportional to the correlation length j in the unsheared suspension:h A~j .(2) In this Letter we present experimental results on both the divergence of the correlation length and the viscosity of a colloid polymer mixture. Without polymer the colloidal particles behave as hard spheres. The addition of nonadsorbing polymer induces an effective attraction be-tween the colloidal particles due to a depletion mechanism [6,7]. The magnitude and range of the attractive forces can be tuned by the concentration and size of the polymer. Different size ratios lead to different kinds of phase behavior [8,9]. In our case the size ratio of polymer͞colloidal particles is 0.48, which leads to a gas-l...

Phase transitions of fluids in shear flow

“…In aqueous solutions of agarose, huge viscosity enhancement was also observed, in which gelation was probably induced upon phase separation [124]. In colloids near the critical point [125] and dense microemulsions near the percolation threshold [126], the viscosity has been reported to grow strongly, in contrast to the weak-viscosity singularity in the usual near-critical fluids. In such systems, near a phase transition, interesting nonlinear rheology might well be expected at high shear and/or in two-phase cases.…”

Shear-banding and microstructure of colloids in shear flow

“…Since an earlier, similar study, 14 we have now built an optical shear cell that allows for quantitative measurements, which can be compared in detail to theoretical predictions. Earlier quantitative studies on shear-induced phenomena close to the gas-liquid critical point relate to integrals over the structure factor, such as the critical divergence of the shear-viscosity, 15 the nonanalytic dependence of turbidity on shear-rate 16 and the critical behavior of flow induced dichroism. 17 These integrated quantities were shown to agree with a theory for the structure factor distortion of near-critical colloids 18,19 (except for the shear-induced dichroism on close approach of the critical point).…”

Microstructure of a near-critical colloidal dispersion under stationary shear flow

“…Earlier quantitative studies on the effect of shear flow on critical correlations are concerned with integrated properties such as the critical divergence of the shear viscosity [15], the shearrate dependence of the turbidity [13], and the critical behaviour of flow-induced dichroism [12]. These integrated quantities were shown to agree with the theory where distortions of shortranged correlations are neglected (except for the shear-induced dichroism on close approach to the critical point).…”