Relaxation of Critical Fluctuations after Cessation of Simple Shear Flow

Abstract

Scattering experiments under shear of a near-critical system show a highly anisotropic static structure factor. We present small angle light scattering experiments on the relaxation of this anisotropic structure factor of a near-critical colloidal system after cessation of a simple shear flow. The structure factor is found to develop a scattering ring, which shifts to smaller wave vectors in time and increases in intensity. A theoretical explanation of this phenomenon is proposed. [S0031-9007(98)05367-8] PACS numbers: 61.20.Lc, 64.60.Ht, 82.20.Mj The deformation of the structure factor under shear has been of considerable interest in the last two decades. Effects of shear flow are especially pronounced for systems near the gas-liquid critical point, due to the long-ranged character of interactions and slow diffusive motion. Scattering experiments under shear have been performed near the critical point for binary fluids [1,2], polymer blends [3-6], polymer solutions [7-9], and colloids [10]. Notable theoretical approaches to describe shear effects on microstructure are due to Onuki et al. [11,12], Schwarzl and Hess [13], Ronis [14], and Wagner and Russell [15]. Only a few experimental results are available on the relaxation of long range critical microstructure. Beysens et al.describe a scattering experiment on a binary fluid at a single angle and observed pure exponential relaxation [2]. Wu et al., Dixon et al., and van Egmond et al. found that in entangled polymer systems shear flow enhances structure, leading to an increase of the turbidity [7-9]. Relaxation studies of these enhanced structures are reported.The latter kind of shear flow effects are absent in simple fluids near the critical point: for binary fluids and spherical colloids the turbidity is found to sharply decrease on applying a shear flow [2,16]. This sharp decrease in turbidity is due to the strong reduction of microstructure, contrary to the polymer systems in Refs. [7][8][9] where entanglement seems to enhance microstructural order.In this Letter we report on the first observation of an unusual equilibration scenario of a near-critical colloidal system after cessation of a simple shear flow: an optimum relaxation rate at some intermediate wave vector is found, giving rise to a scattering ring. Light scattering experiments are described, and a theoretical explanation of this new relaxation phenomenon is presented.Experiments are performed on a colloid polymer mixture. Without polymer the colloidal particles behave as hard spheres. The addition of nonadsorbing polymer induces an effective attraction between the colloidal particles due to a depletion mechanism [17,18]. 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 [19,20]. The control variable in these systems is not the temperature but rather the concentrations of polymer and colloid. Therefore, measurements are performed along so-called dilution lines, where the conce...

Metastability and Depletion-Driven Aggregation

“…The phase-ordering dynamics of such systems have just begun to be investigated, and the underlying physics may have relevance to such things as the crystallization of globular proteins and nematic ordering of rodlike particles in the presence of a nonadsorbing polymer [6,7]. Beyond their broad commercial significance, colloidal suspensions serve as useful models for a variety of condensed matter systems, from crystals [8,9] and glasses [10] to critical fluids [11] and fractal networks [12]. In this Letter, video microscopy is used to study depletion driven selfassembly in confined binary suspensions of nearly hardsphere colloids.…”

Microstructural response of a near-critical colloid-polymer mixture to shear flow

“…The first relaxation contribution is large at large K, while the second contribution is large at small K. This leads to a maximum of the relaxation rate at finite wave vectors. This competition of two relaxation mechanisms is responsible for a maximum in scattering pattern after cessation of shear flow [31].…”

“…In the present paper we investigate, both theoretically and experimentally, the response of a near-critical colloid-polymer mixture to stationary and oscillatory shear flow. Thus we pursue the line set by earlier scattering experiments on critical dispersions of attractive colloids [29][30][31][32][33] under shear. Having established the link between the experimentally determined and theoretically calculated microstructure, one can continue to predict rheological behavior [18], since it has been shown that shearinduced distortions of the critical structure factor of such dispersions are the origin of near-critical viscoelastic behavior [30].…”