Shear suppression of critical fluctuations in a diluted polymer blend

Ek, Hobbie; Ai, Nakatani; Yajima, Hirofumi; Jf, Douglas; Cui, Han

doi:10.1103/physreve.53.r4322

Cited by 18 publications

(9 citation statements)

References 26 publications

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“…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][4][5][6], polymer solutions [7][8][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].…”

Section: Relaxation Of Critical Fluctuations After Cessation Of Simplmentioning

confidence: 99%

Relaxation of Critical Fluctuations after Cessation of Simple Shear Flow

1998

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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...

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Section: Relaxation Of Critical Fluctuations After Cessation Of Simplmentioning

confidence: 99%

Relaxation of Critical Fluctuations after Cessation of Simple Shear Flow

1998

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“…The effect of shear on near-critical systems has been studied for binary fluids [20,21], polymer blends [22][23][24], and polymer [25][26][27] and micellar solutions [28]. 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.…”

Section: Introductionmentioning

confidence: 99%

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

2004

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Long-ranged microstructural order of a near-critical colloid-polymer mixture under stationary and oscillatory shear flow is studied by means of time-resolved small-angle light scattering. The distance from the critical point, the shear rate, and the frequency of oscillation are systematically varied. Unexpected shear-induced distortions of critical microstructural order is observed in directions perpendicular to the flow direction, more so on closer approach of the gas-liquid critical point. The measured distortion of long-ranged order can be quantitatively understood on the basis of an approximate solution of the Smoluchowski equation, provided that a nonanalytical distortion of short-ranged microstructural order is assumed. These short-ranged microstructural changes, induced by the flow, account for the observed distortion perpendicular to the flow direction, and are responsible for the shear-induced shift of the location of the gas-liquid critical point. The importance of short-ranged correlations renders the behavior of critical systems under shear flow nonuniversal. The origin of the nonanalytic dependence of short-ranged distortions on the shear rate remains unclear. Including these short-ranged microstructural distortions, we find a good agreement between theory and experiment for both stationary and oscillatory shear flow.

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“…Critical exponents describing the shape of the coexistence curve and the shift of the critical temperature DT c ͑ ᠨ g͒ were compatible with expectations based on renormalization group and mode-coupling theories, but the introduction of a reduced variable description of DT c suggested a tendency for T c to saturate at high shear rates. The effects of shear on the critical temperature T c has been studied in small molecule binary mixtures as well as polymeric blends [1][2][3][4]. Because of the relatively large influence of shear and the slow dynamics of polymeric systems, it has been possible to perform more comprehensive measurements on bulk [3] and diluted [4] polymer blends using a combination of small angle neutron scattering and dynamic light scattering techniques.…”

mentioning

confidence: 99%

“…The effects of shear on the critical temperature T c has been studied in small molecule binary mixtures as well as polymeric blends [1][2][3][4]. Because of the relatively large influence of shear and the slow dynamics of polymeric systems, it has been possible to perform more comprehensive measurements on bulk [3] and diluted [4] polymer blends using a combination of small angle neutron scattering and dynamic light scattering techniques. A comparison [5] of experimental results obtained in the one phase region with the theory of Onuki and Kawasaki [6] indicates a remarkable consistency.…”

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confidence: 99%

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Shear-Induced “Homogenization” of a Diluted Polymer Blend

Douglas

Hobbie

et al. 1997

Phys. Rev. Lett.

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The miscibility of a diluted polymer blend under steady shear has been investigated in the twophase region using fluorescence and phase-contrast microscopy. Critical exponents describing the shape of the coexistence curve and the shift of the critical temperature DT c ͑ ᠨ g͒ were compatible with expectations based on renormalization group and mode-coupling theories, but the introduction of a reduced variable description of DT c suggested a tendency for T c to saturate at high shear rates.[S0031-9007 (97)02774-9] PACS numbers: 83.80.Es, 64.75. + gThe effects of shear on the critical temperature T c has been studied in small molecule binary mixtures as well as polymeric blends [1][2][3][4]. Because of the relatively large influence of shear and the slow dynamics of polymeric systems, it has been possible to perform more comprehensive measurements on bulk [3] and diluted [4] polymer blends using a combination of small angle neutron scattering and dynamic light scattering techniques. A comparison [5] of experimental results obtained in the one phase region with the theory of Onuki and Kawasaki [6] indicates a remarkable consistency. However, many questions remain about the influence of shear on the phase stability of mixtures. For example, the combined influence of additives (e.g., solvent for the blend) and shear on the phase stability of mixtures and the nature of mode-coupling effects in the unstable regime have received limited attention. The present paper considers these problems based on the existing Onuki-Kawasaki framework.Previous measurements [2] on diluted polymer blends in the unstable regime under conditions of steady shear showed a marked decrease of scattering intensity at high rates of shear. This shear-induced "homogenization" effect was interpreted in terms of a shift of the phase boundary with shear, as in earlier shear-scattering measurements on small molecule mixtures in the one-phase region [1]. The effect of droplet distortion on the shear-scattering measurements was minimized by considering the projection of the scattering data along the vorticity direction. Estimates of the integrated intensity along this direction from shear light scattering measurements showed a tendency to strongly decrease and then to saturate at high shear rates. The characteristic shear rate ᠨ g c , corresponding to this saturation point, was then employed along with theoretical arguments to estimate the shift of the phase boundary with shear. While this procedure led to estimates of the critical temperature shifts with shear that were apparently consistent with the Onuki-Kawasaki (OK) theory [6], it seems unclear to us whether the diminished scattering intensity ("homogenization") reflected a true shift of the phase boundary or merely the breakup of droplets under shear to a scale undetectable by light scattering measure-ments. Since this kind of scattering intensity measurement depends on the history of the shearing measurement, the procedure of extrapolation, and the sensitivity of the detection system, it seems nece...

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Shear suppression of critical fluctuations in a diluted polymer blend

Cited by 18 publications

References 26 publications

Relaxation of Critical Fluctuations after Cessation of Simple Shear Flow

Relaxation of Critical Fluctuations after Cessation of Simple Shear Flow

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

Shear-Induced “Homogenization” of a Diluted Polymer Blend

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