We examine the impact of confinement on the structure, dynamics, and rheology of spherically confined macromolecular suspensions, with a focus on the role played by entropic forces, by comparing the limits of strong hydrodynamics and no hydrodynamics. We present novel measurements of the osmotic pressure, intrinsic viscosity, and long-time self-diffusivity in spherical confinement and find confinement induces strong structural correlations and restrictions on configurational entropy that drive up osmotic pressure and viscosity and drive down self-diffusion. Even in the absence of hydrodynamics, confinement produces distinct short-time and long-time self-diffusion regimes. This finding revises the previous understanding that short-time self-diffusion is a purely hydrodynamic quantity. The entropic short-time self-diffusion is proportional to an entropic mobility, a direct analog to the hydrodynamic mobility. A caging plateau following the short-time regime is stronger and more durable without hydrodynamics, and entropic drift—a gradient in volume fraction—drives particles out of their cages. The distinct long-time regime emerges when an entropic mobility gradient arising from heterogeneous distribution of particle volume drives particles out of local cages. We conclude that entropic mobility gradients produce a distinct long-time dynamical regime in confinement and that hydrodynamic interactions weaken this effect. From a statistical physics perspective, confinement restricts configurational entropy, driving up confined osmotic pressure, viscosity, and (inverse) long-time dynamics as confinement tightens. We support this claim by rescaling the volume fraction as the distance from confinement-dependent maximum packing, which collapses the data for each rheological measure onto a single curve.
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Confined Brownian suspensions: Equilibrium diffusion, thermodynamics, and rheology
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March 2023
Research Article|
March 01 2023
Confined Brownian suspensions: Equilibrium diffusion, thermodynamics, and rheology
Alp M. Sunol
;
Alp M. Sunol
a)
Department of Chemical Engineering, Stanford University
, Stanford, California 94305a)Author to whom correspondence should be addressed; electronic mail: asunol@stanford.edu
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Roseanna N. Zia
Roseanna N. Zia
Department of Chemical Engineering, Stanford University
, Stanford, California 94305
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a)Author to whom correspondence should be addressed; electronic mail: asunol@stanford.edu
J. Rheol. 67, 433–460 (2023)
Article history
Received:
June 08 2022
Accepted:
November 26 2022
Citation
Alp M. Sunol, Roseanna N. Zia; Confined Brownian suspensions: Equilibrium diffusion, thermodynamics, and rheology. J. Rheol. 1 March 2023; 67 (2): 433–460. https://doi.org/10.1122/8.0000520
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