Colloids may be treated as “big atoms” so that they are good models for atomic and molecular systems. Colloidal hard disks are, therefore, good models for 2d materials, and although their phase behavior is well characterized, rheology has received relatively little attention. Here, we exploit a novel, particle-resolved, experimental setup and complementary computer simulations to measure the shear rheology of quasi-hard-disk colloids in extreme confinement. In particular, we confine quasi-2d hard disks in a circular “corral” comprised of 27 particles held in optical traps. Confinement and shear suppress hexagonal ordering that would occur in the bulk and create a layered fluid. We measure the rheology of our system by balancing drag and driving forces on each layer. Given the extreme confinement, it is remarkable that our system exhibits rheological behavior very similar to unconfined 2d and 3d hard particle systems, characterized by a dynamic yield stress and shear-thinning of comparable magnitude. By quantifying particle motion perpendicular to shear, we show that particles become more tightly confined to their layers with no concomitant increase in density upon increasing the shear rate. Shear thinning is, therefore, a consequence of a reduction in dissipation due to weakening in interactions between layers as the shear rate increases. We reproduce our experiments with Brownian dynamics simulations with Hydrodynamic Interactions (HI) included at the level of the Rotne–Prager tensor. That the inclusion of HI is necessary to reproduce our experiments is evidence of their importance in transmission of momentum through the system.
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14 May 2022
Research Article|
May 09 2022
The rheology of confined colloidal hard disks
Special Collection:
Slow Dynamics
Ian Williams
;
Ian Williams
1
Department of Physics, University of Surrey
, Guildford GU2 7XH, United Kingdom
2
H.H. Wills Physics Laboratory
, Tyndall Avenue, Bristol BS8 1TL, United Kingdom
3
Centre for Nanoscience and Quantum Information
, Tyndall Avenue, Bristol BS8 1FD, United Kingdom
4
School of Chemistry, University of Bristol
, Cantock’s Close, Bristol BS8 1TS, United Kingdom
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Erdal C. Oğuz
;
Erdal C. Oğuz
5
School of Mechanical Engineering, Tel Aviv University
, Tel Aviv 6997801, Israel
6
The Sackler Center for Computational Molecular and Materials Science, Tel Aviv University
, Tel Aviv 6997801, Israel
7
School of Chemistry, Tel Aviv University
, Tel Aviv 6997801, Israel
8
Key Laboratory of Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences
, Beijing 100190, China
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Hartmut Löwen
;
Hartmut Löwen
9
Institut für Theoretische Physik II, Heinrich-Heine-Universität
, D-40225 Düsseldorf, Germany
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Wilson C. K. Poon
;
Wilson C. K. Poon
10
SUPA and School of Physics and Astronomy, The University of Edinburgh
, Peter Guthrie Tait Road, Edinburgh EH9 3FD, United Kingdom
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C. Patrick Royall
C. Patrick Royall
a)
2
H.H. Wills Physics Laboratory
, Tyndall Avenue, Bristol BS8 1TL, United Kingdom
3
Centre for Nanoscience and Quantum Information
, Tyndall Avenue, Bristol BS8 1FD, United Kingdom
4
School of Chemistry, University of Bristol
, Cantock’s Close, Bristol BS8 1TS, United Kingdom
11
Gulliver UMR CNRS 7083, ESPCI Paris, Université PSL
, 75005 Paris, France
a)Author to whom correspondence should be addressed: paddy.royall@epsci.psl.eu
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a)Author to whom correspondence should be addressed: paddy.royall@epsci.psl.eu
Note: This paper is part of the JCP Special Topic on Slow Dynamics.
J. Chem. Phys. 156, 184902 (2022)
Article history
Received:
February 05 2022
Accepted:
April 19 2022
Citation
Ian Williams, Erdal C. Oğuz, Hartmut Löwen, Wilson C. K. Poon, C. Patrick Royall; The rheology of confined colloidal hard disks. J. Chem. Phys. 14 May 2022; 156 (18): 184902. https://doi.org/10.1063/5.0087444
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