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How Confinement-Induced Structures Alter the Contribution of Hydrodynamic and Short-Ranged Repulsion Forces to the Viscosity of Colloidal Suspensions

Meera Ramaswamy1,*, Neil Y. C. Lin1, Brian D. Leahy1, Christopher Ness2, Andrew M. Fiore3, James W. Swan3, and Itai Cohen1

  • 1Department of Physics, Cornell University, Ithaca, New York 14850, USA
  • 2Department of Chemical Engineering and Biotechnology, University of Cambridge, Philippa Fawcett Drive, Cambridge CB3 0AS, United Kingdom
  • 3Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA

  • *To whom correspondence should be addressed. mr944@cornell.edu
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Phys. Rev. X 7, 041005 – Published 12 October, 2017

DOI: https://doi.org/10.1103/PhysRevX.7.041005

Abstract

Confined systems ranging from the atomic to the granular are ubiquitous in nature. Experiments and simulations of such atomic and granular systems have shown a complex relationship between the microstructural arrangements under confinement, the short-ranged particle stresses, and flow fields. Understanding the same correlation between structure and rheology in the colloidal regime is important due to the significance of such suspensions in industrial applications. Moreover, colloidal suspensions exhibit a wide range of structures under confinement that could considerably modify such force balances and the resulting viscosity. Here, we use a combination of experiments and simulations to elucidate how confinement-induced structures alter the relative contributions of hydrodynamic and short-range repulsive forces to produce up to a tenfold change in the viscosity. In the experiments we use a custom-built confocal rheoscope to image the particle configurations of a colloidal suspension while simultaneously measuring its stress response. We find that as the gap decreases below 15 particle diameters, the viscosity first decreases from its bulk value, shows fluctuations with the gap, and then sharply increases for gaps below 3 particle diameters. These trends in the viscosity are shown to strongly correlate with the suspension microstructure. Further, we compare our experimental results to those from two different simulations techniques, which enables us to determine the relative contributions of hydrodynamic and short-range repulsive stresses to the suspension rheology. The first method uses the lubrication approximation to find the hydrodynamic stress and includes a short-range repulsive force between the particles while the second is a Stokesian dynamics simulation that calculates the full hydrodynamic stress in the suspension. We find that the decrease in the viscosity at moderate confinements has a significant contribution from both the hydrodynamic and short-range repulsive forces whereas the increase in viscosities at gaps less than 3 particle diameters arises primarily from short-range repulsive forces. These results provide important insights into the rheological behavior of confined suspensions and further enable us to tune the viscosity of confined suspensions by changing properties such as the gap, polydispersity, and the volume fraction.

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Complex fluids consisting of particles suspended in an ambient fluid—such as toothpaste, lubricants, and gels—are ubiquitous in everyday life. Understanding how these complex fluids flow through confined spaces, along with the underlying forces at play, are important; flows of confined suspensions have implications in areas ranging from blood flow in thin capillaries to the design of lubricants for micromachines. Studying these confined fluids is challenging, however, as traditional flow devices do not achieve such small gaps. Here, we use a combination of experimental and simulation techniques to determine the flow behavior under extreme confinement and understand the forces that give rise to the resistance to flow, or viscosity.

We show that at small gaps, the suspension flow response is strongly coupled to the arrangements of the particles in the fluid. When a suspension is confined to a space between 6 and 15 times the particle diameter, the particles arrange themselves into layers, which gives the suspension a low viscosity. At gaps between three and six particle diameters, geometric effects such as the commensurability of the gap with the particle diameter become important, giving rise to fluctuations in the viscosity. Finally, at extreme confinements with gaps less than three particle diameters, the contact forces between particles become increasingly important, and the particles form bridges between the plates that increase the viscosity by over an order of magnitude.

With this understanding, we are able to tune the flow behavior by altering the particle arrangements using dopants or different particle concentrations.

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Phys. Rev. X 7, 041005– Published 12 October, 2017
  • Received 8 March 2017
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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