Abstract
The rotational Brownian motion of colloidal spheres in dense suspensions reflects local hydrodynamics and contact forces, which are both key to nonlinear rheological phenomena such as shear-thickening and jamming, and transport in crowded environments, including intracellular migration and blood flow. To fully elucidate the role of rotational dynamics experimentally, it is crucial to measure the translational and rotational motion of all spheres simultaneously. Here, we directly access hydrodynamic and frictional coupling in colloidal suspensions up to arbitrarily high volume fractions using compositionally uniform colloidal spheres with an off-center, fully embedded core. We reveal interparticle hydrodynamic rotation-rotation coupling in charged colloidal crystals. We also find that higher local crystallinity in denser hard-sphere crystalline sediments enhances rotational diffusivity and that nearly arrested particles exhibit a stick-slip rotational motion due to frictional coupling. Our findings shed new light on the largely unexplored, local rotational dynamics of spherical particles in dense particulate materials.
- Received 1 September 2020
- Revised 15 March 2021
- Accepted 15 April 2021
DOI:https://doi.org/10.1103/PhysRevX.11.021056
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.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Particulate suspensions are everywhere, including in industrial applications, such as paints, abrasives, coatings, and foods, as well in nature, such as silt, blood, and bacteria. To understand how macroscopic properties of these suspensions emerge from microscopic behavior, researchers often turn to suspensions of micrometer-sized spherical particles, which are small enough to be nudged around by the underlying thermal motion of atoms but large enough to be observed. However, certain key interactions have long remained hidden because of an inability to visualize the rotational motion of single particles in dense suspensions. Here, we overcome this barrier by coming up with a way to observe the rotation of single spherical particles, giving direct access to how single-particle rotational dynamics is locally coupled via hydrodynamic and frictional effects.
Each of our rotational “probes” consists of a small sphere embedded off-center in a larger sphere. In fluorescence imaging, the embedded sphere glows differently from the rest of the particle, providing a visible marker for tracking how the larger sphere is spinning. When charged spheres are arranged in a well-separated, crystalline layout, we find that neighboring rotations couple together like gears. We also reveal that in denser sediments, the rotational motion is not only determined by the proximity to neighboring particles, but also by the packing geometry. Importantly, we identify interparticle mechanical contact for the first time, directly observing an arrested or “stick-slip” rotational Brownian motion that is typical of the emergence of friction.
Our findings fundamentally change how we study particulate suspensions, departing from idealized “atomic analogs” to real particles with hydrodynamic and frictional interactions.
