Abstract
The rotational Brownian motion of colloidal spheres in dense suspensions reflects local hydrodynamics and friction, both key to non-linear 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 develop compositionally uniform colloidal spheres with an off-centre, fully embedded core with a different fluorophore to the particle body, allowing access to rotational motion for all particles at the single-particle level. We reveal interparticle hydrodynamic rotational coupling in charged colloidal crystals. We also find that higher local crystallinity in denser crystals enhances rotational diffusivity, and that nearly arrested particles exhibit a stick-slip rotational motion due to frictional coupling. Our method sheds new light on the largely-unexplored local rotational dynamics of spherical particles in dense colloidal materials.
Highlights
The dynamics of colloidal particles is key to connecting the equilibrium phase behavior of particulate suspensions to their atomic analogs [1,2]
Given the lack of a name for such spheres, we call these particles “offcenter core under laser illumination” (OCULI) particles, and hereafter, we refer to the core as the “eye” and to the whole particle as the “body.” we coat these particles with a nonfluorescent layer to make core-shell OCULI, particles compatible with individual particle tracking all the way up to close packing: it is at these high volume fractions that hydrodynamic and contact mechanics begin to play a role
We have observed the onset of stickslip rotational motion, indicating the emergence of contact friction in dense particulate materials
Summary
The dynamics of colloidal particles is key to connecting the equilibrium phase behavior of particulate suspensions to their atomic analogs [1,2]. Efforts to directly image rotational dynamics include nematic liquid crystal droplets with a frozen director [40], Janus (MOON) particles [41], anisotropic fluorescence profiles using photobleaching [42,43], and rough colloidosomes with a subpopulation of fluorescent surface probes [44], but none of these allows confocal microscopy studies at arbitrarily high volume fractions. The particles can be density and index matched in a solvent mixture, allowing a single 3D confocal microscopy snapshot to reveal the coordinates and orientations of all observed particles up to arbitrarily high concentrations Using these probes, we study hydrodynamic coupling in charged colloidal crystals, finding that the rotation of adjacent spheres exhibits transient coupling. We observe a stick-slip dynamics, indicative of the emergence of local contact friction
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