Rotational irreversibility in dense colloidal suspensions under flow.

We study the flow reversibility of dense colloidal suspensions to link micro-structure to bulk material properties and elucidate the connections between fundamental ideas in statistical physics: chaos, reversibility, and predictability. The translational displacements of particles within granular and colloidal systems have been extensively studied under externally applied oscillatory shear [1,2,3]. Using confocal-rheology, we image particles once per cycle under oscillatory shear to experimentally verify the irreversibility threshold for translational motion and measure the corresponding threshold for rotational motion. Our in-house synthesized colloidal OCULI particles [4] have an offset core-shell construction, providing a real-space quantification of the rotational dynamics of each individual particle. This novel construction allows us to independently measure the critical strain amplitude for irreversible translational and rotational motion. Furthermore, we will demonstrate how concentration and OCULI surface roughness shift the threshold of flow irreversibility.

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