Probing nonlinear velocity profiles of shear-thinning, nematic platelet dispersions in Couette flow using x-ray photon correlation spectroscopy

Abstract

We report experiments employing x-ray photon correlation spectroscopy (XPCS) to characterize the velocity profiles of complex fluids in Couette flow. The approach involves modeling the XPCS correlation functions obtained with the incident x-ray beam passing tangentially through the Couette cell gap at various distances from the inner wall. We first demonstrate the technique with measurements on a dilute colloidal dispersion in the Newtonian liquid glycerol, where the expected linear velocity profiles are recovered. We then employ the technique to map the shear-rate-dependent velocity profiles of a shear-thinning dispersion of nematically ordered Gibbsite platelets. The nonlinear velocity profiles of the Gibbsite dispersion include a narrow slip region adjacent to the outer wall and a band with small velocity gradient in the interior of the gap that evolves into a region increasingly resembling plug flow with increasing shear rate. Variations in the velocity profile along the vorticity direction indicate an instability in the interface between this region of small velocity gradient and a region of high velocity gradient near the inner wall. Analysis of the small-angle scattering patterns provides information about the spatial and temporal variations in the nematic order of the Gibbsite dispersion and their coupling to the velocity profile. Additional potential applications of this XPCS-based technique and comparisons with established methods for characterizing velocity profiles are discussed.

Anomalous dynamic response of nematic platelets studied by spatially resolved rheo-small angle x-ray scattering in the 1–2 plane

“…18 The steep decrease in intensity at γ 0 = 0.4 suggest that wall slip could occur at lower shear rates, while the inhomogeneity in the intensity at γ 0 = 12.8 suggests shear banding at high shear rates, both of which were observed in Ref. 34. However, it should be noted that LAOStrain provokes more dramatic structural responses than observed for steady shear flow due to the flow reversal.…”

“…Stress-controlled experiments show a much smoother and more gradual response. The observed structural inhomogeneities suggest that the velocity profile might be non-linear, as some of us recently showed steady shear flow using X-ray photon correlation spectroscopy 34 . These findings are very relevant in view of using clay as flow modifiers in industrial systems, but also when processing other platelet particles, such as graphene oxide 62 .…”

“…33, but notably also under shear flow 23 , where heterogeneities in the flow, as well as the vorticity direction, were observed. In a very recent paper, we have shown that nematic Gibbsite dispersions under steady shear display instability, causing the formation of shear bands in the gradient direction, accompanied by fluctuations in the structure 34 . In these experiments, X-ray photon correlation spectroscopy (XPCS) was used, scanning the gap of a Couette cell by directing the X-ray beam along the tangent direction.…”

Structure and Pattern Formation in Biological Liquid Crystals: Insights From Theory and Simulation of Self-Assembly and Self-Organization

“…Even in the absence of flow, it was shown that to resolve defects in monomeric LCs a mesh smaller than 3 nm is needed (Chen et al, 2021). The platforms include:…”

Selected advances in small-angle scattering and applications they serve in manufacturing, energy and climate change

“…Two other developments should also be mentioned. In SAXS, X-ray photon-correlation spectroscopy (XPCS) or 'speckle' spectrometry, using small, coherent, X-ray beams, allows the internal dynamics of a wide range of material systems to be probed (Jo et al, 2021;Chen et al, 2021). Meanwhile, the development of polarized and spinecho SANS capabilities (Schmitt et al, 2020) has driven advances in studies of magnetic ordering, e.g.…”