Shear-induced reinforcement in boehmite gels: a rheo-X-ray-scattering study

Boehmite, an aluminum oxide hydroxide $\gamma$-AlO(OH), is broadly used in the form of particulate dispersions in industrial applications, e.g., for the fabrication of ceramics and catalyst supports or as a binder for extrusion processes. Under acidic conditions, colloidal boehmite dispersions at rest form gels, i.e., space-spanning percolated networks that behave as soft solids at rest, and yet yield and flow like liquids under large enough deformations. Like many other colloidal gels, the solid-like properties of boehmite gels at rest are very sensitive to their previous mechanical history. Our recent work [Sudreau et al., J. Rheol. 66, 91-104 (2022), and Phys. Rev. Material 6, L042601 (2022)] has revealed such \textit{memory effects}, where the shear experienced prior to flow cessation drives the elasticity of boehmite gels: while gels formed following application of a shear rate $\dot\gamma_{\rm p}$ larger than a critical value $\dot\gamma_{\rm c}$ are insensitive to shear history, gels formed after application of $\dot\gamma_{\rm p}<\dot\gamma_{\rm c}$ display reinforced viscoelastic properties and non-negligible residual stresses. Here, we provide a microstructural scenario for these striking observations by coupling rheometry and small-angle X-ray scattering. Time-resolved measurements for $\dot\gamma_{\rm p} <\dot\gamma_{\rm c}$ show that scattering patterns develop an anisotropic shape that persists upon flow cessation, whereas gels exposed to $\dot\gamma_{\rm p}>\dot\gamma_{\rm c}$ display isotropic scattering patterns upon flow cessation. Moreover, as the shear rate applied prior to flow cessation is decreased below $\dot\gamma_{\rm c}$, the level of anisotropy frozen in the sample microstructure grows similarly to the viscoelastic properties, thus providing a direct link between mechanical reinforcement and flow-induced microstructural anisotropy.Comment: 13 pages, 10 figure

Shear-Induced Memory Effects in Boehmite Gels

International audienceColloidal gels are formed by the aggregation of Brownian particles into clusters that are, in turn, part of a space-spanning percolated network. In practice, the microstructure of colloidal gels, which dictates their mechanical properties, strongly depends on the particle concentration and on the nature of their interactions. Yet another critical control parameter is the shear history experienced by the sample, which controls the size and density of the cluster population, via particle aggregation, cluster breakup, and restructuring. Here, we investigate the impact of shear history on acid-induced gels of boehmite, an aluminum oxide. We show that following a primary gelation, these gels display a dual response depending on the shear rate _ γp used to rejuvenate their microstructure. We identify a critical shear rate _ γc, above which boehmite gels display a gel-like viscoelastic spectrum upon flow cessation, similar to that obtained following the primary gelation. However, upon flow cessation after shear rejuvenation below _ γc, boehmite gels display a glassylike viscoelastic spectrum together with enhanced elastic properties. Moreover, the nonlinear rheological properties of boehmite gels also differ on both sides of _ γc: weak gels obtained after rejuvenation at _ γp . _ γc show a yield strain that is constant, independent of _ γp, whereas strong gels obtained with _ γp , _ γc display a yield strain that significantly increases with _ γp. Our results can be interpreted in light of the literature on shear-induced anisotropy, which accounts for the reinforced elastic properties at _ γp , _ γc, while we rationalize the critical shear rate _ γc in terms of a dimensionless quantity, the Mason number, comparing the ratio of the strength of the shear flow with the interparticle bond force

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Residual Stresses and Shear-Induced Overaging in Boehmite Gels

International audienceColloidal gels respond as soft solids at rest, whereas they flow as liquids under external shear. Starting from a fluidized state under an applied shear rate ˙γp, abrupt flow cessation triggers a liquid-to-solid transition during which the stress relaxes towards a so-called residual stress σres that tallies a macroscopic signature of previous shear history. Here, we report on the liquid-to-solid transition in gels of boehmite, an aluminum oxide, that shows a remarkable nonmonotonic stress relaxation towards a residual stress σres (˙γp) characterized by a dual behavior relative to a critical value γc of the shear rate ˙γp. Following shear at ˙γp>˙γc, the gel obtained upon flow cessation is insensitive to shear history, and the residual stress is negligible. However, for ˙γp<˙γc, the gel encodes some memory of the shear history, and σres increases for decreasing shear rate, directly contributing to reinforcing the gel viscoelastic properties. Moreover, we show that both σres and the gel viscoelastic properties increase logarithmically with the strain accumulated during the shear period preceding flow cessation. Such a shear-induced “overaging” phenomenon bears great potential for tuning the rheological properties of colloidal gels

Investigating mass transfer coefficients in lean methane combustion reaction through the morphological and geometric analysis of structured open cell foam catalysts

In this work, we characterized ceramic open cell foams made of zirconia, silicon carbide and alumina of different nominal pore densities (30–45 ppi) using X-ray computed microtomography. This technique allowed the comprehensive and quantitative extraction of morphological and geometrical characteristics of the structures (pore size, strut diameter and length, node diameter, open porosity and specific geometrical surface area). An empirical model was proposed to determine the specific surface area from parameters easily accessible with standard laboratory equipment. Moreover, lean CH4 combustion tests were performed on ceramic foams coated with PdO/Co3O4 at 3 wt%. Mass transfer coefficients of the foams were measured by monitoring the oxidation reaction under diffusional control conditions. We derived a dimensionless correlation of the type Sh = A•Rem•Sc1/3. Finally, the pressure drop across the foams as a function of surface velocity was evaluated and compared with various experimental data and models available in the literature

Spray drying of colloidal suspensions: Coupling of particle drying and transport models with experimental validations

International audienceA numerically effective approach was developed for the modeling of spray-drying of colloidal suspensions. This approach was based on the integration of two models. The first is a phenomenological and radially symmetric model accounting for the drying of single-droplets, while the second employs computational fluid dynamics (CFD) simulations to account for the gas flow conditions and atomization in a spray dryer. Experiments were also conducted on single suspension droplets trapped in an acoustic field as well as on droplets in a mini-spray dryer. The predictions of the models were found to be in reasonable agreement with the experimental data, in terms of droplet shrinking and buckling, particle yield, and spatial distribution in the spray dryer mockup

Shear-induced stiffening in boehmite gels: A rheo-X-ray-scattering study

International audienceBoehmite, an aluminum oxide hydroxide γ-AlO(OH), is broadly used in the form of particulate dispersions in industrial applications, e.g., for the fabrication of ceramics and catalyst supports or as a binder for extrusion processes. Under acidic conditions, colloidal boehmite dispersions at rest form gels, i.e., space-spanning percolated networks that behave as soft solids at rest, and yet yield and flow like liquids under large enough deformations. Like many other colloidal gels, the solid-like properties of boehmite gels at rest are very sensitive to their previous mechanical history. Our recent work [J. Rheol. 66, 91 (2022); Phys. Rev. Mater. 6, L042601 (2022)] has revealed such memory effects, where the shear experienced prior to flow cessation drives the elasticity of boehmite gels: while gels formed following application of a shear rate ˙γp larger than a critical value ˙γc are insensitive to shear history, gels formed after application of ˙γp˙γc display isotropic scattering patterns upon flow cessation. Moreover, as the shear rate applied prior to flow cessation is decreased below ˙γc, the level of anisotropy frozen in the sample microstructure grows similarly to the viscoelastic properties, thus providing a direct link between mechanical stiffening and flow-induced microstructural anisotropy

Low incidence of SARS-CoV-2, risk factors of mortality and the course of illness in the French national cohort of dialysis patients

International audienceThe aim of this study was to estimate the incidence of COVID-19 disease in the French national population of dialysis patients, their course of illness and to identify the risk factors associated with mortality. Our study included all patients on dialysis recorded in the French REIN Registry in April 2020. Clinical characteristics at last follow-up and the evolution of COVID-19 illness severity over time were recorded for diagnosed cases (either suspicious clinical symptoms, characteristic signs on the chest scan or a positive reverse transcription polymerase chain reaction) for SARS-CoV-2. A total of 1,621 infected patients were reported on the REIN registry from March 16th, 2020 to May 4th, 2020. Of these, 344 died. The prevalence of COVID-19 patients varied from less than 1% to 10% between regions. The probability of being a case was higher in males, patients with diabetes, those in need of assistance for transfer or treated at a self-care unit. Dialysis at home was associated with a lower probability of being infected as was being a smoker, a former smoker, having an active malignancy, or peripheral vascular disease. Mortality in diagnosed cases (21%) was associated with the same causes as in the general population. Higher age, hypoalbuminemia and the presence of an ischemic heart disease were statistically independently associated with a higher risk of death. Being treated at a selfcare unit was associated with a lower risk. Thus, our study showed a relatively low frequency of COVID-19 among dialysis patients contrary to what might have been assumed