Colloidal 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 γ˙p used to rejuvenate their microstructure. We identify a critical shear rate γ˙c, 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 γ˙c, 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 γ˙c: weak gels obtained after rejuvenation at γ˙p>γ˙c show a yield strain that is constant, independent of γ˙p, whereas strong gels obtained with γ˙p<γ˙c display a yield strain that significantly increases with γ˙p. Our results can be interpreted in light of the literature on shear-induced anisotropy, which accounts for the reinforced elastic properties at γ˙p<γ˙c, while we rationalize the critical shear rate γ˙c in terms of a dimensionless quantity, the Mason number, comparing the ratio of the strength of the shear flow with the interparticle bond force.

A numerically effective approach was developed for the modeling of spray-drying of colloidal suspensions. This approach was based on the 14 integration of two models. The first is a phenomenological and radially symmetric model accounting for the drying of single-droplets, while the 16 second employs computational fluid dynamics (CFD) simulations to account for the gas flows conditions and atomization in a spray dryer. 18Experiments were also conducted on single suspension droplets trapped in an acoustic field as well as on droplets in a mini-spray dryer. The 20 predictions of the models were found to be in reasonable agreement with the experimental data, in terms of droplet shrinking and buckling, 22 particle yield, and spatial distribution in the spray dryer mockup.

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Colloidal gels respond like soft solids at rest, whereas they flow like liquids under external shear. Starting from a fluidized state under an applied shear rate γp, abrupt flow cessation triggers a liquidto-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 non-monotonic 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.