Reversible shear thickening is common in concentrated dispersions of Brownian hard-spheres at high-shear rates. We confirm the existence of a well-defined colloidal shear-thickened state through experimental measurements of the shear stress and the first and second normal stress differences in the shear-thickened state as a function of the particle volume fraction for a model dispersion of near hard-spheres. The shear stress and normal stress differences are observed to grow linearly with the shear rate in the shear-thickened state and both normal stress differences are observed to be negative. Our experimental results show the shear-thickened state of colloidal dispersions can be described by three material properties—the shear viscosity and first and second normal stress difference coefficients—that are a function of the volume fraction. All three material properties are found to diverge with a power law scaling as (1−ϕϕmax)−2 close to maximum packing, ϕmax, which is found to be 0.54 ± 0.01. We find ηr,sts > ϒ2,sts > ϒ1,sts. These results are consistent with theoretical predictions for shear thickening by hydrocluster formation and quantitatively comparable to Stokesian Dynamics simulations. We further postulate and show that these material properties are consistent with those measured for non-Brownian suspensions.

The rheological properties of poly(3-hexylthiophene) (P3HT) solutions were investigated in a nonvolatile solvent, 2-ethylnaphthalene, at various P3HT concentrations. A mild viscosity increase was noted during shear for lower concentration P3HT solutions over 24 h, yet the viscosity increase was greater than if the sample was not sheared at all over the same time period. In this case, crystalline fibrils with large aspect ratios were formed during shear. Crystallization was determined to be dictated by Brownian motion and mediated by shear, or in other words Brownian motion brought the molecules together while shear changed their conformation to allow crystallization. Higher concentration P3HT solutions produced a significant viscosity increase, up to 2 orders of magnitude, during shear. Again, if a similar solution was merely aged without shear, a much lower viscosity increase was noted. Simple calculations show that the fibril concentration was above the percolation threshold at the higher concentration accounting for the large viscosity increase. Finally, the crystal structure of P3HT after shear was found to have π–π stacking (b-axis) direction parallel to the crystalline needle axis, as seen for crystals formed under quiescent conditions, yet the crystalline fibrils are micrometers long. The proposed mechanism for crystal formation is that Brownian motion brings the molecules to a growing crystal face while shear removes improperly incorporated chains, allowing longer crystals to form. This phenomenon is expected to apply to other systems, allowing formation of longer, more perfect crystalline fibrils.

Many real-world industrial processes involve non-spherical particles suspended in a fluid medium. Knowledge of the flow behavior of these suspensions is essential for optimizing their transport properties and designing processing equipment. In the present work, we explore and report on the rheology of concentrated suspensions of cubic-shaped colloidal particles under steady and dynamic shear flow. These suspensions exhibit a rich non-Newtonian rheology that includes shear thickening and normal stress differences at high shear stresses. Scalings are proposed to connect the material properties of these suspensions of cubic particle to those measured for suspensions of spherical particles. Negative first normal stress differences indicate that lubrication hydrodynamic forces dominate the stress in the shear-thickened state. Accounting for the increased lubrication hydrodynamic interactions between the flat surfaces of the cubic particles allows for a quantitative comparison of the deviatoric stress in the shear-thickened state to that of spherical particles. New semi-empirical models for the viscosity and normal stress difference coefficients are presented for the shear-thickened state. The results of this study indicate that cubic particles offer new and unique opportunities to formulate colloidal dispersions for field-responsive materials.

The shear flow of non-Brownian glass spheres suspended in a concentrated colloidal dispersion that exhibits non-Newtonian rheology is investigated. At low volume fractions, the addition of non-Brownian spherical particles to the colloidal dispersion leads to an increase in the steady shear viscosity as well as the dynamic moduli. The flow curves of these suspensions are qualitatively similar to the suspending colloidal dispersion medium, and as such, in this semidilute regime, the suspension data can be shifted on to that of the colloidal dispersion medium at constant shear stress with shift factors comparable to those predicted for spherical particles in a Newtonian fluid. At higher volume fractions of non-Brownian spheres, the shear thickening power law exponent increases with the addition of non-Brownian particles. This increase in the shear thickening power law exponent is shown to be consistent with the effects of confinement on the shear thickening colloidal dispersion by the larger non-Brownian particles. V

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