Jamming and unjamming of concentrated colloidal dispersions in channel flow

We investigated the pressure driven flow of concentrated colloidal dispersions in a converging channel geometry. Optical microscopy and image analysis were used to track tracer particles mixed into dispersions of sterically stabilized poly(methyl methacrylate) (PMMA) spheres. The dispersions were drawn into a round \unit[0.5]{mm} capillary at one of two pump speeds ($\equiv$ applied pressure): v_1=\unit[0.245]{ml\,\, min^{-1}} and v_2=\unit[0.612]{ml\,\, min^{-1}}. We observed that the dispersions at particle volume fractions $\phi\leqslant0.50$ followed Hagen-Poiseuille flow for a simple fluid; i.e. the mean flow rate $\langle V\rangle$ is approximately proportional to pressure drop (pump speed) and inversely proportional viscosity $\eta$. Above this concentration ($\phi\geqslant0.505$), the dispersions exhibit granular-like jamming behavior with $\langle V\rangle$ becoming independent of the pressure drop. However, at the highest applied pressure ($v_2$), the dispersions are able to unjam and switch from granular-like behaviour back to a simple hard-sphere liquid like system, due to the formation of rotating vortices in the spatial flow pattern. This mechanism is consistent with computer simulations of granular systems and supports for example proposed explanations of anomalously low friction in earthquake faults

Generality of shear thickening in suspensions

Suspensions are of wide interest and form the basis for many smart fluids. For most suspensions, the viscosity decreases with increasing shear rate, i.e. they shear thin. Few are reported to do the opposite, i.e. shear thicken, despite the longstanding expectation that shear thickening is a generic type of suspension behavior. Here we resolve this apparent contradiction. We demonstrate that shear thickening can be masked by a yield stress and can be recovered when the yield stress is decreased below a threshold. We show the generality of this argument and quantify the threshold in rheology experiments where we control yield stresses arising from a variety of sources, such as attractions from particle surface interactions, induced dipoles from applied electric and magnetic fields, as well as confinement of hard particles at high packing fractions. These findings open up possibilities for the design of smart suspensions that combine shear thickening with electro- or magnetorheological response.Comment: 11 pages, 9 figures, accepted for publication in Nature Material

Shear-banding in a lyotropic lamellar phase, Part 2: Temporal fluctuations

We analyze the temporal fluctuations of the flow field associated to a shear-induced transition in a lyotropic lamellar phase: the layering transition of the onion texture. In the first part of this work [Salmon et al., submitted to Phys. Rev. E], we have evidenced banded flows at the onset of this shear-induced transition which are well accounted for by the classical picture of shear-banding. In the present paper, we focus on the temporal fluctuations of the flow field recorded in the coexistence domain. These striking dynamics are very slow (100--1000s) and cannot be due to external mechanical noise. Using velocimetry coupled to structural measurements, we show that these fluctuations are due to a motion of the interface separating the two differently sheared bands. Such a motion seems to be governed by the fluctuations of $\sigma^\star$, the local stress at the interface between the two bands. Our results thus provide more evidence for the relevance of the classical mechanical approach of shear-banding even if the mechanism leading to the fluctuations of $\sigma^\star$ remains unclear

The effects of nanosilica on the fresh and hardened properties of 3D printable mortars

This study presents the experimental results of an investigation on the effects of nanosilica NS on the material characteristics of printable mortars used for additive manufacturing. Printable cement mortars based on Ordinary Portland Cement, limestone filler and silica sand were modified with different dosages of nanosilica from 2 to 6 by weight of binder and its influence on their hydration, rheological, mechanical and transport properties was assessed. The study showed that NS accelerates significantly the setting and hardening of printable mortar, while reducing its open time. Moreover, an increment of yield stress, together with an increment in NS dosage, was found to have occurred. The incorporation of an optimal NS dosage results in a noticeable increase in the compressive strength and alteration of the pore structure as determined by the MIP measurements. Moreover, transport properties of the produced mortar are significantly improved due to incorporation of NS. In addition to the microstructure refinement, Micro CT and scanning electron microscopy SEM studies revealed that 3D printed mortars exhibit pore anisotropy in accordance with the printing direction. However, incorporation of NS in the mixture resulted in improved buildability, thus decreasing pore anisotrop

Does Reviewing Lead to Better Learning and Decision Making? Answers from a Randomized Stock Market Experiment

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