Letter to the Editor: Wall slip in dispersion rheometry

Whereas our understanding of the role and mechanism of wall slip has improved substantially over the last decade or two, it is still common to see papers on disperse systems appear wherein scant details of the measurement methods are given and wherein no mention of the possibility (probability?) of slip is made. It is argued that there is a need to raise awareness. It takes experience, judgement and skill to make meaningful measurements on disperse systems and it is suspected that the nature of the experimental challenge is under-estimated grossly by too many workers even now.Comment: 10 pages, 2 figure

Wall Adhesion and Constitutive Modelling of Strong Colloidal Gels

Wall adhesion effects during batch sedimentation of strongly flocculated colloidal gels are commonly assumed to be negligible. In this study in-situ measurements of colloidal gel rheology and solids volume fraction distribution suggest the contrary, where significant wall adhesion effects are observed in a 110mm diameter settling column. We develop and validate a mathematical model for the equilibrium stress state in the presence of wall adhesion under both viscoplastic and viscoelastic constitutive models. These formulations highlight fundamental issues regarding the constitutive modeling of colloidal gels, specifically the relative utility and validity of viscoplastic and viscoelastic rheological models under arbitrary tensorial loadings. The developed model is validated against experimental data, which points toward a novel method to estimate the shear and compressive yield strength of strongly flocculated colloidal gels from a series of equilibrium solids volume fraction profiles over various column widths.Comment: 37 pages, 12 figures, submitted to Journal of Rheolog

The non-monotonic shear-thinning flow of two strongly cohesive concentrated suspensions

The behaviour in simple shear of two concentrated and strongly cohesive mineral suspensions showing highly non-monotonic flow curves is described. Two rheometric test modes were employed, controlled stress and controlled shear-rate. In controlled stress mode the materials showed runaway flow above a yield stress, which, for one of the suspensions, varied substantially in value and seemingly at random from one run to the next, such that the up flow-curve appeared to be quite irreproducible. The down-curve was not though, as neither was the curve obtained in controlled rate mode, which turned out to be triple-valued in the region where runaway flow was seen in controlled rising stress. For this first suspension, the total stress could be decomposed into three parts to a good approximation: a viscous component proportional to a plastic viscosity, a constant isostatic contribution, and a third shear-rate dependent contribution associated with the particulate network which decreased with increasing shear-rate raised to the -7/10th power. In the case of the second suspension, the stress could be decomposed along similar lines, although the strain-rate softening of the solid-phase stress was found to be logarithmic and the irreducible isostatic stress was small. The flow curves are discussed in the light of recent simulations and they conform to a very simple but general rule for non-monotonic behaviour in cohesive suspensions and emulsions, namely that it is caused by strain-rate softening of the solid phase stress.Comment: Revised and corrected version accepted by J. non-Newtonian Fluid Mech., this version 24 pages, 9 Figs inc. graphical abstrac

Restructuring of colloidal aggregates in shear flow: Coupling interparticle contact models with Stokesian dynamics

A method to couple interparticle contact models with Stokesian dynamics (SD) is introduced to simulate colloidal aggregates under flow conditions. The contact model mimics both the elastic and plastic behavior of the cohesive connections between particles within clusters. Owing to this, clusters can maintain their structures under low stress while restructuring or even breakage may occur under sufficiently high stress conditions. SD is an efficient method to deal with the long-ranged and many-body nature of hydrodynamic interactions for low Reynolds number flows. By using such a coupled model, the restructuring of colloidal aggregates under stepwise increasing shear flows was studied. Irreversible compaction occurs due to the increase of hydrodynamic stress on clusters. Results show that the greater part of the fractal clusters are compacted to rod-shaped packed structures, while the others show isotropic compaction.Comment: A simulation movie be found at http://www-levich.engr.ccny.cuny.edu/~seto/sites/colloidal_aggregates_shearflow.htm

Regenerated silk and carbon nanotubes dough as masterbatch for high content filled nanocomposites

Regenerated silk (RS) is a natural polymer that results from the aggregation of liquid silk fibroin proteins. In this work, we observed that RS dispersed in aqueous solution undergoes a reversible solid/liquid transition by programmed heating/cooling cycles. Fourier transform infrared, atomic force microscopy imaging and Raman measurements of the RS reveal that the transition fromrandomcoil to b-sheet structures is involved in this liquid–solid transition. The reversible solid-liquid transition of silk fibroin was then found to be helpful to prepare polymer-like carbon nanotube (CNT) dispersions. We demonstrate that the gelation of RS makes the CNTs with the consistency of a dough with polymeric behavior. Such RS can disperse carbon nanotubes at high concentrations of tens of weight percent. Finally, such carbon nanotube dough has been used for the realization of rubber composites. With this method, we pave the way for handling nanopowders (e.g. CNTs or graphene related materials) with safety and reducing the filler volatility that is critical in polymer-processing

Stress overshoot in a simple yield stress fluid: an extensive study combining rheology and velocimetry

We report a large amount of experimental data on the stress overshoot phenomenon which takes place during start-up shear flows in a simple yield stress fluid, namely a carbopol microgel. A combination of classical rheological measurements and ultrasonic velocimetry makes it possible to get physical insights on the transient dynamics of both the stress $\sigma(t)$ and the velocity field across the gap of a rough cylindrical Couette cell during the start-up of shear under an applied shear rate $\dot\gamma$. (i) At small strains ($\gamma <1$), $\sigma(t)$ increases linearly and the microgel undergoes homogeneous deformation. (ii) At a time $t_m$, the stress reaches a maximum value $\sigma_m$ which corresponds to the failure of the microgel and to the nucleation of a thin lubrication layer at the moving wall. (iii) The microgel then experiences a strong elastic recoil and enters a regime of total wall slip while the stress slowly decreases. (iv) Total wall slip gives way to a transient shear-banding phenomenon, which occurs on timescales much longer than that of the stress overshoot and has been described elsewhere [Divoux \textit{et al., Phys. Rev. Lett.}, 2010, \textbf{104}, 208301]. This whole sequence is very robust to concentration changes in the explored range ($0.5 \le C \le 3$ w/w). We further demonstrate that the maximum stress $\sigma_m$ and the corresponding strain $\gamma_m=\dot\gamma t_m$ both depend on the applied shear rate $\dot \gamma$ and on the waiting time $t_w$ between preshear and shear start-up: they remain roughly constant as long as $\dot\gamma$ is smaller than some critical shear rate $\dot\gamma_w\sim 1/t_w$ and they increase as weak power laws of $\dot \gamma$ for $\dot\gamma> \dot\gamma_w$ [...].Comment: 18 pages, 14 figures, accepted for publication in Soft Matte

Delayed collapse of concentrated dispersions flocculated in a secondary minimum

The effect of volume fraction, varied from ca. 0.05 to 0.5 on the rigidity and induction time for collapse are presented. The effect of centrifugal acceleration is examined also. It is argued that scalings of the data are consistent with the idea coming from the LAMPPS simulations of Zia et al. (Journal of Rheology 2014) that coarsening occurs by means of Interfacial diffusion and fluidisation.Comment: An extended abstract of 6 pages with 6 figs. Comments (to RB) are most welcom