Concentrated dispersions of soft particles are shown to exhibit a generic slip behavior near smooth surfaces. Slip results from a balance between osmotic forces and noncontact elastohydrodynamic interaction between the squeezed particles and the wall. A model is presented that predicts the slip properties and provides insight into the behavior of the bulk paste.

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We use X-ray imaging to study viscous resuspension. In a Taylor-Couette geometry, we shear an initially settled layer of spherical glass particles immersed in a Newtonian fluid and measure the local volume fraction profiles. In this configuration, the steady-state profiles are simply related to the normal viscosity defined in the framework of the Suspension Balance Model (SBM). These experiments allow us to examine this fundamental quantity over a wide range of volume fractions, in particular in the semi-dilute regime where experimental data are sorely lacking. Our measurements unambiguously show that the particle stress is quadratic with respect to the volume fraction in the dilute limit. Strikingly, they also reveal a nonlinear dependence on the Shields number, in contrast with previous theoretical and experimental results. This likely points to shear-thinning particle stresses and to a non-Coulomb or velocityweakening friction between the particles, as also evidenced from shear reversal experiments. arXiv:1904.12655v1 [cond-mat.soft]

We extend a previously developed ultrafast ultrasonic technique [Gallot et al., Rev. Sci. Instrum. 84, 045107 (2013)] to concentration field measurements in non-Brownian particle suspensions under shear. The technique provides access to time-resolved concentration maps within the gap of a Taylor-Couette cell simultaneously to local velocity measurements and standard rheological characterization. Benchmark experiments in homogeneous particle suspensions are used to calibrate the system. We then image heterogeneous concentration fields that result from centrifugation effects, from the classical Taylor-Couette instability and from sedimentation or shear-induced resuspension.

Summary Model proppant transport experiments are conducted at the laboratory scale using a Newtonian carrier fluid in a long tube of rectangular cross section. Under the particular flow conditions studied, we observe the buildup of a dense but flowing sediment, which rapidly reaches a steady-state height. The existence of this steady-state flowing sediment implies that the proppant flux leaving the channel equals that entering the channel; that is, “efficient” proppant transport occurs. As soon as the suspension flow is stopped, the fluidized sediment ceases flowing and quickly becomes more compact. This collapse implies that the particle sediment is maintained in an expanded state while under flow, with an average volume fraction considerably lower than that under static conditions. The relevant mechanism of sediment transport is identified as viscous resuspension because the flow is at a low Reynolds number (Re at approximately 0.1). We estimate the average volume fraction of the resuspended sediment from experimental measurements of the “expanded” flowing sediment height, with the assumption that the corresponding compact sediment volume fraction is ϕ0=0.61, the volume fraction at which the suspension viscosity diverges. Predictions of the resuspended sediment heights are made with a simple approach based on the diffusive flux model by Leighton and Acrivos (1986) using the average shear stress across the channel width. A good agreement is found between the predicted and experimental values, indicating that 2D effects remain weak. Microscopic observations show that the sediment is fully fluidized while under flow for all the flow rates studied in our channel, and one does not observe the buildup of static sediment banks that are observed in larger-scale tests during the suspension flow (Kern et al. 1959; Babcock et al. 1967; Schols and Visser 1974; Sievert et al. 1981). This apparent difference is explained in the context of the viscous resuspension model.

We study the phase behavior in water of a mixture of natural long chain fatty acids (FAM) in association with ethylenediamine (EDA) and report a rich polymorphism depending on the composition. At a fixed EDA/FAM molar ratio, we observe upon dilution a succession of organized phases going from a lamellar phase to a hexagonal phase and, finally, to cylindrical micelles. The phase structure is established using polarizing microscopy, SAXS, and SANS. Interestingly, in the lamellar phase domain, we observe the presence of defects upon dilution, which SAXS shows to correspond to intrabilayer correlations. NMR and FF-TEM techniques suggest that these defects are related to an increase in the spontaneous curvature of the molecule monolayers in the lamellae. ATR-FTIR spectroscopy was also used to investigate the degree of ionization within these assemblies. The successive morphological transitions are discussed with regards to possible molecular mechanisms, in which the interaction between the acid surfactant and the amine counterion plays the leading role.

We present experimental studies of two aqueous drops, stabilized by colloidal silica, which are placed close to each other in a bath of toluene, ethanol and surplus colloidal silica. If one of the drops is enriched in ethanol while the other is pure water then we observe the spontaneous formation of small droplets at the surface of the water drop closest to its neighbour. These droplets are then observed to form all along the path to the ethanol enriched drop until they make a complete bridge. We relate this behaviour to the diffusion pathways on the underlying three-fluid phase diagram. We argue that the phenomena is a version of compositional ripening where the transfer of the dispersed phase leads to the spontaneous formation of droplets in the continuous phase. We show that, while the large drops are particle-stabilized, the spontaneously formed droplets are not. Instead the presence of surplus particles leads to the droplets gelling as an elastic bridge. The phenomenology at long times and at low particle concentrations becomes increasingly surprising.

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