We propose a simple model, supported by contact-dynamics simulations as well as rheology and friction measurements, that links the transition from continuous to discontinuous shear-thickening in dense granular pastes to distinct lubrication regimes in the particle contacts. We identify a local Sommerfeld number that determines the transition from Newtonian to shear-thickening flows, and then show that the suspension's volume fraction and the boundary lubrication friction coefficient control the nature of the shear-thickening transition, both in simulations and experiments.Flow non-linearities attract fundamental interest and have major consequences in a host of practical applications [1,2]. In particular, shear-thickening (ST), a viscosity increase from a constant value (Newtonian flow-Nw) upon increasing shear stress (or rate) at high volume fraction φ, can lead to large-scale processing problems of dense pastes, including cement slurries [3]. Several approaches have been proposed to describe the microscopic origin of shear-thickening [4][5][6][7]. The most common explanation invokes the formation of "hydroclusters", which are responsible for the observed continuous viscosity increase [6,8,9] and which have been observed for Brownian suspensions of moderate volume fractions [10,11]. However, this description no longer holds for bigger particles and denser pastes, where contact networks can develop and transmit positive normal stresses [12]. Moreover, the link between hydroclusters and CST for non-Brownian suspensions is still a matter of debate [13]. Additionally, dense, non-Brownian suspensions can also show sudden viscosity divergence under flow [14][15][16][17] with catastrophic effects, such as pumping failures. In contrast to a continuous viscosity increase at any applied rate, defined as continuous shear-thickening (CST), the appearance of an upper limit of the shear rate defines discontinuous shear-thickening (DST). This CST to DST transition is observed when the volume fraction of the flowing suspension is increased above a critical value, which depends on the system properties, e.g. polydispersity or shape, and on the flow geometry [3,18]. An explanation for its microscopic origin is still lacking [19]. Moreover, experiments have demonstrated that the features of the viscosity increase (slope, critical stress) can be controlled by tuning particle surface properties such as roughness [20] and/or by adsorbing polymers [21,22]. These findings suggest that inter-particle contacts play a crucial role in the macroscopic flow at high volume fractions. A more precise description of these contacts is therefore essential to interpret the rheological behavior.In this paper, we present a unified theoretical framework, supported by both numerical simulations and experimental data, which describes the three flow regimes of rough, frictional, non-Brownian particle suspensions (Nw,CST,DST) and links the Nw-ST (in terms of shear) and the CST-DST transitions (in terms of volume fraction) to the local friction. Our micro...

International audienceWe consider here surf zone turbulence measurements, recorded in the Eastern English Channel using a sonic anemometer. In order to characterize the intermittent properties of their fluctuations at many time scales, we analyze the experimental time series using the Empirical Mode Decomposition (EMD) method. The series is decomposed into a sum of modes, each one narrow-banded, and we show that some modes are associated with the energy containing wave-breaking scales, and other modes are associated with small-scale intermittent fluctuations. We use the EMD approach in association with a newly developed method based on Hilbert spectral analysis, representing the probability density function in an amplitude-frequency space. We then characterize the fluctuations in a stochastic framework using a cumulant generating function for all scales, and compare the results obtained from direct and classical structure function analysis, to EMD-Hilbert spectral analysis results, showing that the former method saturates at large scales, whereas the latter method is more precise in its scale approach. These results show the strength of the new EMD-hilbert spectral analysis method for data presenting a strong forcing such as found in shallow water, wave dominated situations

Friction between microscopic objects controls many macroscopic phenomena. For instance, the friction between microasperities determines the tribology of rough surfaces in contact and in relative motion. Additionally, the friction between microparticles is responsible for many aspects of the rheological response of granular media, ranging from microscale contacts at the single-particle level to macroscopic flow properties of sheared, dry granular systems and non-Brownian suspensions. We propose a new, precise, and robust method, based on lateral force microscopy, to measure the coefficient of friction between microspheres quantitatively and without complex data processing. We have successfully applied this method to the contact between silica spheres in liquid with and without a polymer coating.

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