Thin granular layers are stronger, even without friction

We experimentally investigate a model frictionless granular layer flowing down an inclined plane, as a way to disentangle generic collective effects from those arising from frictional interactions. We find that thin frictionless granular layers are devoid of hysteresis of the avalanche angle, yet the layer stability is increased as it gets thinner. Steady rheological laws obtained for different layer thicknesses can be collapsed into a unique master curve, supporting that non-local effects are the consequence of the usual finite-size effects associated to the presence of a critical point.

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How a dye mixes when sheared in a suspension of particles

We build on the recent determination of stretching laws in particulate suspensions [Souzy JFM 2017] to describe how a blob of dye mixes when the suspension is sheared. The mixing is initially dominated by the exponential and log-normally distributed stretching laws. At longer times, the limited growth of the dispersion enveloppe forces a massive coalescence between nearby lamellae, which affect the evolution of the concentration distribution [Turuban JFM Rapids 2021].

CoPerMix ITN Marie Skłodowska-Curie Actions

The CoPerMix project on mixing, led by E. Villermaux and involving many other partners across Europe, has just started. Please check out our new website and also the PhD offers including ours at IUSTI.

3D Porous media from inside

Mathieu Souzy is the winner of the visualization challenge from the Digital Rock Portal of the National Science Foundation. Watch this video to experience the flow within a 3D porous media. Note that this video was generated from 3D experimental measurements of the fluid velocity field within a random stack of spherical particles [data are available here and corresponding paper here]. This data has been used to develop Pore Aventura, a 3D velocity field explorer applicable to any 3D velocity field, see for more on GitHub.

Marie-Curie PhD position available:

We are currently looking for a PhD student, funded by the Marie-Curie European Training Network CoperMix, to study mixing in complex flows. This honorific PhD position is an excellent opportunity to do your research within a European-wide network of researchers, with a tailored plan of training activities and many possibilities of interactions.

The objective of the network is to develop a unified vision, numerical tools, and experimental techniques allowing the description and the quantification of mixing processes in complex flows, such as turbulent atmospheric or oceanic flows and those encountered in geo-porous, granular and biological media. The work at IUSTI will involve model experiments dedicated to understand fundamental mechanisms of mixing.

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The `New Wave’

The way interactions at the microscopic scale influence emerging flow properties in complex fluids at the macroscopic scale is one of the core problems in soft matter physics. This work provides experimental evidence together with a theoretical explanation for ‘Oobleck waves’, an instability arising from the coupling between the flow free surface and the non-monotonic rheological laws of shear-thickening suspensions (Download article).

Cover-page of this month Communications physics website:

Featured by a Communiqué de Presse CNRS, on twitter, in Nature Reviews Physics, see Making waves without inertia and in

Quand les vagues faites par la fécule de maïs bouleversent la mécanique des fluides

Velocity distributions, dispersion and stretching in three-dimensional porous media

Using index-matching and particle tracking, we measure the three-dimensional velocity field in an isotropic porous medium composed of randomly packed solid spheres. This high resolution experimental dataset provides new insights into the dynamics of dispersion and stretching in porous media. Dynamic-range velocity measurements indicate that the distribution of the velocity magnitude, U, is flat at low velocity (p(U) ~ U^0). While such a distribution should lead to a persistent anomalous dispersion process for advected non-diffusive point-particles, we show that the dispersion of non-diffusive tracers nonetheless becomes Fickian beyond a time set by the smallest effective velocity of the tracers. We derive expressions for the onset time of the Fickian regime and the longitudinal and transverse dispersion coefficients as a function of the velocity field properties. The experimental velocity field is also used to study, by numerical advection, the stretching histories of fluid material lines. The mean and the variance of the line elongations are found to grow exponentially in time and the distribution of elongation is log-normal. These results confirm the chaotic nature of advection within three-dimensional porous media. By providing the laws of dispersion and stretching, the present study opens the way to a complete description of mixing in porous media (2020_Souzy_JFM).

 

The Darcytron: A pressure-imposed device to probe the frictional transition in shear-thickening suspensions

We present a new device called the Darcytron, allowing pressure-imposed rheological measurements on dense suspensions made of very small particles, like shear-thickening suspensions. The main idea is to impose and control the particle pressure using a vertical Darcy flow across the settled bed of particles. This new device  provides direct evidence of a transition between a frictionless and a frictional state as the particle pressure is increased, in support of the recent frictional transition scenario for shear thickening (2020_Clavaud_Darcytron).

Friction, Not Inertia, Controls Avalanches

Featured in Physics

The kitchen experiment of tilting a box filled with sugar indicates that a layer of grains starts flowing suddenly above a critical angle, generating a large avalanche. This observation, although seemingly mundane, is a major feature of granular materials (like sand and rocks), which has implications in catastrophic phenomena such as earthquakes or aerial landslides. So far, the general belief was that inertial effects, such as shocks between grains or self-induced acoustic noise, were required to observe such sudden avalanche behavior. This study proves that the same phenomenology also occurs for submarine granular avalanches in which inertial effects are completely negligible. This work is published in PRX  and has been featured in Physics and PhysicsToday.

 

Fully developed and transient concentration profiles of particulate suspensions sheared in a cylindrical Couette cell

Highly resolved index-matching techniques are used to measure the local particulate volume fraction  in a wide-gap Taylor–Couette configuration. We find that the fully developed concentration profiles are well predicted by the suspension balance model of Nott & Brady. Moreover, we provide systematic measurements of the migration strain scale and of the migration amplitude which highlight the limits of the suspension balance model predictions (more here 2019_sarabian_jfm).