This paper theoretically studies, using depth-averaged equations with Wyart-Cates rheology, the instability mechanism at the origin of the surface waves that develop when a shear-thickening suspension flows down an incline (Darbois Texier Comm Phys 2020). While at low concentration we recover the familiar Kapitza (roll-wave) instability of inertial origin, new instability branches appear above the discontinuous shear-thickening transition, due to the negative slope of the suspension flow rule. Analysis of the role of inertia in this regime shows that the waves experimentally observed arise from a purely non-inertial mechanism, coined `Oobleck waves’, which results from the coupling between the free-surface deformation and the negatively-sloped rheology. This result might be relevant to free-surface flows of other complex fluids displaying velocity-weakening rheology (Download the paper here).
The Capillarytron, an osmotic-like rheometer reveals the dual rheology of shear-thickening suspensions
The rheology of dense colloidal suspensions, which may undergo discontinuous shear thickening or shear jamming, is particularly difficult to analyze with conventional rheometers. Here, we develop a new rheometer adapted to colloidal suspensions: the “capillarytron,” which uses the air-suspension capillary interface to impose particle (or osmotic) pressure during shear. More from our article recently published in Physical Review X and featured in Physics.
Mixing in Granular Media (Open PhD)
We have an open PhD Position in collaboration with P. Rognon from Sydney University and funded by AUFRANDE (Australia France Network of Doctoral Excellence). This honoric PhD position is an excellent opportunity to do your research within a European/Australian-wide network of researchers, with a tailored plan of training activities and many possibilities of interactions. More details here and there.
Transient in granular suspensions: key role of Reynolds-like dilatancy
What is the flow resistance of a suspension subjected to a transient change in boundary conditions – such as during an impact? This work shows that the early stress response of the suspension may differ strongly from the prediction of the suspension balance model based on the steady-state rheology. A two-phase model incorporating a Reynolds-like dilatancy law can quantitatively capture the dilation/compaction dynamics of the suspension.
Chemical production under shear
The interplay between chemical reaction and substrate deformation (shear) is investigated experimentally and modelled by adapting Ranz’s formulation to the case of mixing between
two initially segregated reactants.
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.
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.|
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