SOFT – Complex Fluids & Solids

SOFT – Complex Fluids & Solids

We are a collaborative research team working on the physics of complex media, from granular flows to bio-inspired systems. Welcome !

The SOFT team

SOFTNESS disgusts, frightens and annoys. Indecisive and inconstant, it reminds us of our own decadence. From the octopus to the snot and the slacker’s apathy, it is an indisposition. But this much-criticized softness is also a matter of motion, evoking fluidity, buoyancy and smoothness, the comfort of a chewy doughnut. And isn’t the time for relaxation and contemplation the time for creative thinking? Softness is flexible, intangible… dissident!!”  Géraldine Mosna-Savoye, LA FORCE DU MOU.


November 2023

Back on track after a loooong pause… many very good news since, among them:

  • Olivier’s ERC CohPA is starting now ! This opens many exciting possibilities to work on cohesive granular media.
  • Our permanent and special Guest, beloved Babette, is the 2023  Fluid Dynamic Prize recipient! Thumbs up !
  • Murillo Santanna, expert in folding and bi-stable structures, just joined our group as a “Maître de Conférence”. Welcome !
  • Yoël obtained fundings from Amidex Activ’PLANT to work on active motion and signalling in plants.
  • Denis Dumont just joined us to work with Olivier on cohesive granular media. Welcome!
  • Welcome! Jan Siemen, from Twente, is visiting the group for 3 months.
  • Thanks to Mathieu and Murillo, the first two speakers of our SofTherapy new group meeting!

June 2023

  • !!!Congrats to the new assistant Profs!!! Cécile Clavaud (at IPR Rennes), Hugo Perrin (at Fast Orsay), Marie Poulain (in LMFA Lyon), and CNRS researcher Nicolas Levernier (at CINAM Marseille). Always great when our students do so well!

March 2023

  • Nice work on granular collapse of cohesive grains by Adrien, check this out here.
  • Emilie (working with Lolo) promoted DR at CNRS ! Champaign!
  • Everything about Oobleck waves here. Beautiful work Baptiste.
  • Alice moves us towards Brownian suspensions, in collaboration with Olivier Dauchot (The paper is here).
  • More VERY BIG news coming very soon…

February 2023 

    • Olivier, on fire in the lab, trying to keep cool after his Big oral.. crossing fingers 🙂
    • Welcome! Jan Siemen, from Twente, is visiting the group for 3 days.
    • Cargèse Copermix summer school `Frontiers of Mixing’ just annouced!
    • Ilaria, as part of her CoperMix secondment, is visiting Anne DeWitt in Bruxelles for 2 months.
    • Well done Bruno!  The Capillarytron, a new rheometer that can access suspensions properties other rheometers can’t! Just published in PRX, and highlighted in Communiqué de Presse CNRS and in Physics. Hot-buzzing the webArticle has an altmetric score of 23!!!:)
    • The micro-milling machine, just arrived, is now available for the whole lab!
    • Thrilled to hear Joël’s keynote at APS March Meeting on `Morphing with flexible fibers’.


Willing to join the SOFT Team? Below are our current offers. Any questions about the lifestyle in Marseille? Check this out:  Time Magazine 2022, Marseille Secrète, Lonely Planet.

Open (funded) offers:

Many opportunities for internships, so don’t hesitate to contact us!


This theme studies light scattering properties in relation with the characterization of nano- and micro-particle systems (…)

This theme deals with the static and flow behaviors of dense  granular media and cohesive powders (…)

This theme deals with the flow of fluid/particles mixtures, such as dense suspensions, immersed granular flows (…)

This research topic at the cross-road of physics, engineering and biology addresses the biomechanics of plants (…)


Group manager:  Bloen METZGER


Special Guest: Elisabeth GUAZZELLI

PhD Students & Post-Docs:

Simon Hadjade (PhD)

-Working on “Drosophila wing expansion for soft robotic application” with Joël Marthelot, Raphaël Clément (IBDM) & Yoël Forterre.

-Coming from ENS Paris Saclay, France

Ilaria Castaldi (PhD)

-Working on “Mixing fluids of different viscosities” with Henri Lhuissier & Bloen Metzger and funded by ITN Copermix.

-Coming from Roma, Italy.

Francisco Melo da Rocha (Post-Doc)

-Worked on ‘the hydrodynamics of shear thickening suspensions’ with B. Metzger, H. Lhuissier & Y. Forterre, but he likes Marseille so much that he is now working on ‘Cohesive Powders’ with M. Nicolas & O. Pouliquen.

-Coming from Recife, Brazil.

Victor Charpentier (Post-Doc)

-Working on ‘Mechanics of large deformations. Bio-inspiration. Renewable energies’ with J. Marthelot.

-Coming from Princeton and previously from Ecole des Ponts ParisTech.

Denis Dumont (Post-Doc)

-Working on numerical simulations of cohesive granular media with Olivier and Maxime.

-Dens did his PhD thesis at the University of Mons (Belgium) in the InFluX lab under the supervision of Pascal Damman.

Karim Ayoubi (PhD)

-Working on ‘the Settling of flexible Objects’ with L. Bergougnoux & J. Marthelot.

-Coming from Master `Fluid & Solids’, and previously from Beirut Arab Univ., Liban.

Hector Ignacio Urra (Post-Doc)

-Working on suspension pinch-off with Henri Lhuissier

-Coming from Chili and did his PhD on swimming microorganism with Eric Clément.

Valentin Paume (PhD)

-Working on `Indentation forces in Granular media‘ with O. Pouliquen, P. Aussillous & the company Feurst.

-Coming from Polytech Marseille.

Faisal Ahmad (Post-Doc)

-Working on Self-contracting vascular solids with Martin Brandenbourger (IRPHE) and Joël Marthelot

Coming from India and did his PhD on fluid dynamics models of gastrointestinal tract with Clément de Loubens in Grenoble.

Alexis Bougouin (Post-Doc)

-Working on ‘the flow of shear thickening suspension in pipes’ with B. Metzger, H Lhuissier & Y. Forterre.

-Coming from Toulouse too.

Mathieu Rivière (Post-Doc)

-Working on ‘Physics of Fast Actuation in Plants in Mimosa Pudica’ with Y. Forterre and J. Marthelot.

-Coming from Tel Aviv, Israel, and previously MSC, Paris.

Chong Wei Hong (PhD)

-Working on ‘Sediment transport by a laminar flow’ with P. Aussillous in collaboration with E. Guazzelli.

-Coming from Taiwan.

Terence Declaux (Post-Doc)

-Working on ‘ways to put socks on and root penetration in soils’ with Yoel Forterre and Joel Marthelot

-Coming from Imft Toulouse.

Lilian Chabrol (Ater)

-Working on 3D dynamics, shape, size and composition of particulate and biological media with Fabrice Onofri.

-Coming from ENS de Lyon and string theory!

Antoine Weber (Post-Doc)

-Working on ‘probing the microstructure of blood flows’ with Lolo et Emilie Franceschini

-Coming from Paris.


Latest Publications

Transition from granular to Brownian suspension:  An inclined plane experiment

We address the flow down an inclined plane of dense granular suspensions made of micron-sized particles, for which thermal fluctuations cannot be ignored. Using confocal microscopy on a miniaturized setup, we observe that, in contrast with standard granular rheology, the flow properties strongly depend on the particle size through the ratio of the gravity pressure to the thermal pressure. A phenomenological model based on the sum of a thermal contribution describing the glass transition and an athermal contribution capturing the jamming transition is developed, which reproduces well the experimental observations. Notably, the model predicts the existence of a glassy friction angle lower than the granular athermal friction angle, an unusual signature of the glass transition in the framework of a pressure-imposed rheology.

Alice Billon, Yoël Forterre, Olivier Pouliquen, and Olivier Dauchot (2023) Transition from granular to Brownian suspension: An inclined plane experiment, Physical Review Fluids 8, 034302. [pdf]

Capillary-Stress Controlled 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.

Bruno Etcheverry, Yoel Forterre & Bloen Metzger (2023) Capillary-Stress Controlled Rheometer Reveals the Dual Rheology of Shear-Thickening Suspensions, Physical Review X 13, 011024. [pdf]

Shear-thickening suspensions down inclines: From Kapitza to Oobleck waves

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.

Baptiste Darbois Texier, Henri Lhuissier, Bloen Metzger and Yoël Forterre (2023) SHear thickening suspensions down inclines: From Kapitza to Oobleck waves, Journal of Fluid Mechanics vol. 959, A27. [pdf]

Book “Soft Matter in Plants: from Biophysics to Biomimetics”

Plants offer some of the most elegant applications of soft matter principles in Nature. Starting with fundamental concepts around plant biology, physics of soft matter and viscous fluids, readers of this book will be given a cross-disciplinary and expert grounding to the field, from local scale aspects (fluid-solid coupling, cell and tissue growth, water stress and cavitation) to physical interaction with the environment (root/soil, pathogens invasion) to engineering applications (actuators inspired by plant motion).

K.H. Jensen, Y. Forterre (Eds) (2022) “Soft Matter in Plants: from Biophysics to Biomimetics”. Royal Society of Chemistry Book, 244 pages

Chapter 1. Y. Forterre “Basic soft matter for plants” pdf

Transient flows and migration in granular suspensions: key role of Reynolds-like dilatancy [paper in JFM]

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.

Athani et al J. Fluid Mech. 949 A9 (2022) 

Viscous to Inertial Transition in Dense Granular Suspensions [paper in Phys. Rev. Lett.]

Granular suspensions present a transition from a Newtonian rheology in the Stokes limit to a Bagnoldian rheology when inertia is increased. A custom rheometer that can be run in a pressure- or a volume-imposed mode is used to examine this transition in the dense regime close to jamming. By varying systematically the interstitial fluid, shear rate, and packing fraction in volume-imposed measurements, we show that the transition takes place at a Stokes number of 10 independent of the packing fraction. Using pressure-imposed rheometry, we investigate whether the inertial and viscous regimes can be unified as a function of a single dimensionless number based on stress additivity.

Tapia et al Phys. Rev. Lett. 129, 078001 (2022)

First “European ITN project Copermix” Workshop at IUSTI ! 

This workshop aims at providing common high-level phenomenological and theoretical background about the new lamellar description of mixing, and first contacts with state of the art experimental and computational techniques. It will be held at the IUSTI institution in Marseille, with dedicated lab training session on experimental techniques in our team.  

You can download the program here:


Bubble casting soft robotics [paper in Nature]

Inspired by living organisms, soft robots are developed from intrinsically compliant materials, enabling continuous motions that mimic animal and vegetal movement. Here we demonstrate a new all-in-one methodology for the fabrication and the programming of soft machines. Instead of relying on the assembly of individual parts, our approach harnesses interfacial flows in elastomers that progressively cure to robustly produce monolithic pneumatic actuators whose shape can easily be tailored to suit applications ranging from artificial muscles to grippers. 

Jones et al Nature 599, 229-233 (2021) 

Transmission of droplet-conveyed infectious agents such as SARS-CoV-2 by speech and vocal exercises during speech therapy [paper in European Archives of Oto-Rhino-Laryngology]

The Infectious agents, such as SARS-CoV-2, can be carried by droplets expelled during breathing. The spatial dissemination of droplets varies according to their initial velocity. Combining experimental visualization of droplet exhalation and computation fluid dynamic, we determine the velocity of the exhaled air during vocal exercises. Our study revealed that vocal exercises produce a slower airflow than long exhalation. Speech therapy should, therefore, not be associated with an increased risk of contamination when implementing standard recommendations..

Giovanni et al  European Archives of Oto-Rhino-Laryngology. 278 1687-1692 (2021) 10.1007/s00405-020-06200-7

Rheology of Cohesive Granular Media: Shear Banding, Hysteresis, and Nonlocal Effects [paper in PRX]

We provide a comprehensive analysis of the rheology of a cohesive granular medium, sheared in a normal-stress-imposed plane shear cell over a wide range of shear rate, employing numerical simulations. At high imposed shear rates, the flow is homogeneous, and the rheology is well described by the existing scaling laws. However, at low imposed shear rates, the flow is inhomogeneous, exhibiting shear banding. We reveal that the occurrence of shear banding is related to the existence of a nonmonotonic intrinsic rheological curve. A simple theoretical model based on a nonlocal rheological model coupled with a nonmonotonic flow curve successfully reproduce all the key features of the shear banding observed in the numerical simulations.

Mandal et al Phys. Rev. X 11 21017 (2021) 10.1103/PhysRevX.11.021017 

A Model of Plant Gravitropism Linking Statoliths Position and Auxin Transport [paper in Front. Plant Sciences]

The response of plants to gravity implies starch-filled plastids, the statoliths, which sediments at the bottom of the gravisensing cells. We build on recent experimental results showing that statoliths do not act as gravitational force sensor, but as position sensor, to develop a bottom-up theory of plant gravitropism. The main hypothesis of the model is that the presence of statoliths modifies PIN trafficking close to the cell membrane. This basic assumption, coupled with auxin transport and growth in an idealized tissue made of a one-dimensional array of cells, recovers several major features of the gravitropic response of plants..

Levernier et al, Front. Plant Sci. 12 651928 (2021) 

Extensional viscosity and thinning of a fiber suspension thread [paper in Phys Rev Fluids]

Two flow situations involving the extensional dynamics of a rigid fibre suspension are investigated: the gravitational stretching of a quasi-steady jet and the breakup of an unstable capillary bridge.  At high concentration, the extensional viscosity increases much more strongly with increasing φ than predicted by available models assuming purely hydrodynamics interactions between the fibers. 

Chateau et al Phys. Rev. Fluids 6, 44307 (2021)  

Falling clouds of particles in vortical flows [paper in JFM]

The coupling between particle-particle and particle-fluid interactions is examined by studying the sedimentation of clouds of spheres in a model cellular flow at a small but finite Reynolds number. The model flow consists of counter-rotating vortices and is aimed at capturing key features of the vortical effects on particles. The dynamics of clouds settling in this vortical flow is investigated through a comparison between experiments and point-particle simulations.

Marchetti et al J. Fluid Mech. 908 A30 (2021) 10.1017/jfm.2020.883

“The New-Wave”: Surface-wave instability without inertia in shear-thickening suspensions [paper in Communication Physics]

We investigate the flow of a shear-thickening suspension down an inclined plane and show that, at large volume fractions, surface kinematic waves can spontaneously emerge. Curiously, the instability develops at low Reynolds numbers, and therefore does not fit into the classical framework of Kapitza or ‘roll-waves’ instabilities based on inertia. We show that this instability, that we call ‘Oobleck waves’, arises from the sole coupling between the non-monotonic (S-shape) rheological laws of shear-thickening suspensions and the flow free surface.

Darbois Texier et al Communication Physics 3, 232 (2020)

Insight into the Rheology of Cohesive Granular Media [paper in PNAS]

In this paper, we show using discrete numerical simulations that the cohesiveness during flow is not only controlled by the interparticle adhesion, but also by the stiffness and inelasticity of the grains. For the same adhesion, stiffer and less dissipative grains yield a less cohesive flow, i.e., higher “flowability.” This combined effect can be embedded in a single dimensionless number—a result that enriches our understanding of powder rheology.

Mandal et al PNAS – Published 2 April 2020

Velocity distributions, dispersion and stretching in three-dimensional porous media [paper in JFM]

Using index matching and particle tracking, we measure the three-dimensional velocity field in an isotropic porous medium composed of randomly packed solid spheres. Our results confirm the chaotic nature of advection within three-dimensional porous media and, by providing the laws of dispersion and stretching, opens the way to a complete description of mixing in porous media.

Souzy et al J. Fluid Mech. – Published 23 March 2020

The Darcytron: A pressure-imposed device to probe the frictional transition in shear-thickening suspensions [paper in J. Rheology]

In this paper, 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. Our results on a model shear-thickening suspensions of micrometric silica beads provide direct evidence of a transition between a frictionless and a frictional state as the particle pressure is increased, providing support to the recent frictional transition scenario for shear thickening.

Clavaud et al J. Rheology – Published 5 March 2020

Laser-light and Interactions with particles [conference LIP2020]

The next LIP conference will be held on August 22-28th, 2020 in the Institute of Physics, Warsaw, Poland. Main topics: interactions between laser beams and particles, encompassing the following fundamental topics : particle characterization methods, near-field, far-field and time-resolved scattering, plasmonics and other resonances, complex shaped particles and aggregates, multiple scattering and random media, mechanical effects of light, laser beams description (contributions acoustical and quantum beams are also welcomed) and application domains: two-and multiphase-flow characterization, aerosol science and atmospheric environment, plasma and soft matter physics, biomedical optical engineering, remote sensing…

Conveners: D. Jakubczyk, M. Kolwas, F. Onofri, G. Gouesbet

Website: / Flyer (call for abstracts): pdf

Gas-assisted discharge flow of granular media from silos [paper in PRFluids]

We studied experimentally the discharge of a vertical silo filled by spherical glass beads and assisted by injection of air from the top at a constant flow rate. Using a two-phase continuum model with a frictional rheology to describe particle-particle interactions, we reveal the role played by the air-pressure gradient at the orifice and proposed a simple analytical model to predicts the mass flow rate of a granular media discharged from a silo with injection of gas.

Zhou et al Phys. Rev. Fluids – Published 18 December 2019

Avalanches of Brownian granular materials [paper in PRL]

Macroscopic granular materials and colloids are usually studied  by different communities. But what happens if particles in a granular heap are so small that their thermal agitation becomes comparable to their weight? In this paper, we study such ‘Brownian granular flows’ using a microfluidic setup and show that thermal agitation can completely erase the flow threshold, a first step to bridge the gap between the physics of granular matter and colloids.

Bérut et al Phys. Rev. Lett. – Published 12 December 2019

Rheology of immersed and dry frictional spheres [paper in PRFluids]

In this paper, we use pressure-imposed rheometry to study the influence of surface roughness on the rheology of immersed and dry frictional spheres in the dense regime. We show that the quasistatic value of the effective friction coefficient is not significantly affected by particle roughness while the critical volume fraction at jamming decreases with increasing roughness. Collapse of rheological data is obtained by rescaling the volume fraction by the maximum volume fraction.

Tapia et al Phys. Rev. Fluids – Published 17 October 2019

Interparticle friction determines hysteresis in granular flow [paper in PRX]

Hysteresis is a major feature of the solid-liquid transition in granular materials but its origin is still debated. To study this phenomenon, we monitor the avalanche dynamics of non-Brownian suspensions in slowly rotating drums. By using microsilica particles whose interparticle friction coefficient can be turned off, we show that microscopic friction, conversely to inertia, is key to triggering hysteresis in granular suspensions.

Perrin et al Phys. Rev. X – Published 16 August 2019; see Focus story in Physics APS, news in Physics Today

Memory effects and transient gravitropic response in plants [paper in J Exp Bot]

In this paper, we study the plant gravitropic response to transient inclinations at the organ scale and the associated motion of statoliths at the cellular level. Our results reveal the existence of a memory process in the signalling pathway, independent of statolith dynamics. By combining this memory process with statolith motion, we build a mathematical model that unifies the different laws found in the literature and that predicts the early bending response of shoots to arbitrary gravi-stimulations.

Chauvet et al J Exp Bot 70,1955–1967 (2019) – Published 27 March 2019

Breakup of a particulate suspension jet [paper in PRFluids]

Adding solid particles to a liquid, which increases the effective viscosity, can paradoxically shorten the breakup length of a liquid capillary jet accelerated by gravity. This apparent contradiction is rationalized by considering finite-size effects occurring at the scale of a few particles. A model is presented which captures the breakup length of suspension jets for a broad range of conditions.

Château et al Phys. Rev. Fluid 4, 012001(R) – Published 10 January 2019

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