Research overview

MY RESEARCH

I have focused my research to provide answers to fundamental problems consisting in better understanding how the rheological behavior of granular suspensions is related to the structuring and rearrangement dynamics at the particle scale.

Since 2006, my research activities have consisted in developing tools and methods (particle tracking using refractive index matching techniques, Multispeckle diffusing wave sprectroscopy, non-linear acoustic measurements ...) for multi-scale rheological characterization of granular dispersions subjected to vibrations, whether for confined or free surface. The approach of my work is based on a multidisciplinary systemic approach using multi-scale and multi-physical modelling techniques to model the behavior of granular dispersions at the laboratory scale. I launched and initiated a new research team at LEMTA on this topic.

I have extended my collaborations at the University of Lorraine by working with the LIBio laboratory (Laboratoire d'Ingénierie des Biomolécules), specialized in the functionalization of powders for agri-food applications and LRGP laboratory (Laboratoire Réactions Genie des Procédés).  I have therefore grouped together a consortium made up of teams from LEMTA, LRGP and LIBio at the University of Lorraine around a joint project on the study of granular dispersions and powders in particular. In this context, I initiated in 2017 a large scale rapprochement and collaboration between this Lorraine consortium and universities of the Greater Region, also specialized in granular dispersions and internationally recognized for their work in this field. I have therefore set up and I led a European Interreg VA project "PowderReg", which brought together 7 laboratories within 5 universities (University of Lorraine, University of Kaiserslautern, University of Liège, University of Luxembourg, University of Saarland). The aim of this major project was to study the "transport, storage and forming of industrially relevant powders". This project ended in 2021.

I am currently the coordinator of an international PRCI ANR project (French national research agency) in collaboration with the University of Kaiserslautern (DFG program) on the optimization of the spreading of granular pastes by the application of vibrations. This work will enable us to propose ways of improving processes that use gypsum pastes or concrete.

WHAT I FOUND OUT ...

Precursors of avalanches are periodic stick-slip events that mobilize the superficial layers of the pile

Rearrangements at the surface of a slowly tilted pile made of 1 mm  glass beads.
Evolution of the rearranged surface fraction as a function of the inclination angle of the pile.

I was interested in the relaxation dynamics of a granular packing slowly tilted to the avalanche threshold. Different types of rearrangements appear in both the volume and the surface of the system. Small localized events involving a small number of grains appear at small angles followed above a critical angle by the appearance of large events involving a large portion of the particles, called avalanche precursors. These precursors appear periodically and heralds the imminence of an avalanche. This study has made it possible to characterize the dynamics and statistical properties of these events both at the surface and in the volume by coupling classical optical methods for tracking particles with acoustic scattering techniques to probe the dynamics of the contact network during tilting. This study has led to advances in the understanding of the stability of the stacks, and from a rheological point of view on the nature of the flow threshold.

Vibrations can tune the viscosity of granular media! The more viscous the interstitial fluid is, the better the medium flows

Viscosity of a granular suspension as a function of the shear rate obtained in a vibrated Couette device. (1) Newtonian regime, (2) Intermediate regime and (3) Frictional regime.
Influence the interstitial fluid viscosity on the flow curve for a given vibration intensity.
Index matching techniques are used to visualize and track particle trajectories. Mechanical vibrations make appear a Newtonian behavior related to a diffusive behavior of particles.

Whether for vibrated dry or fully liquid saturated granular dispersions, the apparent viscosity exhibits a Newtonian plateau followed by a frictional behavior. The rheology is then similar to a shear thinning behavior with a Newtonian plateau at low shear rate, whose value decreases with the vibration intensity. For saturated dense suspensions, the viscosity at the plateau decreases with the lubrication Peclet number depending on the vibration amplitude and frequency, interstitial fluid viscosity and grain diameter. This lubrication Peclet number can be interpreted as the ratio between the lubrication stress induced by the vibrations and the frictional stress between the grains. Vibrations induce lubricating forces between the grains that remove the apparent threshold stress of these materials and cause its viscosity to drop. In the low-gradient vibration-controlled regime, the system exhibits a diffusive behavior where the root mean square displacement increases linearly with time. The associated diffusion coefficient increases linearly with the intensity of the vibrations, and thus with the lubrication Peclet number. The viscosity scales as the inverse of the plateau viscosity (i.e. the inverse of the lubrication Peclet number).

The European Project "PowderReg" (2017 - 2021)

Academic partners:

University of Liege

Understanding granular matter compaction under humid and charged atmosphere.

Universität des Saarlandes

Powder metallurgy, Powder rheology under confinement.

University of Luxembourg 

Computer simulation methods DEM.

University of Lorraine

Physical and Chemical characterization of granular matter, Powder rheology from a fluid dynamics perspective. 

University of Kaiserslautern

Powder technology, Particle Process Engineering.

Industrial partners: NovaCarb (Nancy), Granutools (Belgium).

In 2017, I initiated the creation of an international research network on "powders" in the Grande Région. We have gathered more than 15 years of research and development in granular matter applications in a new European team.  

Problems to which we can provide solutions: variability of powders behaviors depending on surrounding conditions, powders degradation by industrial processes, difficulties to predict flowabillity, formulation dependence of segregation, fragmentation, agglomeration …

Make bidisperse grains vibrate... for a better lubrication!

Vibrations promote the segregation of a bidisperse granular medium flowing on an inclined plane. For a critical volume fraction of small particles (~30%), the latter acts as a lubricant to limit the apparent friction with the bottom. Under these conditions, a bidisperse material flows faster than the monodisperse flows corresponding to each particle size.

This is a new discovery that allows to better understand some of the mechanisms at work when additives are added to increase the flowability of powders.

Left: Sketch of the experimental setup (3D and top view) used to study the free surface flow of a bimodal granular mixture. Right: Evolution of the surface velocity 𝑢𝑠 as a function of the concentration of large particles 𝛷𝐿 for different mixtures. From scientific publication https://doi.org/10.1016/j.powtec.2023.118219 

When the grains swell... they reach their final size before absorbing their maximum water capacity!

We have studied the swelling of couscous particles using a grain-scale experimental approach. During the swelling phase, both the evolution of particle size using image analysis and the spatially resolved field of diffusion coefficient of water inside the grains were studied using MRI measurements.

A concentrically penetration of water in the cross section of the grains was shown on images. The experimental results were compared to classical models from the literature used to predict the swelling of polymeric materials. A simple first-order kinetic equation was shown to be adequate to describe the size evolution during swelling. 

Thus, the results suggested that the swelling dynamics was a water uptake-limiting mechanism because of the existence of a gelatinous layer at the grain surface formed during the manufacturing process.

Top left: Typical time evolution of the normalized diameter of a couscous grain. Top right: Final diffusion map from MRI measurements by suppressing the background signal coming from the surrounding water. Bottom: MRI intensity images showing the long-time swelling dynamics of six couscous grains. From scientific publication https://doi.org/10.1016/j.jfoodeng.2021.110910 

Ongoing research ...