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 recently 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 am currently leading a European Interreg VA project "PowderReg", which brings 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 is to study the "transport, storage and forming of industrially relevant powders".

I am also 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).

ONGOING RESEARCH: 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 …