Growing of hexagonal compact domains in a granular compaction experiment. Watch the movie

Flowing of metallic balls in a rotating drum

Vibrated granular layer. Watch the movie

By chance my professional activities are combining my passions for sciences and technologies. In particular, I'm using extensively competencies (mainly developed in the framework of my hobbies) in electronics, programmation and mechanics to develop original measurement devices for research projects and also to illustrate my lectures.

### Position

• Associate Professor at the University of Liège
Chair : Soft Matter Physics and Complex Systems
Research Unit : CESAM
Research Group : GRASP

• Co-founder of GranuTools company developing and commercializing laboratory instruments for powder characterization: flow, density, cohesiveness, electrostatic properties, ...

### Lectures

• General Physics (for students in pharmacy)
• Thermodynamics (for bio-engineers)
• Waves and Quanta (for engineers)
• Techniques of Experimental Physics (for students in physics)
• Numerical tools for Soft Matter Physics (for students in physics)
Some pictures of the experiments performed to illustrate my lectures:

### Research

Soft matter - Cohesive granular materials - Static, quasistatic and dynamical properties of granular materials - Effect of a magnetic and electric field on a granular material - Powder electrostatics - Self Assembly processes - Active materials - Colloids

Research strategy - Our strategy to study a physical phenomenon is based on the development of original experimental set-ups to obtain original experimental results. Afterward, physical models are proposed to describe the physical mechanisms and to extract the main parameters. If needed, numerical simulations are conducted to investigate parameters which are difficult to control experimentally. Sometimes, laboratory prototypes developed to perform fundamental studies are becoming commercial instruments.

Physics of powders and granular materials. - A granular material is a conglomeration of discrete solid particles. Granular materials behavior is influenced by (1) steric repulsions, (2) friction forces (3) cohesive forces and (4) interaction with the surrounding gas. The steric repulsion is related to the grain geometry. Friction forces are influenced by both the surface state (rough or smooth surface) and the chemical nature of the grains. Cohesive forces may be induced by the presence of liquid bridges, by electrostatic charges, by van der Waals interactions or more rarely by magnetic dipole-dipole interactions. The predominance of one of these forces depends on both the environmental conditions and the physico-chemical properties of the grains. When the weight of one grain is higher than the cohesive forces, the material is considered as non-cohesive. These materials have been intensively studied during the last decades because of the rich variety of their physical properties. On the other hand, if the cohesive forces acting on a grain are higher than the weight of the grain, these cohesive forces will drastically modify the properties of the pile. Among these cohesive granular materials, fine powders are used in many research domains : chemistry, pharmacy, engineering... Nowadays, the processes used for the manipulation of powders are still mainly based on empirical knowledge. However, the complexity of the methods used in these domains induces the necessity of more rigorous knowledge of these materials. Therefore, fundamental studies of cohesive powders are still essential.

Model cohesive powders. - The difficulty to quantify and to control cohesion between the grains of a powder makes their experimental study very complex. Therefore, we first used a controlled cohesive powder made of metallic grains in an adjustable magnetic field B [18, 21]. In this controlled system, the cohesion between the grains can the tuned easily through the magnetic field. This system has been used during to study the influence of the cohesion on packing fraction, repose angle, heap shape, flow in silos and on the flow in a rotating drum.

Real cohesive powders. - After the study of model cohesive powders with a fundamental approach, we analyzed (and are still analyzing) the behavior of powders used in the industry with a more practical approach [25]. For that, we developped a range of original set-ups to measure packing dynamics, powder rheology, powder electrostatic properties, cohesiveness,... At the beginning, these set-ups were laboratory prototypes. After repetitive expressions of interest from industries, these methods were adapted to become commercial laboratory instruments: GranuFlow, GranuPack, GranuDrum, GranuHeap, GranuMidity and GranuCharge. These instruments are now commercialized by the company GranuTools.

Combined effect of humidity and electrostatic charges on powders. - When two materials are rubbed, electric charges are exchanged at the surfaces. This contact electrification is an old fundamental scientific subject. However, despite the numerous studies dedicated to this subject, the fundamental mechanisms behind the triboelectric effect are not fully understood in powders and granular materials. The electric charges created by triboelectric effects lead to uncontrolled electric field, electrostatic forces between the grains and/or between the grains and the container. Moisture is known to affect both static and dynamic behaviors of granular materials. Moreover, the effect of moisture is far from obvious due to the interplay with electrostatic effects. Indeed, moisture influences both surface grains conductivity and capillary bridges formation. For low relative air humidity, the electrical conductivity necessary for charge dissipation is reduced. For high relative air humidity, the electrical conductivity increases and liquid bridges may be formed at the contacts between the grains, resulting in sticking. Therefore, the electrical charges are dissipated more easily. However, the apparition of liquid bridges also induces cohesive forces inside the packing. At intermediate relative humidity values the cohesion is expected to be lower. We performed different studies on that topic. In particular, we analyzed the effect of powder flow aid additives (fumed silica, mesoporous silica, stearate, ...) on these cohesive forces.

Self assembly processes - My present research project includes a second axis dedicated to the study of self-assembly processes leading to the formation of mesostructures. Mesostructures are microscopic (typically from 100 nanometers to 100 microns) architectures with complex arrangements which confer them remarkable physical properties. Static and dynamic properties of such structures are investigated using model systems of Soft Matter. This activity is based on expertise acquired during my experimental works on collective motions, and patterning in granular materials. These self-organization processes take place in assemblies of micro and nano particles placed in an external field (magnetic and/or electric) and submitted to geometrical, mechanical, capillary and hydrodynamic constraints. In order to identify and to control the relevant self-assembly processes, the interactions between the particles have to be studied precisely. With a better fundamental understanding of these interactions, the self-organization processes will be obtained through a bottom-up method instead of the classical empirical methods. Then, we will be able to improve the long-range organization in mesostructures, catalyst, porous materials, sintered materials, ... Moreover, future studies will be dedicated to reversible self-organized systems where the order could be modified in order to obtain smart reconfigurable materials.

#### Publications

1Compaction of anisotropic granular materials: Experiments and simulations
Phys. Rev. E 70, 051314 (2004)
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2Grain mobility and hexagonal domains formation in 2d granular compaction
Powders & Grains 1, 343 (2005)
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3Experimental Study of Granular Compaction Dynamics at Different Scales: Grain Mobility, Hexagonal Domains, and Packing Fraction
Phys. Rev. Lett. 95, 028002 (2005)
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4Experimental study of the compaction dynamics for two-dimensional anisotropic granular materials
Phys. Rev. E 74, 021301 (2006)
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5Compaction of granular materials: experiments and contact dynamics simulations
J. Phys.: Conf. Ser. 40, 133 (2006)
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6Linking compaction dynamics to the flow properties of powders
Appl. Phys. Lett. 89, 093505 (2006)
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7Precursors to avalanches in a granular monolayer
Phys. Rev. E 74, 031311 (2006)
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8The influence of grain shape, friction and cohesion on granular compaction dynamics
Eur. Phys. J. E 22, 241 (2007)
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9Kinetic Energy Fluctuations and Diffusivity in a 2D Vibrated Granular Packing
TRAFFIC AND GRANULAR FLOW 2007 , 597 (2007)
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10TUNABLE RANDOM PACKINGS
New J. of Phys. 9, 406 (2007)
G.Lumay and N.Vandewalle
We present an experimental protocol that allows to tune the packing fraction $\eta$ of a random pile of ferromagnetic spheres from a value close to the lower limit of random loose packing $\eta_{RLP} \simeq 0.56$ to the upper limit of random close packing $\eta_{RCP} \simeq 0.64$. This broad range of packing fraction values are obtained under normal gravity, but we apply a magnetically controlled cohesion between the grains during the formation of the pile. After the formation of the pile, the induced cohesion is decreased continuously along a linear decreasing ramp. The controlled collapse of the pile is found to generate various and reproducible values of the random packing fraction $\eta$.
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11Swarming and swirling in self-propelled polar granular rods
Phys. Rev. Lett. 100, 058001 (2008)
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12Stationary states in 1D system of inelastic particles
Ukr. Journ. Phys. 53, 1128 (2008)
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13Controlled flow of Smart Powders
Phys. Rev. E 78, 061302 (2008)
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14Mullite coatings on ceramic substrates: stabilisation of Al2O3-SiO2 suspensions for spray drying of composite granules suitable for reactive plasma spray
J. of Eur. ceramic soc. 29, 2169 (2008)
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15Motion of carbon nanotubes in a rotating drum: The dynamic angle of repose and a bed behavior diagram
Chem. Eng. J. 146, 143 (2009)
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16Packing fraction and compaction dynamics of magnetic powders
AIP Conf. Proc. 1145, 131-134 (2009)
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17Flow properties and heap shape of magnetic powders
AIP Conf. Proc. 1145, 135-138 (2009)
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18Compaction dynamics of a magnetized powder
Phys. Rev. E 80, 041302 (2009)
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19Effect of an electric field on an intermittent granular flow
Phys. Rev. E 81, 041309 (2010)
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20Compaction dynamics of wet granular assemblies
Phys. Rev. Lett. 105, 048001 (2010)
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21Flow of magnetized grains in a rotating drum
Phys. Rev. E 82, 040301(R) (2010)
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22A pendulum test as a tool to evaluate viscous friction parameters in the equine fetlock joint
The Veterinary Journal 188, 204 (2011)
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23Influence of a reduced gravity on the volume fraction of a monolayer of spherical grains
Phys. Rev. E 84, 041305 (2011)
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24Granular gas in a periodic lattice
Eur. Phys. J. E 32, 1465 (2011)
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25Measuring the flowing properties of powders and grains
Powder Technology 224, 19 (2012)
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26How relative humidity affects random packing experiments
Phys. Rev. E 85, 031309 (2012)
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27Hysteretic behavior in three-dimensional soap film rearrangements
Phys. Rev. E 83, 021403 (2011)
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28Symmetry breaking in a few-body system with magnetocapillary interactions
Phys. Rev. E 85, 041402 (2012)
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29Cascade of flows for characterizing segregation of granular mixtures
Powder Technology 234, 32 (2013)
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30Breaking arches with vibrations: the role of defects and friction
Phys. Rev. Lett. 109, 068001 (2012)
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31Flow abilities of powders and granular materials evidenced from dynamical tap density measurement
Powder Technology 235, 842 (2013)
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32Experimental study of a vertical column of grains submitted to a series of impulses
European Physical Journal E 36, 16 (2013)
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33Influence of the gravity on the discharge of a silo
Granular Matter 15, 263 (2013)
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34Self-assembled magnetocapillary swimmers
Soft Matter 9, 2420 (2013)
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35Melting of a confined monolayer of magnetized beads
Phys. Rev. E 87, 062201 (2013)
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36Mesoscale structures from magnetocapillary self-assembly
Eur. Phys. J. E 36, 127 (2013)
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37Customizing mesoscale self-assembly with three-dimensional printing
New J. of Phys. 16, 023013 (2014)
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38Quantitatively mimicking wet colloidal suspensions with dry granular media
Scientific reports 5, 10348 (2015)
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39Bernal random loose packing through freeze-thaw cycling
Phys. Rev. E 92, 010202 (2015)
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40Rheological behavior of β-Ti and NiTi powders produced by atomization for SLM production of open porous orthopedic implants
Powder Technology 283, 199 (2015)
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41Linking flowability and granulometry of lactose powders
International Journal of Pharmaceutics 494, 312 (2015)
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42Remote control of self-assembled microswimmers
Scientific reports 5, 16035 (2015)
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43Flow of magnetic repelling grains in a two-dimensional silo
Papers in Physics 7, 070013 (2015)
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44Ribbons of superparamagnetic colloids in magnetic field
The European Physical Journal E 39, 47 (2016)
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45Statics and dynamics of magnetocapillary bonds
Phys. Rev. E 93, 053117 (2016)
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46Effect of relative air humidity on the flowability of lactose powders
Journal of Drug Delivery Science and Technology 35, 207 (2016)
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47Magnetoelastic instability in soft thin films
The European Physical Journal E 40, 29 (2017)
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48Relating Brownian motion to diffusion with superparamagnetic colloids
American Journal of Physics 85, 265 (2017)
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49Frustrated crystallization of a monolayer of magnetized beads under geometrical confinement
Phys. Rev. E 95, 062120 (2017)
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50Self-assembly processes of superparamagnetic colloids in a quasi-two-dimensional system
Phys. Rev. E 96, 012608 (2017)
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51Superparamagnetic colloids in viscous fluids
Scientific Reports 7, 7778 (2017)
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52Self-assembly of smart mesoscopic objects
The European Physical Journal E 40, 108 (2017)
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53Discharge of repulsive grains from a silo: experiments and simulations
EPJ Web of Conferences 140, 03089 (2017)
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54Combined effect of moisture and electrostatic charges on powder flow
EPJ Web of Conferences 140, 13009 (2017)
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55Transitional bulk-solutal Marangoni instability in sessile drops
Phys. Rev. E 98, 062609 (2018)
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56Remote-controlled deposit of superparamagnetic colloidal droplets
Phys. Rev. E 98, 062608 (2018)
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57Decompaction of wet granular materials under freeze-thaw cycling
Phys. Rev. E 99, 012901 (2019)
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58From jamming to fast compaction dynamics in granular binary mixtures
Scientific Reports 9, 7281 (2019)
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59Tribo-electrification of pharmaceutical powder blends
Particulate Science and Technology 37, 1020 (2019)
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60How to gain a full understanding of powder flow properties, and the benefits of doing so
ONdrugDelivery 102, 42 (2019)
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61Influence of mesoporous silica on powder flow and electrostatic properties on short and long term
Journal of Drug Delivery Science and Technology 53, 101192 (2019)
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62Combined effects of Marangoni, sedimentation and coffee-ring flows on evaporative deposits of superparamagnetic colloids
Colloid and Interface Science Communications 32, 100198 (2019)
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63Effect of volume fraction on chains of superparamagnetic colloids at equilibrium
European Physical Journal E 42, 123 (2019)
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