Publications of Nicolas Vandewalle
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See detailFrom jamming to fast compaction dynamics in granular binary mixtures
Pillitteri, Salvatore ULiege; Lumay, Geoffroy ULiege; Opsomer, Eric ULiege et al

in Scientific Reports (2019), 9

Mixing particles of different sizes is an efficient method to increase the density of granular media. While the density of mixtures made of two sizes of glass beads has been highly studied, the dynamics ... [more ▼]

Mixing particles of different sizes is an efficient method to increase the density of granular media. While the density of mixtures made of two sizes of glass beads has been highly studied, the dynamics of the compaction process remains poorly investigated. In our study, we measured the typical compaction time for various mixtures depending on the size ratio and the relative fraction of small and large beads. We observed a diverging behaviour of the compaction time close to the percolation threshold and we highlighted a fast compaction dynamics when the size ratio is large enough. [less ▲]

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See detailJamming transition in binary mixture
Pillitteri, Salvatore ULiege; Lumay, Geoffroy ULiege; Opsomer, Eric ULiege et al

Conference (2019, April 11)

Binary mixtures of spherical particles is a well known topic in granular physics and mathematics. While the bulk density has been highly studied, the compaction dynamics of mixtures remains poorly ... [more ▼]

Binary mixtures of spherical particles is a well known topic in granular physics and mathematics. While the bulk density has been highly studied, the compaction dynamics of mixtures remains poorly investigated. In our work, we measured the compaction dynamics of various mixtures with different compositions of large and small particles and size ratios. We observed a diverging behaviour of the typical time of compaction depending on the size ratio. [less ▲]

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See detailRemote-controlled deposit of superparamagnetic colloidal droplets
Darras, Alexis ULiege; Mignolet, Florence ULiege; Vandewalle, Nicolas ULiege et al

in Physical Review. E ,Statistical, Nonlinear, and Soft Matter Physics (2018), 98(6),

Evaporation of sessile droplets is a way to organize suspended particles and create surface coating. Many studies have demonstrated that suspensions with various composition can give rise to qualitatively ... [more ▼]

Evaporation of sessile droplets is a way to organize suspended particles and create surface coating. Many studies have demonstrated that suspensions with various composition can give rise to qualitatively different dried patterns, often by focusing on the radial density pro le of deposited particles. We demonstrate that a single suspension of superparamagnetic colloids can give rise to several dried patterns thanks to an external magnetic eld applied during the evaporation process. We show the various patterns obtained with zero, constant, rotating and oscillating magnetic elds, and evidence the continuous control given by the intensity of a constant magnetic eld. We also show this magnetic control has a substantial e ect on the morphological details of the deposits. [less ▲]

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See detailTransitional bulk-solutal Marangoni instability in sessile drops
Darras, Alexis ULiege; Vandewalle, Nicolas ULiege; Lumay, Geoffroy ULiege

in Physical Review. E ,Statistical, Nonlinear, and Soft Matter Physics (2018), 98(6),

Evaporation of sessile droplets is a method to organize suspended particles on solid substrates. Many studies have demonstrated that Marangoni flows caused by surface adsorbed molecules or temperature ... [more ▼]

Evaporation of sessile droplets is a method to organize suspended particles on solid substrates. Many studies have demonstrated that Marangoni flows caused by surface adsorbed molecules or temperature gradients can strongly a ect the dried deposit. In the present paper, we show how transitional Marangoni instabilitiy can be triggered by bulk-diluted tensio-active ions. Thanks to PIV analysis, we identify four different flow stages. The transition between them can be understood by considering the competition between the Marangoni flow and the mass conservation flow, usually responsible for the coffee-ring pattern. We also demonstrate that the initial ionic concentration can select a coffee-ring pattern or a more homogeneous dried deposit. [less ▲]

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See detailStudy of compaction due to horizontal and vertical vibrations
Pillitteri, Salvatore ULiege; Vandewalle, Nicolas ULiege; Lumay, Geoffroy ULiege et al

Conference (2018, September 11)

Shaking a granular medium increases its packing fraction. While extensively studied, this phenomenon, called compaction, remains puzzling. Indeed, compaction is determined by many parameters like the ... [more ▼]

Shaking a granular medium increases its packing fraction. While extensively studied, this phenomenon, called compaction, remains puzzling. Indeed, compaction is determined by many parameters like the grain’s shapes, sizes and the acceleration of vibration. In our study, we address the effect of vibration’s direction. We present experimental results for different experimental conditions (horizontal versus vertical vibrations). [less ▲]

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See detailBubbles and beads : The same problem of sphere packing ?
Pillitteri, Salvatore ULiege; Lumay, Geoffroy ULiege; Opsomer, Eric ULiege et al

Poster (2018, July 10)

Ones can observe that bubbles in a foam are organized and deformed according to the Plateau's laws. This phenomenon is related to a problem of surface optimization : the system minimizes its surface ... [more ▼]

Ones can observe that bubbles in a foam are organized and deformed according to the Plateau's laws. This phenomenon is related to a problem of surface optimization : the system minimizes its surface energy. However, in wet foams, it is different. Bubbles are spheres of different sizes with low deformations. The organization of the foam seems to be closer to a problem of sphere packing. [less ▲]

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See detailThreshold of gas-like to clustering transition in driven granular media in low-gravity environment
Noirhomme, Martial ULiege; Cazaubiel, Annette; Darras, Alexis ULiege et al

in Europhysics Letters (2018), 123(14003),

Strongly driven granular media are known to undergo a transition from a gas-like to a cluster regime when the density of particles is increased. However, the main mechanism triggering this transition is ... [more ▼]

Strongly driven granular media are known to undergo a transition from a gas-like to a cluster regime when the density of particles is increased. However, the main mechanism triggering this transition is not fully understood so far. Here, we investigate experimentally this transition within a 3D cell filled with beads that are driven by two face-to-face vibrating pistons in low gravity during parabolic flight campaigns. By varying large ranges of parameters, we obtain the full phase diagram of the dynamical regimes reached by the out-of-equilibrium system: gas, cluster or bouncing aggregate. The images of the cell recorded by two perpendicular cameras are processed to obtain the profiles of particle density along the vibration axis of the cell. A statistical test is then performed on these distributions to determinate which regime is reached by the system. The experimental results are found in very good agreement with theoretical models for the gas-cluster transition and for the emergence of the bouncing state. The transition is shown to occur when the typical propagation time needed to transmit the kinetic energy from one piston to the other is of the order of the relaxation time due to dissipative collisions. [less ▲]

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See detailInfluence of grain size distribution on both packing fraction and compaction dynamics
Pillitteri, Salvatore ULiege; Lumay, Geoffroy ULiege; Opsomer, Eric ULiege et al

Poster (2018, June 27)

Shaking a granular media increases its packing fraction. While extensively studied, this phenomenon, called compaction, remains puzzling. Some studies have shown that we can link measurements performed ... [more ▼]

Shaking a granular media increases its packing fraction. While extensively studied, this phenomenon, called compaction, remains puzzling. Some studies have shown that we can link measurements performed during a compaction experiment with some flow properties of a powder. [less ▲]

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See detailStructures macroscopique et microscopique et formation des aegagropiles de Posidonia oceanica (L.) Delile (1813)
Lefebvre, Laurence ULiege; Gobert, Sylvie ULiege; Compère, Philippe ULiege et al

Poster (2018, April 11)

Il est courant de retrouver le long de plages de Méditerranée des agglomérations de débris végétaux originaire de l’herbier à Posidonies. Ces agglomérats portent le nom d’aegagropiles de P. oceanica ... [more ▼]

Il est courant de retrouver le long de plages de Méditerranée des agglomérations de débris végétaux originaire de l’herbier à Posidonies. Ces agglomérats portent le nom d’aegagropiles de P. oceanica , ceux-ci arborent des formes sphériques et ellipsoïdes. Les objectifs principaux de ce travail étaient d’identifier les différents composants des aegagropiles de P. oceanica et de déterminer la formation de ceux-ci. Pour ce faire, différentes analyses microscopiques (MET, MEB et MO) ont été effectuées ainsi que l’usage d’un mécanisme simulant la houle. Les observations en microscopie électronique à balayage ont été réalisées à l’aide de différents détecteurs (SSD et ETD) de SE et BSE, ce qui additionnés à diverses techniques de colorations et de contrastants ont permis de répondre à de nombreux objectifs. Les aegagropiles se forment par l’hydrodynamisme qu’engendre la houle et se composent d’un assemblage de débris végétaux provenant de P. oceanica ainsi que de grains de sable. Au fil des différentes étapes du cycle de la plante et de l’hydrodynamisme, les organes de la plante se rompent, s’accumulent au sein de la litière et subissent une combinaison de dégradations biotique et abiotique. Ces organes sont majoritairement des feuilles et des morceaux de rhizomes. La dégradation biotique est notamment engendrée par des champignons noirs septés nommés « Dark Septate Endophyte - DSE » qui, en s’attaquant spécifiquement à la lamelle mitoyenne des cellules permet des scissions longitudinales. La dégradation abiotique est quant à elle engendrée par les forces hydrodynamiques. Cette combinaison entraîne dès lors un lessivage important des différents organes végétaux qui se scindent et se courbent formant finalement de fines « fibres » qui s’accumuleront et s’entremêleront ensemble des ripples-marks afin d’élaborer des aegagropiles de P. oceanica. Les aegagropiles de P. oceanica se formeront initialement avec des débris intacts de rhizomes ou bien avec des faisceaux de fibres lignifiés ronds provenant également de ces rhizomes où de nombreux grains de nature minérale s’enchevêtreront à ces débris végétaux afin de constituer une structure initiale consolidée et résistante. Ensuite, d’autres débris végétaux, majoritairement de formes plates et provenant des feuilles et de gaines foliaires de P. oceanica , s’entremêleront à cette structure afin d’obtenir une agrégation allongée. Enfin, via une fragmentation aléatoire et des mouvements suivant préférentiellement une direction, des aegagropiles de P. oceanica de formes majoritairement ellipsoïdales s’échoueront le long des plages. [less ▲]

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See detailEvaporation of colloids droplets with PBS
Darras, Alexis ULiege; Vandewalle, Nicolas ULiege; Lumay, Geoffroy ULiege

Conference (2018, March 19)

The evaporation of colloidal droplets is currently a topic of intensive research due to its wide range of potential applications, extending from blood analysis via inkjet printing, paint and polymers , up ... [more ▼]

The evaporation of colloidal droplets is currently a topic of intensive research due to its wide range of potential applications, extending from blood analysis via inkjet printing, paint and polymers , up to and including nanotechnology. The main feature of colloidal droplets evaporation is the socalled coffee-ring effect. This effect implies that particles tend to be deposited at the edge of the final dried pattern. However, several studies have shown various mechanisms preventing this effect. Notably, capillary interactions of ellipsoidal particles and the presence of surface-adsorbed polymers allow a uniform coating of the particles. More recently, some researchers have shown that more complex deposits’ structures can be achieved thanks to various concentration of liquid crystals. In our research, we demonstrate how magnetic interactions between superparamagnetic colloidal particles can be used to control the properties of colloidal droplets’ deposits. This actually requires the right chemical composition of the suspension. But then, a single suspension can result in various dried patterns. The actual pattern is selected from the application of an external magnetic field during the evaporation. To our knowledge, this is the first time that such a tunable control of evaporative deposits is described in the literature. [less ▲]

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See detailMagnetocapillary self-assemblies: locomotion and micromanipulation along a liquid interface
Grosjean, Galien ULiege; Hubert, Maxime ULiege; Vandewalle, Nicolas ULiege

in Advances in Colloid and Interface Science (2018), 255

Review article on magnetocapillary self-assemblies, and in particular microswimmers, which includes new results concerning the manipulation of small objects and the mixing fluid at low Reynolds number.

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See detailSurface swimmers, harnessing the interface to self-propel
Grosjean, Galien ULiege; Hubert, Maxime ULiege; Collard, Ylona ULiege et al

in European Physical Journal E. Soft Matter (2018), 41

In the study of microscopic flows, self-propulsion has been particularly topical in recent years, with the rise of miniature artificial swimmers as a new tool for flow control, low Reynolds number mixing ... [more ▼]

In the study of microscopic flows, self-propulsion has been particularly topical in recent years, with the rise of miniature artificial swimmers as a new tool for flow control, low Reynolds number mixing, micromanipulation or even drug delivery. It is possible to take advantage of interfacial physics to propel these microrobots, as demonstrated by recent experiments using the proximity of an interface, or the interface itself, to generate propulsion at low Reynolds number. This paper discusses how a nearby interface can provide the symmetry breaking necessary for propulsion. An overview of recent experiments illustrates how forces at the interface can be used to generate locomotion. Surface swimmers ranging from the microscopic scale to typically the capillary length are covered. Two systems are then discussed in greater detail. The first is composed of floating ferromagnetic spheres that assemble through capillarity into swimming structures. Two previously studied configurations, triangular and collinear, are discussed and contrasted. A new interpretation for the triangular swimmer is presented. Then, the non-monotonic influence of surface tension and viscosity is evidenced in the collinear case. Finally, a new system is introduced. It is a magnetically powered, centimeter-sized piece that swims similarly to water striders. [less ▲]

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See detailSelf-assembly of smart mesoscopic objects
Metzmacher, Jean ULiege; Poty, Martin ULiege; Lumay, Geoffroy ULiege et al

in European Physical Journal E. Soft Matter and Biological Physics (2017), 40

Self-assembly due to capillary forces is a common method for generating 2D mesoscale structures made of identical particles floating at some liquid-air interface. We show herein how to create soft ... [more ▼]

Self-assembly due to capillary forces is a common method for generating 2D mesoscale structures made of identical particles floating at some liquid-air interface. We show herein how to create soft entities that deform or not the liquid interface as a function of the strength of some applied magnetic field. These smart floating objects self-assemble or not depending on the application of an external field. Moreover, we show that the self-assembling process can be reversed opening ways to rearrange structures. [less ▲]

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See detailSuperparamagnetic colloids in viscous fluids
Darras, Alexis ULiege; Opsomer, Eric ULiege; Vandewalle, Nicolas ULiege et al

in Scientific Reports (2017), 7

The influence of a magnetic field on the aggregation process of superparamagnetic colloids has been well known on short time for a few decades. However, the influence of important parameters, such as ... [more ▼]

The influence of a magnetic field on the aggregation process of superparamagnetic colloids has been well known on short time for a few decades. However, the influence of important parameters, such as viscosity of the liquid, has received only little attention. Moreover, the equilibrium state reached after a long time is still challenging on some aspects. Indeed, recent experimental measurements show deviations from pure analytical models in extreme conditions. Furthermore, current simulations would require several years of computing time to reach equilibrium state under those conditions. In the present paper, we show how viscosity influences the characteristic time of the aggregation process, with experimental measurements in agreement with previous theories on transient behaviour. Afterwards, we performed numerical simulations on equivalent systems with lower viscosities. Below a critical value of viscosity, a transition to a new aggregation regime is observed and analysed. We noticed this result can be used to reduce the numerical simulation time from several orders of magnitude, without modifying the intrinsic physical behaviour of the particles. However, it also implies that, for high magnetic fields, granular gases could have a very different behaviour from colloidal liquids. [less ▲]

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See detailVibration Induced Phenomena in Granular Media in Microgravity
Opsomer, Eric ULiege; Noirhomme, Martial ULiege; Vandewalle, Nicolas ULiege

in EPJ Web of Conferences (2017, June 30)

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See detailFrustrated crystallization of a monolayer of magnetized beads under geometrical confinement
Schockmel, Julien ULiege; Vandewalle, Nicolas ULiege; Opsomer, Eric ULiege et al

in Physical Review. E ,Statistical, Nonlinear, and Soft Matter Physics (2017)

We present a systematic experimental study of the confinement effect on the crystallization of a monolayer of magnetized beads. The particles are millimeter-scale grains interacting through the short ... [more ▼]

We present a systematic experimental study of the confinement effect on the crystallization of a monolayer of magnetized beads. The particles are millimeter-scale grains interacting through the short range magnetic dipole-dipole potential induced by an external magnetic field. The grains are confined by repulsing walls and are homogeneously distributed inside the cell. A two-dimensional (2d) Brownian motion is induced by horizontal mechanical vibrations. Therefore, the balance between magnetic interaction and agitation allows investigating 2d phases through direct visualization. The effect of both confinement size and shape on the grains’ organization in the low-energy state has been investigated. Concerning the confinement shape, triangular, square, pentagonal, hexagonal, heptagonal, and circular geometries have been considered. The grain organization was analyzed after a slow cooling process. Through the measurement of the averaged bond order parameter for the different confinement geometries, it has been shown that cell geometry strongly affects the ordering of the system. Moreover, many kinds of defects, whose observation rate is linked to the geometry, have been observed: disclinations, dislocations, defects chain, and also more exotic defects such as a rosette. Finally, the influence of confinement size has been investigated and we point out that no finite-size effect occurs for a hexagonal cell, but the finite-size effect changes from one geometry to another. [less ▲]

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See detailCoarsening dynamics of a granular lattice gas
Opsomer, Eric ULiege; Noirhomme, Martial ULiege; Ludewig, François et al

Poster (2017, March 23)

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See detailRealization of the Najafi-Golestanian microswimmer
Hubert, Maxime ULiege; Grosjean, Galien ULiege; Lagubeau, Guillaume et al

Conference (2017, March 14)

The development of artificial microswimmers, microscopic robots that swim in a fluid like sperm cells and motile bacteria, could cause a leap forward in various fields such as microfluidics, microsystems ... [more ▼]

The development of artificial microswimmers, microscopic robots that swim in a fluid like sperm cells and motile bacteria, could cause a leap forward in various fields such as microfluidics, microsystems, or minimally invasive medicine. Nature provides plenty of examples of efficient microswimmers. However, a bottom-up approach, looking at the simplest ingredients needed to generate a microswimmer, can lead to a deeper under- standing of the swimming problem. First described by Najafi and Golestanian 1 , a paradigmatic microswimmer is the three-linked-spheres model, which follows a minimalist approach for propulsion by shape shifting. In this presentation, we describe the experimental realisation of this microswimmer using self-assembled ferromagnetic particle at an air-water interface, powered by an uniform oscillating magnetic field 2 . A model, using two har- monic oscillators, reproduces the experimental findings. Because the model remains general, the same approach could be used to design a variety of efficient microswimmers. [less ▲]

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