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See detailSelf-Assembly and Evaporation of Superparamagnetic Colloids
Darras, Alexis ULiege

Doctoral thesis (2019)

The evaporation of colloidal droplets is an area of intensive research. From paint coating to blood analysis on crime scene, applications of patterning from evaporation of colloids are numerous and ... [more ▼]

The evaporation of colloidal droplets is an area of intensive research. From paint coating to blood analysis on crime scene, applications of patterning from evaporation of colloids are numerous and various. In our work, we aimed to bring highlight on how interactions between colloidal particles influence the eventual deposit's pattern. To do this we used superparamagnetic colloids as a way to have a tunable interaction with these particles. We first studied the influence of dipolar interactions on the suspension of particles. We performed systematic experiments to characterize the thermodynamic equilibrium reached by the suspensions. We showed that tuning the viscosity parameter could be used to speed-up numerical simulations. We used this process in sped-up simulations in order to study a new range of volume fraction. We showed that high volume fraction conditions led to higher chains' length than expected. We proposed a modification of current models. We then focused on evaporating droplets. We began by reviewing the liquid flows in our suspensions. We showed there was a competition between coffee-ring flow and Marangoni instability. We then characterized the deposits left after evaporating suspensions under magnetic fields, and showed the influence of both this parameter and the Marangoni instability on the eventual deposits. We evidenced a transition in the deposit's behaviour when the DLVO interaction between the particles and the substrate becomes attractive. [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 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 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 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 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 detailSelf-assembly processes of superparamagnetic colloids in a quasi-two-dimensional system
Rojas, Nicolas; Darras, Alexis ULiege; Lumay, Geoffroy ULiege

in Physical Review. E ,Statistical, Nonlinear, and Soft Matter Physics (2017), 96(1), 012608

Superparamagnetic colloids gather depending on the magnitude of the magnetic field applied, forming chains and ribbons in a quasi-two-dimensional chamber. The results presented in this work are in good ... [more ▼]

Superparamagnetic colloids gather depending on the magnitude of the magnetic field applied, forming chains and ribbons in a quasi-two-dimensional chamber. The results presented in this work are in good agreement with recent experimental multistable data for the mean length of the aggregates in thermodynamic equilibrium. [less ▲]

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See detailRibbons of superparamagnetic colloids in magnetic field
Darras, Alexis ULiege; Fiscina, Jorge; Pakpour, Maryam et al

Conference (2017, March)

While the aggregation process of superparamagnetic colloids in strong magnetic eld is well known on short time since a few decades, recent theoretical works predicted an equilibrium state reached after a ... [more ▼]

While the aggregation process of superparamagnetic colloids in strong magnetic eld is well known on short time since a few decades, recent theoretical works predicted an equilibrium state reached after a long time. In this talk, we present experimental observations of this equilibrium state with a twodimensional system and we compare our data with the predictions of a pre-existing model. Above a critical aggregation size, a deviation between the model and the experimental data is observed. This deviation is explained by the formation of ribbon-shaped aggregates. The ribbons are formed due to lateral aggregation of chains. An estimation of the magnetic energy for chains and ribbons shows that ribbons are stable structures when the number of magnetic grains is higher than N=30. [less ▲]

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See detailRibbons of superparamagnetic colloids
Darras, Alexis ULiege; Fiscina, Jorge; Pakpour, Maryam et al

Poster (2016, October 12)

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See detailStatics and dynamics of magnetocapillary bonds
Lagubeau, Guillaume; Grosjean, Galien ULiege; Darras, Alexis ULiege et al

in Physical Review. E ,Statistical, Nonlinear, and Soft Matter Physics (2016), 93

When ferromagnetic particles are suspended at an interface under magnetic fields, dipole-dipole interactions compete with capillary attraction. This combination of forces has recently given promising ... [more ▼]

When ferromagnetic particles are suspended at an interface under magnetic fields, dipole-dipole interactions compete with capillary attraction. This combination of forces has recently given promising results towards controllable self-assemblies as well as low-Reynolds-number swimming systems. The elementary unit of these assemblies is a pair of particles. Although equilibrium properties of this interaction are well described, the dynamics remain unclear. In this paper, the properties of magnetocapillary bonds are determined by probing them with magnetic perturbations. Two deformation modes are evidenced and discussed. These modes exhibit resonances whose frequencies can be detuned to generate nonreciprocal motion. A model is proposed that can become the basis for elaborate collective behaviors. [less ▲]

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See detailRibbons of superparamagnetic colloids in magnetic field
Darras, Alexis ULiege; Fiscina, Jorge; Pakpour, Maryam et al

in European Journal of Physics (2016), 39(47),

While the aggregation process of superparamagnetic colloids in strong magnetic field is well known on short time since a few decades, recent theoretical works predicted an equilibrium state reached after ... [more ▼]

While the aggregation process of superparamagnetic colloids in strong magnetic field is well known on short time since a few decades, recent theoretical works predicted an equilibrium state reached after a long time. In the present paper, we present experimental observations of this equilibrium state with a two-dimensional system and we compare our data with the predictions of a pre-existing model. Above a critical aggregation size, a deviation between the model and the experimental data is observed. This deviation is explained by the formation of ribbon-shaped aggregates. The ribbons are formed due to lateral aggregation of chains. An estimation of the magnetic energy for chains and ribbons shows that ribbons are stable structures when the number of magnetic grains is higher than N = 30. [less ▲]

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See detailRemote control of self-assembled microswimmers
Grosjean, Galien ULiege; Lagubeau, Guillaume ULiege; Darras, Alexis ULiege et al

in Scientific Reports (2015)

Floating magnetic particles can self-assemble into structures. These structures are periodically deformed in a non reciprocal way using magnetic fields, which leads to controllable low Reynolds number ... [more ▼]

Floating magnetic particles can self-assemble into structures. These structures are periodically deformed in a non reciprocal way using magnetic fields, which leads to controllable low Reynolds number locomotion. [less ▲]

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