Ref HAL: hal-01307989_v1
DOI: 10.1039/C5FD00117J
WoS: 000374440100019
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13 Citations
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Polymer nanocomposites are used widely, mainly for the industrial application of car tyres. The rheological behavior of such nanocomposites depends in a crucial way on thedispersion of the hard filler particles – typically silica nanoparticles embedded in a soft polymer matrix. It is thus important to assess the filler structure, which may be quitedifficult for aggregates of nanoparticles of high polydispersity, and with strong interactions at high loading. This has been achieved recently using a coupled TEM/SAXSstructural model describing the filler microstructure of simplified industrial nanocomposites with grafted or ungrafted silica of high structural disorder. Here, wepresent an original method capable of reducing inter-aggregate interactions by swelling of nanocomposites, diluting the filler to low-volume fractions. Note that this isimpossible to reach by solid mixing due to the large differences in viscoelasticity between the composite and the pure polymer. By combining matrix crosslinking,swelling in a good monomer solvent, and post-polymerization of these monomers, it isshown that it is possible to separate the filler into small aggregates. The latter have then been characterized by electron microscopy and small-angle X-ray scattering,confirming the conclusions of the above mentioned TEM-SAXS structural model applied directly to the highly loaded cases.

Ref HAL: hal-01304654_v1
DOI: 10.1615/AtomizSpr.2015013630
WoS: WOS:000375749100006
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3 Citations
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Agricultural spraying involves atomizing a liquid stream through a hydraulic nozzle, thus forming a liquid sheet that is subsequently destabilized into drops. Standard adjuvants such as dilute oil-in- water emulsions are known to influence the spray drop size distribution. Although being documented, the physical mechanisms at the origin of the size increase remain unclear. To elucidate the mechanisms causing the changes on the drop size distribution, we investigate the influence of dilute emulsions on the destabilization mechanisms of liquid sheets. Model laboratory experiments based on the collision of a liquid tear on a small solid target are used to produce and characterize liquid sheets. With dilute oil-in-water emulsions, the liquid sheet is destabilized during its expansion by the nucleation of holes that perforate the sheet and grow. The emulsion concentration and the size of the oil droplet of the emulsion are varied to rationalize their influence on the sheet destabilization mechanisms. The results obtained with the model laboratory experiments are compared to the measurement of the drop size distribution resulting from a conventional agricultural spray. The very good correlation between the number of perforation events and the volume fraction of small drops in the spray suggests (i) that the model experiment on liquid sheet is appropriate to investigate and gain an understanding of the physical mechanisms governing the spray drop size distribution and (ii) that the perforation destabilization mechanism of liquid sheets, which dominates for dilute emulsions, is at the origin of the increase of the size of the spray drops.

Commentaires: [Departement_IRSTEA]Ecotechnologies [TR1_IRSTEA]INSPIRE

Ref HAL: hal-01303522_v1
DOI: 10.3390/ijms17020223
WoS: WOS:000371830800032
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13 Citations
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Surface activity and micelle formation of alkylguanidinium chlorides, containing 10, 12, 14, and 16 carbon atoms in the hydrophobic tail have been studied by combining conductivity and surface tension measurements with isothermal titration calorimetry. The purity of the resulting surfactants, their temperatures of Cr → LC and LC → I transitions, as well as their propensity of forming birefringent phases were assessed based on the results of 1H and 13C NMR, differential scanning calorimetry (DSC), and polarizing microscopy studies. Whenever possible, the resulting values of Krafft temperature (TK), critical micelle concentration (CMC), minimum surface tension above the CMC, chloride counter-ion binding to the micelle, and the standard enthalpy of micelle formation per mole of surfactant (micH°), were compared to those characterizing alkyltrimethylammonium chlorides or bromides with the same tail lengths. The value of TK ranged between 292 and 314 K and increased strongly with increase in the chain length of the hydrophobic tail. Micellization was described as both entropy and enthalpy-driven. Based on the direct calorimetry measurements, the general trends in the CMC with the temperature, hydrophobic tail length and NaCl addition were found to be similar to those of other types of cationic surfactants. The particularly exothermic character of micellization was ascribed to the hydrogen-binding capacity of the guanidinium head-group.

Ref HAL: hal-01303466_v1
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Material failure is ubiquitous on length scales ranging from a few nanometers, as in fracture of atomic or molecular systems, up to geological scales, as in earthquakes. The detection of any precursors that may point to incipient failure is the Holy Grail in many disciplines, from material science to engineering and geology. Material failure is particularly relevant to soft matter systems (colloids, emulsions, polymers etc.), which are an ideal benchmark to investigate how mechanical stress impacts condensed matter. Indeed, soft matter is very susceptible to even modest mechanical loads and most soft materials can be conveniently investigated by powerful optical methods such as microscopy or light scattering.We have built up a shear-cell that allows us to couple rheology to light scattering measurements on soft solids. In this talk I will focus on a fractal colloidal gel produced by in-situ aggregation of silica nanoparticles in water and show how reversible and irreversible (microscopic and macroscopic) deformations, build up when the gel is submitted to a shear stress. Thanks to our set-up, we are able to elucidate the physical processes at play at the onset of yielding.

Ref HAL: hal-01303444_v1
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Designing stable liquid crystal (LC) composites is one of the main tasks pursued by several members of theIC1208 Cost action. Liquid crystal composites are however colloidal dispersions which have very specificproperties compared to the usual colloidal dispersions in simple liquids. LC matrices give indeed rise to longrangeattractive multipolar interactions between colloidal dispersions [1]. Strong dipolar interactions are mainlyobserved at large scale, but even at the nanometer scale the interactions between two nanoparticles arequadrupolar, weaker but often sufficient to yield aggregates in many systems [1]. Aggregation phenomena undersuch multipolar interactions are still not fully understood, so to get a deeper understanding we have considered a2D model system describing the classical dynamics of elongated particles at a liquid interface.
When elongated particles are trapped at a liquid interface they distort it and then interact via quadrupolarcapillary interactions. Furthermore the interactions between a group of already aggregated particles and a singleone located at a large distance strongly depend on the spatial arrangement of the aggregated particles. Thisphenomenon is further complicated by the presence of possible direct solid-solid interactions, which would arisewhen the particles enter into contact and possibly freeze the aggregate shapes. Currently we are focusing onmodeling the above described aggregation phenomena by developing a numerical code that is sufficientlyaccurate in order to catch the many-body interactions mentioned above in a simple way. Once this is achievedextensive comparison with experiments will be possible and the model system may be used to address openquestions arising from the experiments. Our model is based on the 2D solution of the Young-Laplace equation togain the forces acting on the particles, then moving each particle individually by solving the Newton equationsbased on classical discreet element methods. In this presentation I would like to give an insight into the currentstate of the development of the numerical model and our future objectives.
[1] Blanc, C., Coursault, D., Lacaze, E., Ordering nano-and microparticles assemblies with liquid crystals, Liquid Crystals Reviews, 1(2),pp. 83-109, (2013).12

Ref HAL: hal-01303434_v1
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Liquid crystal (LC) colloidal dispersions have been shown to promote complex ordered organizations due to the local distortion of the host by the particles and the resulting long-range elastic interactions mediated by the Frank elasticity. The key components responsible for these colloidal patterns have been well understood at the micron scale and are now increasingly studied at the sub-micron scale. One of the main challenges in order to design useful colloidal dispersions in a nematic however remains the fine control of the multipolar elastic interactions. The attractive components are indeed usually sufficiently strong to lead to a rapid aggregation of dispersed particles. We will first discuss some strategies to prevent this phenomenon in bulk.
We will also discuss the case of particles trapped at liquid crystal interfaces where the elastic interactions are strongly modified by the confinement [1,2] but also might compete with capillary interactions, as we detailed recently [3]. When confined at LC interfaces, spherical particles with homeotropic anchoring interact with repulsive interactions only, then forming regular arrays with large lattice parameters. When they are confined in a thin nematic film the colloidal behaviour is much more complex depending on the boundary conditions at the interfaces. For example we have shown that the capillary attraction expected for particles confined in thin films with hybrid anchorings is counteracted by the nematic texture and the presence of topological defects.
[1] M.A. Gharbi, M. Nobili, M. In, G. Prévot, P. Galatola, J.-B. Fournier, and Ch. Blanc, "Behavior of colloidal particles at an air/nematic liquid crystal interface", Soft Matter, 7, 1467-1471, (2011).
[2] M. A. Gharbi, M. Nobili, Ch. Blanc, "Use of topological defects as templates to direct assembly of colloidal particles at nematic interfaces",J. Colloid. Interface Science, 417 ,250 (2014).
[3] Jeridi Haifa, Gharbi M. A., Othman Tahar, and Blanc C., "Capillary-induced giant elastic dipoles in thin nematic films", PNAS, 112 , 14771–14776 (2015).

Résumé:

Liquid sheets that freely expand in air can be produced by the impact of a small drop of liquid on a cylindrical target of size comparable to that of the drop. In this talk I will illustrate with two examples how the imaging with a fast camera of the dynamics of the sheets thus formed can shed light on fundamental and industrial issues. On the one hand, we quantify the interplay between elasticity and viscosity and the effects of a strong and fast deformation on viscoelastic droplets. On the other hand, we rationalize the role of dilute emulsions in the destabilization mechanism of liquid sheets and its consequence for anti-drift phenomena in agricultural sprays.