Ref HAL: hal-02339367_v1
Exporter : BibTex | endNote
Résumé:

Les techniques de microrhéologie basées sur le suivi optique de particules microniques sont très intéressantes lorsqu’il s’agit de mesurer les propriétés viscoélastiques de fluides complexes de petit volume. Elles peuvent également paraître très intéressantes pour sonder localement des milieux hétérogènes. Dans le cas de milieux anisotropes comme les cristaux liquides nématiques, ces techniques doivent cependant être utilisées avec précaution. Nous l’illustrerons à travers le cas d’un cristal liquide lyotrope, le cromoglycate de sodium (DSCG) pour lequel nous avons examiné la pertinence des données rhéologiques extraites à partir de la dynamique de microparticules.Le mouvement Brownien, inhabituel, de microsphères dans la phase nématique de ce système avait en effet attiré récemment l’attention [1,2] suggérant un couplage complexe entre les fluctuations du directeur et les fluctuations de positions des billes. Bien qu’il ait été récemment proposé que des mesures par suivi de particules puissent être utilisées pour obtenir les différentes viscosités de la phase nématique [3], ce phénomène peut en effet conduire à des mesures rhéologiques incohérentes. Afin d’explorer ces différents problèmes, nous avons étudié le mouvement Brownien de particules dans des échantillons alignés en cellules minces (épaisseur 50µm) tout en variant à la fois la concentration de DSCG et la taille des particules (0.5, 1, 3, and 6µm de diamètre).Des propriétés viscoélastiques effectives ont été extraites de nos mesures de microrhéologie passive ainsi que de mesures actives basées sur l’utilisation de pinces optiques et le mouvement forcé des particules[4]. Nos résultats expliquent pourquoi différents comportements viscoélastiques ont été obtenus précédemment sur ce même système et l’origine des modules élastiques mesurés.

Ref HAL: hal-02321405_v1
DOI: 10.1364/AO.58.00G127
WoS: 000502062900019
Exporter : BibTex | endNote
Résumé:

In previous work [Opt. Lett. 44, 2827 (2019)], we presented a method based on digital holography and orthogonal matching pursuit, which is able to determine the 3D positions of small objects moving within a larger motionless object. Indeed, if the scattering density is sparse in direct 3D space, compressive sensing algorithms can be used. The method was validated by imaging red blood cell trajectories in the trunk vascular system of a zebrafish (Danio rerio) larva. We give here further details on the reconstruction technique and present a more robust version of the algorithm based on multiple illuminations.

Ref HAL: hal-02318177_v1
DOI: 10.1016/j.colsurfa.2019.04.081
WoS: 000471655500022
Exporter : BibTex | endNote
Résumé:

Soft adhesion to a horizontal flat substrate of micron-sized colloids coated by acontrolled molar fraction f of PLL-g-PNIPAM is tuned and triggered by controllingthe temperature of the sample. This mechanism is possible becausePNIPAM is a thermo-responsive polymer, which undergoes a phase transitionat a LCST (lower critical solution temperature) Tc = 32 +- 1°C. In order tocapture the very final events before the immobilization of colloids the T-rampprotocol is designed: the particles suspension is injected in cell at room temperature,the temperature is increased at a constant rate up to 38°C> Tc, andkept constant until the end of the acquisition. Attraction between beads andthe flat substrate is thereby triggered when crossing the critical temperatureTc. Ascending and descending ramp experiments are performed in order to accessboth adsorption and desorption kinetics. 3-D motion of beads is real-timetracked using slightly defocused microscopy in parallel illumination. We usetrack records to have access to pre-adsorption dffusion coeffcients and to characterizeadsorption and desorption dynamics. Present results corroborate thatadsorption is controlled by PNIPAM doping, and indeed dominated by rollingand memory (aging) effect on the contact domain. On the contrary, the desorption kinetics is independent of doping. Moreover, the sharpness (in T) ofthe PNIPAM transition is quantified in a set of experiments at different rampspeeds. We show that the transition smearing can reach up to +-1°C for higherPNIPAM coverage, while it is not detectable for the lowest ones. The combinationof all these observations paves the way to practical applications. Asan example, we will discuss a soft adhesion-based method to sort and separatecolloids in microfluidic channels.

Ref HAL: hal-02316058_v1
DOI: 10.1063/1.5116026
WoS: 000503995300015
Exporter : BibTex | endNote
2 Citations
Résumé:

The temperature-dependence of the piezoelectric properties of trigonal -GeO2 single-crystals obtained by the high-temperature flux method was measured by the resonance technique of the electrical impedance in the 20°C-600°C range. To approach the values of the two independent piezoelectric coefficients d11 and d14, we first measured as a function of temperature the elastic coefficients S11, S14 and S66 and the dielectric permittivity 11 which are involved in the coupling coefficient k of both the thickness shear mode and the transverse mode. A Y-cut plate with a simple +45°-rotation ((YXtwl) +45°/0°/0°) was used to measure the coupling coefficient of the thickness shear mode, and two X-turned plates ((XYtwl) +45°/0°/0° and (XYtwl)-45°/0°/0°) were prepared to characterize the coupling coefficient of two transverse modes. From the whole experimental measurements, the piezoelectric coefficients of -GeO2 were calculated up to 600 °C. They show that this crystal is one of the most efficient in regard of the -quartz-like family at room temperature, and that its thermal comportment retains large piezoelectric properties up to 600°C.

Ref HAL: hal-02311614_v1
Ref Arxiv: 1810.12199
DOI: 10.1007/s10404-019-2195-0
WoS: 000457868900001
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
3 Citations
Résumé:

Secondary flows are ubiquitous in channel flows, where small velocity components perpendicular to the main velocity appear due to the complexity of the channel geometry and/or that of the flow itself such as from inertial or non-Newtonian effects, etc. We investigate here the inertialess secondary flow of viscoelastic fluids in curved microchan-nels of rectangular cross-section and constant but alternating curvature: the so-called "serpentine channel" geometry. Numerical calculations (Poole et al, 2013) have shown that in this geometry, in the absence of elastic instabilities, a steady secondary flow develops that takes the shape of two counter-rotating vortices in the plane of the channel cross-section. We present the first experimental visualization evidence and characterization of these steady secondary flows, using a complementarity of µPIV in the plane of the channel, and L. Ducloué · L. Casanellas · A. Lindner confocal visualisation of dye-stream transport in the cross-sectional plane. We show that the measured streamlines and the relative velocity magnitude of the secondary flows are in qualitative agreement with the numerical results. In addition to our techniques being broadly applicable to the character-isation of three-dimensional flow structures in microchan-nels, our results are important for understanding the onset of instability in serpentine viscoelastic flows.

Ref HAL: hal-02301216_v1
Exporter : BibTex | endNote
Résumé:

Heliconal nematic twist bend (NTB) phases are usually not well oriented in planar commercial cells, even in thin (~1 µm) ones where a micron-sized periodic texture is usually observed [1,2,3] (see Fig.1-a). The origin of the stripe patterns is attributed to the rapid shrinking of the pseudo-layers below the N-NTB phase transition temperature, which yields an undulation instability. The observed periodicity is found to be twice the cell gap [1].We have explored the effect of the strength of the anchorings of the substrates on the resulting patterns. We used polymer aligning layers of tunable azimuthal and zenithal strengths to design liquid crystal cells with surface anchorings ranging from ultraweak to strong. We then have studied the patterns formed by the NTB phase of 1,7-bis-4-(4-cyanobiphenyl) heptane (CB7CB). We will show how the period of the stripes can be varied in a large range in cells of the same thickness but with different anchoring preparations. With adequate anchoring conditions (see Fig.1-b-c), an excellent and spontaneous planar alignment of the NTB phase of CB7CB can be obtained much below the phase transition temperature.
References:
[1] Panov V.P. et al, Phys. Rev. Lett., 2010, 105; 167801.
[2] Challa P. K. et al, Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2014, 89, 060501.
[3] Vaupotič N. et al., Chem. Phys. Chem., 2018, 19,2566.

Ref HAL: hal-02301192_v1
Exporter : BibTex | endNote
Résumé:

Confinement of colloidal particles in thin liquid films is typically accompanied with interfacial deformations which give rise to long-range capillary forces, attractive in most of the cases [1]. In simple fluids, these long-range capillary forces compete mostly with the isotropic electrostatic interactions. In nematic[2] or smectic[3] liquid crystal free films, other long-range anisotropic interactions related to bulk distortions of the surrounding fluid appear and new equilibrium positions or the possibility of forming new types of 2D colloidal assemblies appear via original elasto-capillarity couplings.
In a previous work [4], using spatially resolved retardation and easy-axis maps, we have analysed quantitatively the 2D interfaces deformation and the nematic textures observed around isolated particles (see Fig.1) embedded in thin nematic films with hybrid anchoring conditions. Using numerical methods, we have explained quantitatively the different textures and the formation of topological defects. Contrary to what is classically observed in aligned nematic cells, the equilibrium patterns (such as the elastic dipole) due to a microsphere can be much larger than the particle size (as shown in Fig.1).
Here we have focused on the overall colloidal interactions and the equilibrium positions between spherical or anisotropic particles. We have experimentally characterized the equilibrium configuration of isolated colloidal pairs. Using a 2D model of the nematic elasticity and finite element approaches, we have also numerically characterized the pair interaction and explained the textures observed. These new spontaneous organizations in thin nematic films offer new ways to self-assemble complex colloidal systems in 2D.
References:
[1] P.A. Kralchevsky, K. Nagayama, Adv Colloid Interface Sci ,2000, 85, 145.
[2] H.Jeridi, M.A.Gharbi, T.Othman, Ch.Blanc, Proc Natl Acad Sci USA, 2015, 112, 14771.
[3] M.A.Gharbi et al., Langmuir, 2018, 34, 2006.
[4] H. Jeridi, M. Tasinkevych, T. Othman, and Ch. Blanc, Langmuir, 2016, 32, 9097.