To investigate the interplay between microscopic dynamics and macroscopic rheology insoft matter, we couple a stress-controlled-rheometer equipped with a Couette cell to a lightscattering setup in the imaging geometry, which allows us to measure both the deformationfield and the microscopic dynamics. To validate our setup, we test two model systems. For anelastic solid sample, we recover the expected deformation field within 1 μm. For a pure viscousfluid seeded with tracer particles, we measure the velocity profile and the dynamics of thetracers, both during shear and at rest. The velocity profile is acquired over a gap of 5 mm witha temporal and spatial resolution of 1 s and 100 μm, respectively. At rest, the tracer dynamicshave the expected diffusive behavior. Under shear, the microscopic dynamics corrected for theaverage drift due to solid rotation scale with the local shear rate, demonstrating that our setupcaptures correctly the relative motion of the tracers due to the affine deformation.

Janus colloidal particles show remarkable properties in terms of surface activity, self-assembly and wetting. Moreover they can perform autonomous motion if they can chemically react with the liquid in which they are immersed. In order to understand the self-propelled motion of catalytic Janus colloids at the air-water interface, wetting and the orientation of the catalytic surface are important properties to be investigated. Wetting plays a central role in active motion since it determines the contact between fuel and catalytic surface as well as the efficiency of transduction of chemical reaction into motion. Active motion is not expected to occur either when the catalytic face is completely out of the aqueous phase or when the Janus boundaries are parallel to the interfacial plane. The design of a Janus colloid possessing two hydrophilic faces is required to allow the catalytic face to react with the fuel (e.g. H2O2 for Platinum) in water and to permit some rotational freedom of the Janus colloid in order to generate propulsion parallel to the interfacial plane.Here, we discuss some theoretical aspects that should be accounted when studying Janus colloids at the surface of water. The free energy of ideal Janus colloidal particles at the interface is modeled as a function of the immersion depth and the particle orientation. Analytical expressions of the energy profiles are established. Energetic aspects are then discussed in relation to the particle ability to rotate at the interface. By introducing contact angle hysteresis we describe how the effects of contact line pinning modifies the scenario described in the ideal case. Experimental observations of the contact angle hysteresis of Janus colloids at the interface reveal the effect of pinning; and orientations of silica particles half covered with a platinum layer at the interface do not comply with the ideal scenarios. Experimental observations suggest that Janus colloids at the fluid interface behave as kinetically driven system, where the contact line motion over defects decorating the Janus faces rules the orientation and rotational diffusion of the particle.

The interface between two fluids is never flat at the nanoscale, and this is important for transport across interfaces. In absence of any external field, the surface roughness is due to thermally excited capillary waves possessing subnanometric amplitudes in the case of simple liquids. Here, we investigate the effect of ultrasound on the surface roughness of liquid-gas and liquid-liquid interfaces. MHz frequency ultrasound was applied normal to the interface at relatively low ultrasonic pressures (< 0.6 MPa), and the amplitudes of surface fluctuations have been measured by light reflectivity and ellipsometry. We found a dramatic enhancement of surface roughness, roughly linear with intensity, with vertical displacements of the interface as high as 50-100 nm. As a consequence, the effective contact area between two fluids can be increased by ultrasound. This result has a clear impact for enhancing interface based processes such as mass or heat transfer.

Microvessel blood flow imaging techniques are widely used in biomedical research and clinical diagnostics where many diseases have a vascular etiology or involvement. For testing purposes, zebrafish embryo provides an ideal animal model to achieve high-resolution imaging of superficial and deeply localized vessels. Moreover, the study of the formation of a closed circulatory system in vertebrates is a topic of recent interest in biophysics. However, most of the existing techniques are invasive due to the use of a contrast agent for imaging purposes. Recent developments in Digital Holography and Laser Doppler Holography techniques can be considered to alleviate this issue. Laser Doppler holography and transmission microscopy can be coupled to analyze blood flow in fish embryos by adapting a laser Doppler holographic setup to a standard bio-microscope: the two beams of the holographic interferometer (illumination of the object and reference), whose frequency offset is controlled, were addressed to the microscope by optical fibers. Multimodal acquisition and analysis of the data is made by acting on the frequency offset of the two beams, and on the location of the Fourier space filtered zone. In this work, we show that it is possible to select the signal of moving scatterers, and to image Red Blood Cells (RBCs) and blood vessels. Individual RBCs are imaged, and movies showing the RBC motion are obtained. Microsc. Res. Tech., 2016. © 2016 Wiley Periodicals, Inc.

La diffusion hyper-Raman (DHR) est une spectroscopie optique des vibrations qui présente des règles de sélection différentes du celles du Raman et de l'infrarouge. Après avoir décrit les principes de la DHR, nous ferons un état de l'art de la dynamique vibrationnelle (IR, Raman, neutrons) du système relaxeur modèle Pb1/3Mg2/3NbO3 (PMN). Nous insisterons sur le comportement des modes mous polaires dont l'observation est uniquement possible par diffusion hyper-Raman. Les résultats ont été capturés dans un modèle qui offre une vision cohérente des propriétés dynamiques de toute une famille de matériaux. Ils relient par exemples les vibrations à la structure polaire locale, et donnent une origine microscopique aux propriétés diélectriques géantes de ces systèmes.

Une introduction à la théorie de la réponse linéaire nous permettra dans un premier temps de définir les notions de susceptibilité, de spectre des fluctuations, de fonction d’auto-corrélation,… et de relier ces grandeurs à travers le théorème de fluctuation-dissipation. Nous déterminerons ensuite l’origine de la diffusion de la lumière et calculerons l’intensité Raman et Brillouin diffusée dans le cadre d’une théorie macroscopique (classique). Ces calculs feront apparaitre des règles de sélection qui seront mises en pratique à travers quelques exemples simples pris parmi les solides cristallins et les liquides.

The fluctuations of electric polarization in a disordered ferroelectric substance, relaxor crystal PbMg_{1/3}Nb_{2/3}O_{3} (PMN), were studied using a nonlinear inelastic light-scattering technique, hyper-Raman scattering, within a 5-100  cm^{-1} spectral interval and in a broad temperature range from 20 to 900 K. The split ferroelectric mode reveals a local anisotropy of up to about 400 K. Spectral anomalies observed at higher temperatures are explained as due to avoided crossing of the single primary polar soft mode with a temperature-independent, nonpolar spectral feature near 45  cm^{-1}, known from Raman scattering. The temperature changes of the vibrational modes involved in the measured fluctuation spectra of PMN were captured in a simple model that accounts for the temperature dependence of the dielectric permittivity as well. The observed slowing down of the relaxational dynamics directly correlates with the huge increase of the dielectric permittivity.