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.

A very active branch of materials research based on allotropic carbon nanostructures has recentlyflourished, mainly due to their large potential of applications. Among the most outstanding structures aregraphene and carbon nanotubes which both possess interesting properties at the nanoscale, such as:electronic, mechanical, optical etc... When combined with various nanoparticles (NPs), their potential iseven larger. Exploiting these properties, which are mostly anisotropic, requires to transfer them to themacroscopic world through the preparation of ordered nanostructured composites materials. During theirprocessing, liquid crystal phases present an opportunity to arrange them into macroscopic assemblies ofNPs and nanocarbons with long-range ordering [1]. Nevertheless, achieving high orientational orderparameter and large monodomains remains a challenge. In this work we present some of our approaches toachieve large and well-ordered domains oflyotropic liquid crystals. For example the orderparameter of carbon-nanotubes based materialscan be finely tuned by controlling the length andentanglement of the nanotubes [2] and processesbased on shear are sufficient to easily achieve amacroscopic ordering. For graphene, it is knownthat graphene oxide (GO) flakes easily disperse inwater and spontaneously form liquid crystals athigh concentrations. However, most of their electronic functionalities are lost during the oxidationtreatments. Reduced graphene oxide (RGO) is of greater interest but chemical reduction of GO in watergenerally results in the aggregation of the flakes. We recently showed how to obtain water-based RGOliquid crystals stabilized by surfactant molecules [3]. Structural and thermodynamic characterizationsprovide indirect but statistical information on the dimensions of the graphene flakes. Combined with NPs,these graphene based liquid crystals are also useful to design novel coatings and functional materials.[1] Yuan J. et al. Nat. Commun. 6:8700 doi: 10.1038/ncomms9700 (2015).[2] Zakri, C. et al. Phil. Trans. R. Soc. A., 371, 20120499, pp 1-15 (2013)[3] Zamora-Ledezma, C. et al. J. Phys. Chem. Lett., 3 (17), pp 2425–2430 (2012)

Dynamics of anisotropic particles at a fluid interface governs trapping and particles subsequentassembly. Measuring and controlling this viscous dynamics open the way to new materials [1], andapplications [2]. Despite these domains of applications, measurements of the viscous coefficient ofanisotropic particles were mainly restricted to the bulk [3-4] or in quasi-2D confinement between twoglass plates [5]. Here we measure the complete set of viscous coefficients of single micrometricprolate ellipsoids near and trapped at an air-water interface. These coefficients are obtained from theBrownian motion of the particle coupling interferometry and particle tracking. For particles trapped atthe interface we measured anomalously large drags even larger than in the bulk water (ratio of surface/bulk rotational drags rR > 1 in Figure) [6]. Such an intriguing behaviour is discussed in terms ofadditional random forces due to thermally activated fluctuations of the interface and their coupling tothe particle drag through the fluctuation–dissipation theorem.Figure: Rotational viscous-drag ratio rR (surface/bulk) of polystyrene ellipsoidal particles at anair/water interface versus particle’s aspect ratio φ. Inset: mean square angular displacements (MSADs)versus the time lag τ.[1] M. Cavallaro, L. Botto, E. Lewandowski, M. Wang, and K. Stebe, PNAS 108, 20923 (2011).[2] Z. Huang, D. Legendre, and P. Guiraud, Chem. Eng. Sci. 66, 982 (2011).[3] D. Mukhija and M. Solomon, J. Colloid Interface Sci. 314, 98 (2007).[4] T. H. Besseling, M. Hermes, A. Kuijk et al., Journal of Physics: Condensed Matter 27, 194109 (2015).[5] Y. Han, A. Alsayed, M. Nobili, J. Zhang, T. Lubensky, and A. Yodh, Science 314, 626 (2006).[6] G. Boniello, C. Blanc, D. Fedorenko, M. Medfai, N. Ben Mbarek, M. In, M. Gross, A. Stocco, M. Nobili,Nature Materials 14, 908 (2015).

by solid substrates, the particle creates a hyperbolic hedgehogdefect, usually located near the bead (at a distance of order of its size) and the pair particle-defectforms a neutral unit, stable in time. Here we show theoretically and experimentally how capillaryeffects strongly modify the behaviour of particles trapped in a thin nematic film with hybrid anchoringconditions at free surfaces. For a certain range of thickness values (films thinner than the particles’size) two new interesting patterns are formed by isolated particles: the giant dipole [3] and the“butterfly” texture (see Fig.1). In the giant dipole, a micron-sized sphere is accompanied by a pointdefect which is located at a distance up to several hundreds microns. The situation is quite different inthe “butterfly” texture: the particle still produces an accompanying defect in its close neighbourhood,but a p-wall is formed on the opposite side. Using spatially resolved retardation and easy-axis maps,we analysed quantitatively and separately the 2D interfaces deformation and the nematic textures.Both behaviours are due to the same axisymmetric capillary deformation of the thin film around thebeads but with different boundary conditions for the polar 2D c-director. Using a simple 2D Ansatz,we were able to reproduce the c-director patterns found in the films. The local 3D textures have beenalso investigated in the framework of the Landau-de Gennes theory. Although capillary interactionsbetween inclusions in a thin film are always attractive [4], these new spontaneous organizations in thinnematic films offer new ways to self-assemble complex colloidal systems in 2D.Figure1: The two different birefringence patterns formed by microparticles trapped in a thin nematicfilm, observed a) between crossed polarizers and b) with an Abrio birefringence measurement system.[1] I. Musevic, M.Skarabot, U.Tkaler, M.Ravnik and S.Zumer, Science, 313,954 (2006).[2] P.Poulin, H.Stark, T.Lubensky and D.Weitz, Science, 275,1770 (1997).[3] H.Jeridi, M.A.Gharbi, T.Othman, C.Blanc, Proc Natl Acad Sci USA, 112,14771 (2015).[4] P.A. Kralchevsky, K. Nagayama K, Adv Colloid Interface Sci , 85, 145–192 (2000).