The 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 capillary forces compete mostly with electrostatic interactions. In thin complex fluid films such as a liquid crystal, other long-range interactions related to bulk distortions of the surrounding fluid appear. We have recently shown that the presence of a nematic order drastically changed the long-range interactions between particles [2] via an original elasto-capillarity coupling. In this work, we focus on the variety of distortions produced by the colloidal inclusions. In nematic films much thicker than the particles' size, a particle gives rise to elastic multipoles formed by the particle and nearby topological defects [3]. In thin nematic films with hybrid anchoring conditions at free surfaces, capillary effects strongly modify the behaviour of the elastic dipoles. For a certain range of film thickness values, two new interesting patterns emerged around isolated particles: the giant dipole [2] and the " butterfly " texture (see Fig.1). In the giant dipole, a micron-sized sphere is accompanied by a point defect located at a distance up to several hundreds microns. The situation is quite different in the " butterfly " texture: the particle still produces an accompanying defect in its close neighbourhood, but a pi-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 the beads but with different boundary conditions for the polar 2D c-director. Using numerical simulations, we were able to reproduce the patterns found in the films. Although capillary interactions between inclusions in a thin film are always attractive [4], these new spontaneous organizations in thin nematic films offer new ways to self-assemble complex colloidal systems in 2D. Figure 1: The two different birefringence patterns formed by microparticles trapped in a thin nematic film, observed a) between crossed polarizers and b) with an Abrio birefringence measurement system.[1] P.A. Kralchevsky, K. Nagayama K, Adv Colloid Interface Sci , 2000, 85, 145.[2] H.Jeridi, M.A.Gharbi, T.Othman, C.Blanc, Proc Natl Acad Sci USA, 2015, 112,14771.[3] I. Musevic, M.Skarabot, U.Tkaler, M.Ravnik and S.Zumer, Science, 2006, 313,954.

Interfacial tension between immiscible phases is a well-known phenomenon, which manifests itself in everyday life, from the shape of droplets and foam bubbles to the capillary rise of sap in plants or the locomotion of insects on a water surface. More than a century ago, Korteweg generalized this notion by arguing that stresses at the interface between two miscible fluids act transiently as an effective, nonequilibrium interfacial tension, before homogenization is eventually reached. In spite of its relevance in fields as diverse as geosciences, polymer physics, multiphase flows, and fluid removal, experiments and theoretical works on the interfacial tension of miscible systems are still scarce, and mostly restricted to molecular fluids. This leaves crucial questions unanswered, concerning the very existence of the effective interfacial tension, its stabilizing or destabilizing character, and its dependence on the fluid's composition and concentration gradients. We present an extensive set of measurements on miscible complex fluids that demonstrate the existence and the stabilizing character of the effective interfacial tension, unveil new regimes beyond Korteweg's predictions, and quantify its dependence on the nature of the fluids and the composition gradient at the interface. We introduce a simple yet general model that rationalizes nonequilibrium interfacial stresses to arbitrary mixtures, beyond Korteweg's small gradient regime, and show that the model captures remarkably well both our new measurements and literature data on molecular and polymer fluids. Finally, we briefly discuss the relevance of our model to a variety of interface-driven problems, from phase separation to fracture, which are not adequately captured by current approaches based on the assumption of small gradients.

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.