The nature of the interface between two fluids has been the subject of an extensive investigation for more than two centuries [1]. From a mechanical point of view, surface tension can be calculated as an integral of unbalanced tangential stresses over the interface thickness [2] and it is thus of no surprise that, within the assumption of temporal and local equilibrium, such a tension can be ascribed to an interface between two miscible fluids [3] and assumes a nonzero value. The idea that there are capillary forces at work in the layer between miscible liquids goes back at least to an 1871 report of J. Bosscha, cited in a paper of Korteweg [1901], who was the first [4] to introduce stresses due to concentration inhomogeneities in the Navier-Stokes equation, aiming to mimic capillarity effects between two miscible liquids. Only much more recently experimental effort has been made to investigate the existence and the nature (the sign) of such off-equilibrium tension [5-11]. In all cases the authors have pointed out that such interfacial stresses mimic a positive interfacial tension. In analogy with the surface tension of an equilibrium interface between immiscible fluids, the effective surface tension has been rationalized within an approximation of small concentration gradients via the so called square gradient model [6,8,9]. The latter predicts a quadratic dependence of such tension on the compositional mismatch between two miscible fluids.Only very recently colloids and polymer suspensions in contact with a reservoir of solvent have been considered as a model two-fluid system to study off-equilibrium surface tensions [11, 12]: indeed, diffusion is much slower in colloids and polymers as compared to atomic systems, leaving a wider temporal window for probing the transient interface between miscible fluids. In a recent work [11] we have investigated off-equilibrium surface tension between microgel suspensions and their solvent (water) showing that equation (1) holds for low concentration gradients. On the other hand we have also pointed out that the square gradient model is far to catch the volume fraction dependence of the tension for compact hard sphere-like particles [12] up to concentrations close to the maximum packing for spheres. The scenario is then currently controversial.By visualizing viscous fingering instability (VF), appearing when a solvent displaces a suspension in radial Hele-Shaw geometry, we measure interfacial tensions in function of the volume fraction of the suspended colloids or polymers, showing that the internal degrees of freedom of the particles drive the low volume fraction behavior. Our results support the existence of a positive tension between miscible fluids, confirm the quadratic scaling predicted by Korteweg [4] for long linear and crosslinked polymers and show a positive rapidly growing tension for hard sphere suspensions, whose description necessitates a theoretical framework going beyond the classic square gradient model. We rationalize our findings assuming the suspension/solvent interface in local thermodynamic equilibrium, computing explicitly the square gradient contribution to the interfacial tension for polymer/solvent and simple molecular liquid mixtures and proposing a new phenomenological model capturing the compositional dependence of the interfacial tension for large concentration gradients. In addition to that we include and analyze data reported in literature and obtained via spinning drop tensiometry, validating our approach. We conclude proposing very generally the analysis of hydrodynamic instabilities as a new tool to measure interfacial stresses in complex fluids and characterize transient interfacial tensions.

We report the measurement of non-equilibrium interfacial tension of polymer and hard sphere suspensions in contact with their own solvent. By visualizing fingering instability (VF) in radial Hele-Shaw geometry, we measure interfacial tension in function of the volume fraction of the suspensions showing that the internal degrees of freedom of the particles drive the magnitude of such tension. We rationalize our findings proposing a phenomenological model capturing the compositional dependence of the non equilibrium interfacial tension for the whole range of concentration gradients and for all the systems at hand. More generally we propose the analysis of fluid dynamic instabilities as a new tool to measure interfacial tensions between complex fluids.

An holographic microscopy setup using two illumination beams is proposed to image blood microcirculation in preclinical models. Greedy algorithm reconstruction of the two resulting holograms leads to a 3D reconstruction of the sample over time.

A holographic microscopy algorithm is proposed enabling to deal with high numerical aperture holograms. Hologram reconstruction kernel performs under afocal conditions , which allows keeping the full NA over wide defocus range.

Ultrasound-modulated optical tomography (UOT) is a technique that combines light and ultrasound able to image deep inside scattering media. A pulsed version of technique involving holography is proposed and discussed. Ultrasound-modulated optical tomography (UOT) [1] also called acousto-optic imaging [2] is a technique that combines light and ultrasound able to image deep inside scattering media. The ultimate goal of the technique is breast cancer imaging. Heterodyne holography combined with UOT is a powerful tool to detect the UOT tagged photons. Previous experiments are made with phantom samples. Since the sample does not move, the phase of the field remains correlated. Holographic detection is then efficient and shot noise sensitivity is reached [3]. To perform imaging with breast, a new setup with light and ultrasound pulses is proposed. Numerical simulations, that fits with the results of previous work, are made to extrapolate the phantom results to breast imaging with the new setup. They show that breast tumor imaging is possible. Fig. 1. Typical UOT setup (a) and pulsed modified setup to detect signal whose phase correlation is short (b). BS1, BS2: beam splitter; M: mirror; AOM1,AOM2: acousto optic modulator; PZT: piezoelectric transducer that generates the ultrasonic beam US; a: absorber imbedded in the diffusing sample; A: rectangular aperture; L: lens of focal d; C: camera; E LO , E T , E U : LO, tagged and untagged fields. Figure 1 (a) shows the heterodyne holography UOT setup of the phantom experiment [3]. The main laser beam L (frequency ω L) is split by the beam splitter BS1 into a local oscillator (LO), and a signal beam that is scattered by the diffusing sample. The sample is explored by an ultrasonic (US) beam of frequency ω US generated by a piezoelectric

Open channels are peculiar incidents modes that are transmitted by a diffusing medium with high efficiency (up to 100%). Digital holography is used to analyze the correlations of the transmitted field, and to test open channel theory. Quantitative agreement is obtained.