The physical behavior of colloids suspended in a fluid close to a solid wall has been well characterized in the past[1]. Conversely the dynamical behavior of a particle close to a fluid interface is a relatively new field with strong implications in green and food processing [2]. Previous focus was limited to spherical particle’s viscous drag close to the interface [3] and wetting dynamics after interface breaching [4]. In this paper we present experimental results on the dynamics of micrometric colloids(spheres and ellipsoids)close to an interface between water based solution and air prior to the particle’s interfacebreaching. In order to study such dynamics we use a home-made pending-drop setup where particles dispersed in water based solutions (mixture with heavy water and/or with salt at different molarity) sediment towards the interface with air of the drop.The drop curvature is adjusted via the water solution pressure in order to obtain a flat interface at the particle scale. All the particle’s degreesof freedom (i.e. distance from the interface, position in a plane parallel to the interface and, for ellipsoids,orientation) are measured tracking the particle-interface interference patterns using incoherent ledswith three different wavelengths. The typical dynamics of a particle approaching the interface shows an initial sedimentation regimedriven by gravity and viscous drag. Closer to the interface a second regime arises which strongly depend on the solution molarity and density and particle morphology. For both particles morphologies the sedimentation dynamics stops at a distance from the interface of the order of hundreds nanometers. At small molarity (0.5 mM) both type of particles fluctuate around this position during the entire experimental observation time interval (10^3s). Increasing the molarity, ellipsoids more easily then spheres start to escape from this position approaching rapidly the interface till a distance of few nanometers. Such observations point out the importance of the electrostatic and morphology in the interaction with the interface. A calibration of thisinteraction will be presented by taking advantage of the buoyancy driving force changing the solution density. Building up on all these experimental observations, amodel considering the interplay between all the contributing interactions (i.e. gravity with buoyancy, electrostatics and Van der Walls) will be discussed and compared to the experimental results. [1]M. Hosoda, K. Sakai, and K. Takagi, Phys. Rev. E 58, 6275 (1998).[2] V. Sarrot, Z. Huang, D. Legendre, P. Guiraud, Chem. Eng. Sci. 62, 7359–7369 (2007).[3] G. M. Wang, R. Prabhakar and E. M. Sevick, Phys. Rev. Lett. 103, 248303 (2009).[4] A. Wang, W.B. Rogers, V.N. Manoharan, arXiv:1607.08167 [cond-mat.soft].

From the session Tomography and Computational Imaging (JSII_1) (invited) 4D Digital Holographic Microscopy (4D-DHM) is a new technique capable of performing 3D imaging of moving objects. 3D reconstruction is made from a single hologram, but is restricted to sparse objects that occupy only small part of the volume. For a sequence of holograms, 4D-DHM therefore performs 4D (x; y; z and t) imaging. 4D-DHM is well suited to image micro circulation, and has been validated imaging red blood cells circulating in a zebrafish embryo in vivo [1-3]. Fig. 1 (a) Holographic setup. (b, c and d) Images of blood flow within a zebrafish embryo: blood vessel (purple) and RBCs (green). The displayed 2D images correspond to the projections along z of the 3D images rotated along the y direction by-30° (b), 0° (c) and +30° (d). Bar is 50 m, The principle of the 4D DHM is illustrated by Fig. 1 (a). The setup is an upright microscope, which has been modified to perform Digital Holographic Microscopy (DHM) in transmission. The white light illumination is replaced by a laser, which is split in two beams of different directions. To perform holographic detection a reference arm (ELO field) is added on the detection side. The field E scattered by the object interferes with ELO , and a camera records sequences of frames. These frames are used to calculate the holograms, and to reconstruct in 3D the field scattered by the sample. By a proper analysis of this information, 3D images of the blood vessels, and 4D images of the moving red blood cells (RBCs) can be obtained see Fig,1 (b) to (d). To get these images, we have proceeded by the following way [2,3]. First, we calculate the holograms which differences of frames, and select by the way the signal from the moving components of the object (i.e.) the RBCs signal. We have then extract from the data, the holograms corresponding to the two illumination. For each RBC, and each illumination, the field that can be reconstructed by holography as a narrow cone of light (narrow because RCBs are much larger than the wavelength), which is angularly tilted like the illumination. The location of RBCs is then obtained by considering the intersection of the two cones. To prevent the signals of the different RBCs to mix together, the positions of the RBCs that give the largest signals are calculated first. The contributions of these RBCs are then removed from the hologram, and the positions of the following RBCs are calculated from the hologram thus cleaned. The reconstruction by cleaning is highly non-linear (unlike conventional holographic reconstructions), since it is the algorithm that decides to place a RBC in a plane and to remove the field diffused by this RBC from the other planes. In order to converge successively it requires to image sparses objects, that occupy only small part of the holographic reconstructed volume. 4D-DHM is thus capable of performing 3D reconstruction is made from a single hologram, but is restricted to sparse objects that occupy only small part of the volume.

Open channels are peculiar incidents modes of the optical field that are transmitted by a highly diffusing medium with high efficiency (100 % in the ideal case). The existence of these modes has been predicted by Dorokhov et al. [1] more than 25 years ago. Although many experiments were made to prepare these modes [2,3], the maxima of transmission obtained in experiments, are ever much lower than 100 %. To test the theory, we have considered here another Dorokhov's prediction: the average transmission of the modes that sustain the transmitted field must be TT=66%, where TT is defined by N2 TT= N1 T (where T is the averaged transmission of energy, and N1 and N2 the number of incident and transmitted modes i.e. the size of the incident and transmitted mode basis). Fig.1 (a) Holographic experimental setup. (b) Residual correlation of the transmitted field. The horizonal axis is SQ'. The sample transverse size is L=70 µm red, 50 µm green, 35 µm blue.... The horizontal grey line is the residual correlation expected for N1=N2. To verify this prediction, we have measured N2 on a slab sample of ZNO powder (transverse size L<=70 µm, thickness 20 µm) [4], by assuming that N1 is given by the sample geometry. The experimental setup is seen in Fig. 1(a). The incident fields are random (there are scrambled by a diffusing liquid DL) and digital holography is used to measure the complex transmitted field E(x,y) for the two polarisations. N2 is measured by analysing the residual correlations of the transmitted field measured for uncorrelated incident fields. Since the average transmission is T=1/25, we expect N2=N1/16. To get this optimal result, the transverse size of sample L must be infinite and the defocusing SQ' of the microscope objective MO2 must be zero (where S is the sample outgoing surface and Q' the plane imaged by MO2). Figure 1 (b) shows the residual correlation (which is proportional to 1/N2) as a function these two parameters. We get N2=N1/12 without extrapolation, and N2=N1/16 with extrapolation to L infinite. Here, the open channel effect is huge (factor 16) and the aggreement with the Dorokhov's prediction is excellent. We acknowledge Labex Numev (convention ANR-10-LABX-20) for funding. References 1. O.N. Dorokhov. On the coexistence of localized and extended electronic states in the metallic phase. Solid state communications, 51(6): 381-384, (1984).

A series of double hydrophilic block copolymers DHBCs has been synthesized by controlled radical polymerization (Figure 1). Poly(ethylene oxide)-b-poly(acrylic acid) PEO-b-PAA was prepared by ATRP in acetone. PEO-b-PAA and PEO-b-Poly(N-isopropyl acrylamide) PEO-b-PNIPAM were prepared by RAFT using a PEO-based dithiobenzoate macro-RAFT agent, in water and dioxane respectively. Poly(poly(ethylene glycol) methyl ether acrylate)-b-PAA PmPEGA-b-PAA was prepared by RAFT in water using a trithiocarbonate RAFT agent. Poly(acrylamide)-b-PAA PAM-b-PAA and PAM-b-poly(3-acrylamidopropyl)trimethylammonium chloride) PAM-b-PAPTAC were prepared by RAFT/MADIX using xanthate as control agent, in water/ethanol for the PAM block and directly in water for the second block. The block copolymers have been characterized by SEC, pH titration, conductimetry, and capillary electrophoresis (CE).The reversible formation of micelles by variation of temperature in the case of PEO-b-PNIPAM, or by the mixing of polyelectrolyte DHBCs with a polyelectrolyte of opposite charge in the right range of pH (e.g. anionic PEO-b-PAA associated with cationic oligochitosan) has been studied. The micelles have been characterized by various scattering techniques and CE.Such stimuli-responsive (T, pH, ionic strength) micellar systems have been involved as structuring agents in the synthesis of nanostructured mesoporous silica by sol-gel process. A relationship between the composition of the DHBCs, the composition of the micelles, the experimental conditions of micelle formation and the final mesostructure of the silica materials is foreseen. This strategy has been applied to the preparation of drug-loaded silica for biomedical applications.

Gliadins are edible wheat storage proteins well known for their surface active properties1, 2. Although their foaming3 and emulsifying4 capability is useful in food industry, the physical mechanisms of their adsorption at an interface are not well understood and the resulting structure of the protein interfacial films not yet well characterized. In this talk, we will present experimental results on the interfacial properties of acidic gliadin solutions studied over 4 decades of concentrations from 0.01 up to 110 mg/ml.Dynamic pendant drop tensiometry reveals that the surface pressure of gliadin solutions builds up in a multistep process. We show that the series of curves of the time evolution of alpha.time collected at different protein concentrations can be merged onto a single master curve when surface pressure is plotted as a function of alpha.time where t is the time elapsed since the formation of the air/water interface and alpha is a shift parameter that varies with the bulk protein concentration as a power law with an exponent 1.6. The existence of a master curve indicates that the surface pressure is the relevant parameter to characterize the interfaces covered with gliadin proteins.Optical and elastic properties of the interfaces are studied by dilatational rheology and ellipsometry and provide a consistent physical picture of the adsorption processes. Correlated evolutions of ellipsometry and rheological properties show a simple and progressive coverage of the interface by individual proteins at low surface pressure. This stage is followed, at higher surface pressure, by conformational rearrangements of the protein film, which are identified by a strong increase of the dissipative viscoelastic properties of the film concomitantly with an anomalous evolution of its optical profile that we have quantitatively rationalized. In the last stage, at even higher surface pressure, the optical profile is not modified while the elasticity of the film dramatically increases resulting from the gelation of the film due to the formation of intermolecular bonds. [1] J.Ornebro, T. Nylander, A-C Eliasson (2000). Journal of Cereal Science 31, 195-221[2] R. C. A Keller, R. Orsel, R. J. Hamer (1997). Journal of Cereal Science 25, 175-183[3] Martin A. Bos, Bertus Dunnewind, Ton van Vliet (2003). Colloids and Surfaces B 31, 95-105[4] Y.Popineau, F.Pineau, P. Evon, S. Bérot (1999). Nahrung 6, S. 361-367

Controlling the interface between TiO2 nanocrystals and ionic liquids is of high fundamental and applied interest for energy storage and conversion devices. Phase transfer of nanoparticles from a synthesis medium to a processing or an application medium plays a significant role in nanotechnology. Here we demonstrate that surface modification with phosphonic acids bearing cationic end-groups can trigger the phase transfer of TiO2 nanoparticles from an aqueous sol to a typical water-immiscible ionic liquid, [Emim][NTf2]. The transfer involves both the grafting of the phosphonic acid moiety and the exchange of the counter ion of the cationic end-group by NTf2 anions, as demonstrated by solid-state NMR, elemental analysis and independent grafting and ion exchange experiments. Furthermore, the colloidal stability of the TiO2 sols in [Emim][NTf2] strongly depends on the hydrophobic character of the cationic end-groups.

Biological or artificial microswimmers move performing trajectories of different kinds such as rectilinear, circular, or spiral ones. Here, we report on circular trajectories observed for active Janus colloids trapped at the air−water interface. Circular motion is due to asymmetric and nonuni form surface properties of the particles caused by fabrication. Motion persistence is enhanced by the partial wetted state of the Janus particles actively moving in two dimensions at the air−water interface. The slowing down of in plane and out of plane rotational diffusions is described and discussed.