In our work 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, appearing when the solvent displaces suspensions of colloids or polymers, we measure interfacial tensions in function of the volume fraction of the suspended objects, showing that the internal degrees of freedom of the particles drive the low volume fraction behavior (Figure 1). 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 up to maximum packing, 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 phenomenological model capturing the compositional dependence of the interfacial tension for large concentration gradients. Finally we include and analyze data reported in literature and obtained via spinning drop tensiometry that validate the model and we propose the analysis of fluid dynamic instability as a new tool to probe interfacial stresses.

Despite being by far among the most widespread interface on Earth, with a pervasive presence in environment, biology, industry and daily life, physical phenomena at Air-Water Interface (AWI) are still largely elusive. This fact is related to the complexity of AWI with its peculiar electrostatic and flow boundary conditions. In this paper, we present a study of the dynamics and interactions of charged microparticles diffusing at submicrometric distances from an AWI. An interferometric set-up mounted on a microscope gives access to an highly resolved trajectory of the colloid (5-10 nm). Contrary to other established techniques (like TIRM and holography) our technique has the great advantage of allowing the measure of the absolute particle-interface distance. In order to control the electrostatic we use different concentration of salt and different pH in association with pH dependent charged microparticles.Surprisingly new equilibrium distances from the AWI are measured not accounted by established theories. In addition different drags corresponding to different boundaries conditions are found for same combination of particle and AWI depending on the particle’s motion direction namely perpendicular or parallel to the AWI.

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 long-range capillary forces compete mostly with the isotropic electrostatic interactions. In nematic[2] or smectic[3] liquid crystal free films, other long-range anisotropic interactions related to bulk distortions of the surrounding fluid appear and new equilibrium positions or the possibility of forming new types of 2D colloidal assemblies appear via original elasto-capillarity couplings.In a previous work [4], using spatially resolved retardation and easy-axis maps, we have analysed quantitatively the 2D interfaces deformation and the nematic textures observed around isolated particles (see Fig.1) embedded in thin nematic films with hybrid anchoring conditions. Using numerical methods, we have explained quantitatively the different textures and the formation of topological defects. Contrary to what is classically observed in aligned nematic cells, the equilibrium patterns (such as the elastic dipole) due to a microsphere can be much larger than the particle size (as shown in Fig.1). Here we have focused on the overall colloidal interactions and the equilibrium positions between spherical or anisotropic particles. We have experimentally characterized the equilibrium configuration of isolated colloidal pairs. Using a 2D model of the nematic elasticity and finite element approaches, we have also numerically characterized the pair interaction and explained the textures observed. These new spontaneous organizations in thin nematic films offer new ways to self-assemble complex colloidal systems in 2D.References[1] P.A. Kralchevsky, K. Nagayama, Adv Colloid Interface Sci , 85, 145 (2000).[2] H.Jeridi, M.A.Gharbi, T.Othman, Ch.Blanc, Proc Natl Acad Sci USA, 112, 14771(2015).[3] M.A.Gharbi et al., Langmuir, 34, 2006 (2018).[4] H. Jeridi, M. Tasinkevych, T. Othman, and Ch. Blanc, Langmuir, 32, 9097 (2016).

Aqueous dispersions of graphene-derived monolayers form liquid crystal at rather low concentrations due to the high aspect ratio of these flakes [1,2]. If graphene oxide dispersions are easily obtained in water, reduced graphene oxide necessitates the use of surfactants [3]. Preparing organized materials and thin films from these dispersions then requires a good control of the liquid crystal ordering during the deposition and the drying of the films.The flakes easily align at high shear [4,5], which has allowed us to complete previous optical characterizations of the various graphene layers (birefringence and linear dichroism). However at low shear or under small dispalcements, thin films often show peculiar patterns (such as the periodic textures shown in Fig.1). We have shown how to create and stabilize large-sized periodic textures. The patterns have been characterized under optical and electronic microscopies. Their stability can be explained by the competition between the anchoring field of the substrate and the presence of a yield stress resulting from the peculiar elastic and rheological properties of the graphene oxide liquid crystals. Our results also clarify why long-standing hypotheses on the presence of exotic phases [1] at large concentrations are present in the literature.References[1] Z. Xu, C. Gao Nat Commun., 2, 571 (2011).[2] C. Zakri et al., Phil. Trans. A, 371, 20120499 (2013).[3] C. Zamora et al, J. Phys. Chem. Lett. 3, 2425 (2012).[4] P. Poulin et al, Proc Natl Acad Sci USA, 113, 11088 (2016).[5] S-H Honga, T.Z. Shena and J. K. Songa, Liquid Crystals, 42, 261 (2015)

Thin and ultrathin metal coatings are increasingly required by the electronics and optics industries onall types of substrates. However, the adhesion of noble metals is very poor on non-metallic substrates like glass,other oxide materials and polymers. By contrast, CVD graphene has a strong adhesion energy (~0.31-0.45 J.m-2depending on the number of layers) on glass [1]. On the other hand, graphene oxide (GO) was recently shown topromote adhesion of ZnO nanoparticles and subsequent ZnO films on glass or silicon substrates [2].In this work we use GO/Au nanoparticles (NP) composite thin films as bifunctional materials to bothpromote adhesion on glass and assist the deposition of copper in electroless wet industrial coating processes. Wediscuss the influence of the NP density, surface treatment and deposition parameters on the homogeneity andadhesion of the GO/Au NP and copper layers.[1] S. P. Koenig, N. G. Boddeti, M. L. Dunn, and J. S. Bunch, 2011, Nat. Nano. 6, 543‑546[2] Q. Ma, X. Zhu, D. Zhang, and S. Liu, 2014, J. Mater. Chem. C 22, 8956‑8961