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.

We report measurements and theoretical predictions on the effect of an aligning magnetic field on the orientational disorder of a nematic liquid crystal in contact with isotropic solid substrates. Different types of substrates present a similar disorder and a similar dependency on the magnetic field amplitude, i.e. a decreasing of the angular distribution widths and spatial correlation lengths. Measurements are in qualitative agreement with a theory where the orientational disorder emerges from the competition between the aligning magnetic torque and the disorienting thermal fluctuations during the adsorption of the liquid crystal molecules on the substrate.

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].

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.

Colloidal suspensions of anisotropic nanoparticles can spontaneously self-organize in liquid-crystalline phases beyond some concentration threshold. These phases often respond to electric and magnetic fields. At lower concentrations, usual isotropic liquids are observed but they can display very strong Kerr and Cotton-Mouton effects (i.e., field-induced particle orientation). For many examples of these colloidal suspensions, the solvent is water, which hinders most electro-optic applications. Here, for goethite (α-FeOOH) nanorod dispersions, we show that water can be replaced by polar aprotic solvents, such as N-methyl-2-pyrrolidone (NMP) and dimethylsulfoxide (DMSO), without loss of colloidal stability. By polarized-light microscopy, small-angle X-ray scattering and electro-optic measurements, we found that the nematic phase, with its field-response properties, is retained. Moreover, a strong Kerr effect was also observed with isotropic goethite suspensions in these polar aprotic solvents. Furthermore, we found no significant difference in the behavior of both the nematic and isotropic phases between the aqueous and non-aqueous dispersions. Our work shows that goethite nanorod suspensions in polar aprotic solvents, suitable for electro-optic applications, can easily be produced and that they keep all their outstanding properties. It also suggests that this solvent replacement method could be extended to the aqueous colloidal suspensions of other kinds of charged anisotropic nanoparticles.

In this critical review we compare two geometries in partial wetting: a liquid drop on a planar substrate and a spherical particle at a planar liquid interface. We show that this comparison is far from being trivial even if the same physical interactions are at play in both geometries. Similarities and differences in terms of free energies and frictions will be discussed. Contact angle hysteresis, the impact of surface roughness and line pinning on wetting will be described and compared to selected experimental findings.

Nematic Liquid Crystals (CL) dominate the market for high-resolution display devices.Unfortunately, their relatively slow response times (> 1 ms) greatly limit their use in thedevelopment of more efficient screens (3D screens) and components (switches, couplers,filters). In this context, newly discovered Twist-Bend Nematic (TBN) similar to the usualnematic phases, but spontaneously deformed at the level of a few nanometers, due to thecurved shape of the molecules, are very promising [1]. In fact, the more rigid structure ofTBN makes it possible to achieve ultra-short electrical field response times (<1 μs). Thestudy of TBN and their potential in new applications requires to master their anchoring, i.e.their orientation on the solid confinement walls.
In this paper we present an experimental study of a newly synthesized TBN the: N,N0 -Bis(4,40-cyanobiphenymethylidene)-nonane-1,9-diamine (CBI-9-ICB), under well controlledanchoring and confinement conditions.
In the experiment, the compound is inserted by capillarity in a cell made by two parallel glassplates with few microns gap. Different anchoring conditions have been examined. We showthat rubbed polyvinyl alcohol (PVA) and Dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DMOAP) treated substrates give different anchoring and texture fromthat given by usual nematic as 4-Cyano-4'-pentylbiphenyl (5CB). In Figure 1 is represented atypical TBN texture observed under Polarizing Optical Microscope in a cell withhomogeneous planar anchoring obtained with rubbed PVA. Stripes parallel to the anchoringorientation having a wavelength proportional to the cell thickness are measured. The sametexture is also found in rubbed Polyimide substrates. Currently, two models are proposed toexplain this texture. The first is related to the Helfrich-Hurault instability due to the presenceof TBN pseudo-layers [2]. The second is based on the peculiar elasticity of the TBN directorfield [3]. Although it is not clear how these models could reproduce the observed lineardependence of pattern periodicity as a function of the cell gap. In Figure 2 is reported theTBN texture for silane treated glass plates that gives, for ordinary nematic, homeotropic (i.e.perpendicular to the plate) anchoring. The resulting anchoring for TBN is surprisingly planardegenerated as one can infer from the presence of half-strength disclinations. A mosaictexture with homogeneously oriented domains reminiscent of SmC mosaic also appears.Such textures will be discussed in the framework of the elastic theory of TBN.
References:
[1] C. Meyer, G. R. Luckhurst and I. Dozov, Phys. Rev. Lett., 2013,111 (6), 0031-9007.
[2 P. K. Challa,V. Borshch,O. Parri, Phys. Rev. Lett., 2014, 89 (6), 1539-3755.
[3] V. P. Panov, M. Nagaraj, J. K. Vij, Phys. Rev. Lett., 2010, 105 (16), 0031-9007.