1. Introduction
Spontaneously distorted nematic states have been theoretically predicted long ago for bent-shaped mesogens [1-3]. Recent experiments [4] confirmed the existence of the twist-bend nematic phase, NTB, with short-pitch heliconical precession of the director n. Another predicted state [3], the splay-bend nematic, NSB, with n oscillating in a plane, is still elusive. However, a recent birefringence study [5] suggested NSB structure of the defect wall separating two NTB domains with opposite chiralities.
2. Twist-bend to splay-bend transition under strong electric field
Here we investigate the NTB state of the bent-shaped dimer CB7CB under strong a.c. electric field, (E ~10 V/µm) parallel to the helix axis. This in-plane field is applied in a 2 µm cell using two ITO electrodes (Fig.1). Crossing the N- NTB transition under field, we observe in the interelectrode region slightly lower transition temperature and much stronger birefringence than that reported in the NTB phase [5], suggesting an electric-field induced NTB – NSB transition.
3. Conclusion
A strong electric field applied to the NTB phase induces a transition to the splay-bend nematic phase, instead to the usual uniform nematic N. We explain this in the framework of the elastic-instability model [3] of the NTB and NSB phases by taking into account the strong biaxiality of the bent-shape nematic.
References
[1]R. B. Meyer, in Molecular Fluids, edited by R. Balian and G. Weill (Gordon and Breach, New York 1976), pp. 273
[2]V. L. Lorman and B. Mettout, Phys. Rev. Lett. 82, 940 (1999).
[3]I. Dozov, EPL (Europhysics Letters) 56, 247 (2001).
[4]M. Cestari, S. Diez-Berart, D. A. Dunmur, A. Ferrarini, M. R. de la Fuente, D. J. B. Jackson, D. O. Lopez, G. R. Luckhurst, M. A. Perez-Jubindo, R. M. Richardson, J. Salud, B. A. Timimi and H. Zimmermann, Physical Review E 84, 031704 (2011).
[5]C. Meyer, G. R. Luckhurst and I. Dozov, Journal of Materials Chemistry C 3, 318 (2015).

We have investigated the active motion of self-propelled colloids confined at the air–water interface and explored the possibility of enhancing the directional motion of self-propelled Janus colloids by slowing down their rotational diffusion. The two dimensional motion of micron-sized silica–platinum Janus colloids has been experimentally measured by particle tracking video-microscopy at increasing concentrations of the catalytic fuel, i.e. H2O2. Compared to the motion in the bulk, a dramatic enhancement of both the persistence length of trajectories and the speed has been observed. The interplay of colloid self-propulsion, due to an asymmetric catalytic reaction occurring on the colloid, surface properties and interfacial frictions controls the enhancement of the directional movement. The slowing down of the rotational diffusion at the interface, also measured experimentally, plays a pivotal role in the control and enhancement of active motion.

Nanomaterials and solutions: examples and scattering methods for characterization

L’élasticité orientationelle des matrices cristal liquide nématique conduit à l’auto-organisation bidimensionnelle de dispersions colloïdales nématiques à l’échelle du micron [1]. Les mécanismes d’assemblages reposent sur l’existence de paires défauts/particules (dipôles élastiques). Ainsi, une microsphère à ancrage homéotrope, confinée dans une cellule de nématique aligné perturbe le champ du directeur et impose la création d’un défaut topologique. Dans une géométrie plane comme celle d’une cellule à ancrage planaire, le défaut topologique associé à une particule (même de forme quelconque [2]) est localisé à une distance comparable à sa taille. Ce n’est cependant pas le cas dans des géométries complexes comme la géométrie sphérique des coques minces de nématique, où le défaut peut se localiser à grande distance en raison de contraintes topologiques[3]. Nous avons mis en évidence une autre situation physique où les défauts se positionnent également à grande distance des particules en raison d’effets capillaires. Nous avons ainsi étudié des microparticules piégées dans un film nématique mince libre, étalé sur un fluide isotrope. Quand l’épaisseur du film nématique est de l’ordre de la taille des colloïdes, des dipôles élastiques géants sont observés. Pour des particules de 4µm, le défaut point peut ainsi se positionner à une distance supérieure à 100µm de la particule. La taille de ce dipôle élastique géant est contrôlée via l’épaisseur du film. Il existe une épaisseur critique pour laquelle on observe une transition du dipôle élastique classique vers le dipôle géant. Nous avons expliqué l’ensemble des phénomènes en tenant compte des déformations interfaciales du film. L’exploitation de la mesure du retard optique pixel par pixel, permet de mesurer la déformation capillaire autour du colloïde; celle-ci peut-être expliquée dans une approche scalaire simplifiée du problème. Dans un second temps, à profil d’épaisseur du film fixé, la minimisation numérique de l’énergie élastique permet de reproduire les champs directeurs observés. Ce couplage original élasto-capillaire pourrait être utilisé pour contrôler l’assemblage 2D de particules dans des films minces nématiques [4].

We propose a simple and original way to prepare surfactant-free SWNT/hydrosoluble polymer composites with high concentrations of individual SWNT. We first disperse single-walled carbon nanotubes (SWNT) in aqueous suspensions using bile salts (BS), and we mix them with a solution of hydrosoluble polymer, poly(vinylpyrrolidone) (PVP) or poly(vinyl alcohol) (PVA). We measure the yield using visible–NIR absorption spectroscopy, and we probe both the chemical environment of the nanotubes and the effectiveness of individualization from coupled Raman/photoluminescence studies. We evidence a direct exchange of BS molecules and PVA chains at the surface of SWNT. By contrast, no direct exchange is observed with PVP. On the other hand, we show that a simple dialysis process leads to the preparation of aqueous suspensions of SWNT covered by PVP or PVA with high yields and an effective individualization of the nanotubes.

The dynamics of colloidal particles at interfaces between two fluids plays a central role in microrheology, encapsulation, emulsification, biofilm formation, water remediation and the interface-driven assembly of materials. Common intuition corroborated by hydrodynamic theories, suggests that such dynamics is governed by a viscous force lower than that observed in the more viscous fluid. Here, we show experimentally that a particle straddling an air/water interface feels a large viscous drag that is unexpectedly larger than that measured in the bulk. We suggest that such a result arises from thermally activated fluctuations of the interface at the solid/air/liquid triple line and their coupling to the particle drag through the fluctuation–dissipation theorem. Our findings should inform approaches for improved control of the kinetically driven assembly of anisotropic particles with a large triple-line-length/particle-size ratio, and help to understand the formation and structure of such arrested materials.

Active colloids are an emerging and promising class of colloidal particles, which are designed to perform autonomous motion by transforming chemical or other form of energies into work and propulsion.Here, we have investigated the active motion of self-propelled colloids confined at the air-water interface. The two dimensional motion of micron-sized Silica-Platinum Janus colloids has been experimentally measured by particle tracking video-microscopy under increasing concentration of the catalytic fuel, i.e. H2O2. Comparing to the motion in bulk, a dramatic enhancement of both the persistence length of trajectories and the speed has been observed. The interplay of colloid self-propulsion, due to an asymmetric catalytic reaction occurring on the colloid, and interfacial frictions controls the enhancement of the directional movement. The slowing down of the rotational diffusion at the interface, also measured experimentally, plays a pivotal role in the control and enhancement of active motion.