HypothesisAlthough the dynamics of colloids in the vicinity of a solid interface has been widely characterized in the past, experimental studies of Brownian diffusion close to an air–water interface are rare and limited to particle-interface gap distances larger than the particle size. At the still unexplored lower distances, the dynamics is expected to be extremely sensitive to boundary conditions at the air–water interface. There, ad hoc experiments would provide a quantitative validation of predictions.ExperimentsUsing a specially designed dual wave interferometric setup, the 3D dynamics of 9 μm diameter particles at a few hundreds of nanometers from an air–water interface is here measured in thermal equilibrium.FindingsIntriguingly, while the measured dynamics parallel to the interface approaches expected predictions for slip boundary conditions, the Brownian motion normal to the interface is very close to the predictions for no-slip boundary conditions. These puzzling results are rationalized considering current models of incompressible interfacial flow and deepened developing an ad hoc model which considers the contribution of tiny concentrations of surface active particles at the interface. We argue that such condition governs the particle dynamics in a large spectrum of systems ranging from biofilm formation to flotation process.

In this article, we present the mobilities of prolate ellipsoidal micrometric particles close to an air–water interface measured by dual wave reflection interference microscopy. Particle's position and orientation with respect to the interface are simultaneously measured as a function of time. From the measured mean square displacement, five particle mobilities (3 translational and 2 rotational) and two translational–rotational cross-correlations are extracted. The fluid dynamics governing equations are solved by the finite element method to numerically evaluate the same mobilities, imposing either slip and no-slip boundary conditions to the flow at the air–water interface. The comparison between experiments and simulations reveals an agreement with no-slip boundary conditions prediction for the translation normal to the interface and the out-of-plane rotation, and with slip ones for parallel translations and in-plane rotation. We rationalize these evidences in the framework of surface incompressibility at the interface.

Avec la plupart des couches d’alignement classiques des cellules commerciales, il est difficile d’obtenir une texture uniforme des phases nématiques twist bend (NTB) en géométrie planaire, y compris dans des cellules très fines (~1 μm). Même lorsqu’un composé présente une phase nématique N bien alignée, on observe en effet la formation spontanée de bandes lorsque l’on passe en phase NTB [1,2,3] (voir Fig.1-a). Cette texture en rayures est caractérisée par un pas égal à deux fois l’épaisseur de la cellule [1]. Son origine est attribuée à l’amincissement rapide des pseudo-couches NTB en dessous de la température de transition N-NTB qui conduit à une instabilité d’ondulation.Nous avons étudié en détail le rôle de la force d’ancrage des couches d’alignement sur les textures NTB d’un dimère cyanophyphényle, le CB7CB (1,7-bis-4-(4-cyanobiphenyl) heptane). Pour cela nous avons utilisé un polymère dont la force d’ancrage sur les nématiques cyanobiphényles (monomères) était connue pour varier fortement selon les conditions de préparation. Une étude systématique de l’ancrage de ce polymère sur le CB7CB a été faite en phase nématique et nous avons étudié l’influence des énergies d’ancrages zénithale et azimutale sur les textures obtenues en phase NTB. Dans des cellules symétriques, nous avons ainsi montré que la périodicité des instabilités dépendait fortement de l’ancrage et notamment de sa composante azimutale.Par ailleurs, ce travail a permis de montrer qu’avec des conditions d’ancrage adéquates, il était possible d’obtenir un alignement spontané uniforme de la phase NTB (voir Fig.1-b-c). Cet alignement est conservé même en s’éloignant de la température de transition, ouvrant la possibilité future d’utiliser les phases NTB dans des dispositifs électro-optiques.

When a microparticle is trapped at a fluid interface, particle's electrical charge and weight combine to deform the interface. Such deformation is expected to affect the particle diffusion via hydrodynamics boundary conditions. Using available models of particle-induced electrostatic deformation of the interface and particle dynamics at the interface, we are able to analytically predict particle diffusion coefficient values in a large range of particle's contact angle and size. This might offer a solid background of numerical values to compare with for future experimental studies in the field of particle diffusion at a fluid interface.

Nematic twist-bend phases (N-TB) are new types of nematic liquid crystalline phases with attractive properties for future electro-optic applications. However, most of these states are monotropic or are stable only in a narrow high temperature range. They are often destabilized under moderate cooling, and only a few single compounds have shown to give room temperature N-TB phases. Mixtures of twist-bend nematic liquid crystals with simple nematogens have shown to strongly lower the nematic to N-TB phase transition temperature. Here, we examined the behaviour of new types of mixtures with the dimeric liquid crystal [4 ',4 '-(heptane-1,7-diyl)bis(([1 ',1 ''-biphenyl]4 ''-carbo-nitrile))] (CB7CB). This now well-known twist-bend nematic liquid crystal presents a nematic twist-bend phase below T approximate to 104 degrees C. Mixtures with other monomeric alkyl or alkoxy -biphenylcarbonitriles liquid crystals that display a smectic A (SmA) phase also strongly reduce this temperature. The most interesting smectogen is 4 '-Octyl-4-biphenylcarbonitrile (8CB), for which a long-term metastable N-TB phase is found at room and lower temperatures. This paper presents the complete phase diagram of the corresponding binary system and a detailed investigation of its thermal, optical, dielectric, and elastic properties.

In this article, we review both theoretical models and experimental results on the motion of micro-and nano-particles that are close to a fluid interface or move in between two fluids. Viscous drags together with dissipations due to fluctuations of the fluid interface and its physicochemical properties affect strongly the translational and rotational drags of colloidal particles, which are subjected to Brownian motion in thermal equilibrium. Even if many theoretical and experimental investigations have been carried out, additional scientific efforts in hydrodynamics, statistical physics, wetting and colloid science are still needed to explain unexpected experimental results and to measure particle motion in time and space scales, which are not accessible so far.

We report the measurement of the interaction energy between a charged Brownian polystyrene particle and an air–water interface. The interaction potential is obtained from the Boltzmann equation by tracking particle interface distance with a specifically designed Dual-Wave Reflection Interference Microscopy (DW-RIM) setup. The particle has two equilibrium positions located at few hundreds of nanometers from the interface. The farthest position is well accounted by a DLVO model complemented by gravity. The closest one, not predicted by current models, more frequently appears in water solutions at relatively high ions concentrations, when electrostatic interaction is screened out. It is accompanied by a frozen rotational diffusion dynamics that suggests an interacting potential dependent on particle orientation and stresses the decisive role played by particle surface heterogeneities. Building up on both such experimental results, the important role of air nanobubbles pinned on the particle interface is discussed.