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.

Separation efficiency is ideally controlled by molecular diffusion in capillary electrophoresis (CE). However, other adverse phenomena, such as solute adsorption on capillary surface, tend to increase the peak dispersion. An interesting alternative to limit the solute adsorption is to avoid as much as possible the contact of the solute with the capillary surface by elaborating superhydrophobic (SH) coatings on fused silica capillary surfaces. This work describes an optimized protocol to get non-wettable SH coating using hydrophobically modified silica nanoparticle suspensions (Glaco TM), based on simple capillary flushes and thermal stabilization. In this protocol, the control of the air flushing after the introduction of the Glaco TM suspension in the capillary was found crucial to get optimized coating coverage and reproducibility. The SH coating was characterized by ellipsometry, atomic force microscopy, scanning electron microscopy, contact angle (about 159°) and the observation of the meniscus of water in the coated capillary. The hydrodynamic behavior of the SH coated capillary was investigated by plotting the Poiseuille law. Finally, electrophoretic separations of a peptide mixture in acidic conditions demonstrated the interest of this approach with an increase by a factor 2 of the separation efficiency compared to fused silica capillary.

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.

Hard and soft proteins at fluid interfaces

We have investigated the wetting and surface diffusion of mesoporous colloidal silica particles at the water surface and the adsorption of cationic cetyltrimethylammonium (CTA+) surfactant on these particles. Porous silica colloids diffuse at the surface of water and in the volume, interacting with cationic surfactants that can adsorb inside the pores of the particles. We observed that surfactant adsorption on mesoporous silica depends dramatically not only on the particle pore size but also on specific counterion effects. We measured striking differences both on a macroscopic property of the interface, i.e., surface tension, and also at a single particle level by evaluating the translational diffusion of partially wetted particles at the fluid interface. We varied the pore size from 2 to 7 nm and explored the effects of ions possessing different hydration number and kosmotropic/chaotropic character. At concentrations lower than the critical micellar concentration, we evidence that cationic surfactants adsorb on silica as surface micelles and surfactant adsorption inside the pores occurs only if the pore diameter is larger than the size of surface micelles. With a view to understand the surprising different adsorption behavior of CTA+OH– and CTA+Br– on porous silica particles, we investigated the effect of counterions on the surfactant adsorption on porous silica colloids by tuning the pH and the counterion properties.

Despite the relevance to environmental, biological and industrial processes, the motion of a colloidal particle close to a fluid interface and the way it interacts with the water surface are still largely elusive and intriguing physical phenomena.We explored the system of individual colloidal particles close to an air-water interface by recording highly resolved 3D trajectories through a dual wave reflection interference setup (DW-RIM). Contrary to other established experimental techniques, DW-RIM allows accurate measurements of the absolute particle-interface distance and thus does not require any calibration or assumption to know the location of the interface. The particle-interface system shows two different potential energy landscapes resulting to two different equilibrium particle-interface distances. The larger one can be fairly explained by Van der Waals and electrostatic interactions combined with gravity. The shorter one highlights the existence of an unexpected additional attractive interaction. Surface heterogeneities as possible origin of such an interaction are considered and discussed.Concerning the friction the particle undergoes, the air-water interface peculiarly acts as a slip boundary for the particle motion parallel to the interface and as a no-slip boundary for the perpendicular motion. This experimental result is rationalized considering recent models based on surface incompressibility.

Challenges of public health and sustainable development require replacing in food products animal proteins by plant proteins. In this optics, it is crucial to understand the structure and kinetic of formation of a film of plant proteins in order to improve the control of emulsions and foams stabilized by these proteins.In this talk we will present experimental results on the behaviour interfacial properties of wheat gluten, sunflower and rapeseed proteins at liquid interfaces. Thanks to a combination of tensiometry, dilatational rheology and ellipsometry, rational physical pictures of the dynamics of the interfacial properties are achieved, for the various proteins and at both air/water and oil/water interfaces. For gluten proteins, a time-concentration superposition of the data is evidenced whatever the subphase concentration, which reveals that the kinetics of protein adsorption at the interface is dominated by bulk diffusion. We propose a consistent physical picture of the multistep diffusion-controlled irreversible adsorption of the gliadin proteins at an air/water interface, and evidence surface-induced conformational changes of the proteins followed by film gelation [1]. Sunflower and rapeseed proteins by contrast do not reorganize once adsorbed at an interface and display a simpler dynamics of film formation. In addition the failure at high concentration of the time-concentration superposition of the tensiometry and viscoelastic data strongly suggest a surface-induced aggregation process, which we confirm with turbidity measurements. By quantitatively comparing the surface pressure dependence viscoelasticity of the various interfaces, we hightlight the crucial role on the behavior of plant proteins at liquid interfaces of the solvent quality and of the protein softness, that is discussed in regard to the protein structure.