The interface between a liquid and a fluid affects dramatically both the interactions and the motion of colloidal particles. In this talk, the impact of partial wetting dynamics on the motion of passive and active colloids will be presented. First, experimental results on the Brownian dynamics of micrometric spherical silica colloids andpolymeric ellipsoids trapped at a planar air-water interface will be described. Partial wetting defines a contact angle which sets the immersion depth of the colloid. Particle motion is confined in the interfacial plane. For spherical colloids, the contact angle is finely tuned in the range 30°-140° by surface treatments and measured in situ. Translational and rotational diffusion coefficients of colloids trapped at the water interface are obtained by particle tracking video-microscopy. Counter-intuitively, the friction felt by the colloid increases when the contact angle increases; i.e. when particles are less immersed in water and more in air, which has a negligible viscosity. To explain the slowing down of the translational motion for spheres and rotational diffusion for ellipsoids, an extra friction term originating from contact line fluctuations will be introduced.The second part of the talk deals with the motion of isolated active Janus colloids at the surface of water. Spherical catalytic Janus colloids have been prepared coating half surface of silica particles by a thin platinum layer. Immersion depth of the Janus colloids as well as their orientation with respect to the water surface reveal thecomplex wetting properties of Janus particles. The active motion of Janus colloids at the interface in the presence of various concentration of hydrogen peroxide has been studied. The types of trajectories, directional and circular ones observed revealed the effective force and torque induced by the catalytic decomposition of H2O2. At the water surface, active colloids perform more persistent directional motions as compared to the motions performed in the bulk. This has been interpreted as due to the loss of degrees of freedom resulting from the confinement at interfaceand also to the partial wetting conditions that possibly bring new contributions to the rotational friction at interface.

Janus colloidal particles show remarkable properties in terms of surface activity, self-assembly and wetting. Moreover they can perform autonomous motion if they can chemically react with the liquid in which they are immersed. In order to understand the self-propelled motion of catalytic Janus colloids at the air-water interface, wetting and the orientation of the catalytic surface are important properties to be investigated. Wetting plays a central role in active motion since it determines the contact between fuel and catalytic surface as well as the efficiency of transduction of chemical reaction into motion. Active motion is not expected to occur either when the catalytic face is completely out of the aqueous phase or when the Janus boundaries are parallel to the interfacial plane. The design of a Janus colloid possessing two hydrophilic faces is required to allow the catalytic face to react with the fuel (e.g. H2O2 for Platinum) in water and to permit some rotational freedom of the Janus colloid in order to generate propulsion parallel to the interfacial plane.Here, we discuss some theoretical aspects that should be accounted when studying Janus colloids at the surface of water. The free energy of ideal Janus colloidal particles at the interface is modeled as a function of the immersion depth and the particle orientation. Analytical expressions of the energy profiles are established. Energetic aspects are then discussed in relation to the particle ability to rotate at the interface. By introducing contact angle hysteresis we describe how the effects of contact line pinning modifies the scenario described in the ideal case. Experimental observations of the contact angle hysteresis of Janus colloids at the interface reveal the effect of pinning; and orientations of silica particles half covered with a platinum layer at the interface do not comply with the ideal scenarios. Experimental observations suggest that Janus colloids at the fluid interface behave as kinetically driven system, where the contact line motion over defects decorating the Janus faces rules the orientation and rotational diffusion of the particle.

We study the dynamics of individual polystyrene ellipsoids of different aspect ratios trapped at the air-water interface. Using particle tracking and in situ vertical scanning interferometry techniques we are able to measure translational drags and the protrusion in air of the ellipsoids. We report that translational drags on the ellipsoid are unexpectedly enhanced: despite the fact that a noticeable part of the ellipsoid is in air, drags are found larger than the bulk one in water.

Colloids and nanoparticles possessing two different faces with distinct properties are called Janus as the Roman God depicted with two faces looking in opposite directions. In this talk a fabrication of Janus particles by droplet based microfluidics will be presented. Silica colloids dispersed in the oil phase and gold nanoparticles dispersed in the aqueous phase meet at the droplet oil-water interface. The contact angle of the silica colloid defines the area of the silica face immersed in water, which can be decorated by the gold nanoparticles, forming the Janus silica-gold particle.Many physical and physicochemical aspects take part in this process. Wetting of the silica and gold particles set the contact angle of the particles at the droplet interface, which depends strongly on the chemical groups used to stabilize the particles, and in principle allow the fabrication of Janus particles of different area ratios. Adsorption onto the oil-water interface is also a key process, which depends not only on the concentration but also on the hydrodynamic flows and geometry present in the microfluidic channels. The role of long range surface forces such as Van der Waals and electrostatic will be also highlighted both for the adsorption kinetics and for the effect on the particle size dependent contact angle.

Commercial carboxymethylcellulose was used to prepare dispersible multi-walled carbon nanotubes-based composites. These composites were employed to prepare Pickering oil-in-water emulsions. Emulsion-templated macroporous materials were then prepared by embedding the oil droplets into a polymer resin arising from the polycondensation of furfural and phloroglucinol within the continuous aqueous phase in the presence of FeCl3 as catalyst. Polymerization afforded organic–inorganic nanocomposite materials in the form of capsules. After pyrolysis, highly microporous, magnetic and electrically conductive micrometric capsules could be obtained. This approach opens interesting prospects for catalysis, separation and electrochemistry applications.

A mild and simple way to prepare stable aqueous colloidal suspensions of composite particles made of a cellulosic material (Sigmacell cellulose) and multiwalled carbon nanotubes (MWCNTs) is reported. These suspensions can be dried and redispersed in water at pH 10.5. Starting with rather crude initial materials, commercial Sigmacell cellulose and MWCNTs, a significant fraction of composite dispersed in water could be obtained. The solid composites and their colloidal suspensions were characterized by electronic microscopy, thermal analyses, FTIR and Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and light scattering. The composite particles consist of tenuous aggregates of CNTs and cellulose, several hundred nanometers large, and are composed of 55 wt % cellulose and 45 wt % CNTs. Such particles were shown to stabilize cyclohexane-in-water emulsions. The adsorption and the elasticity of the layer they form at interface were characterized by the pendant drop method. The stability of the oil-in-water emulsions was attributed to the formation of an elastic network of composite particles at interface. Cyclohexane droplet diameters could be tuned from 20 to 100 μm by adjusting the concentration of composite particles. This behavior was attributed to the limited coalescence phenomenon, just as expected for Pickering emulsions. Interestingly, cyclohexane droplets were stable over time and sustained pH modifications over a wide range, although acidic pH induced accelerated creaming. This study points out the possibility of combining crude cellulose and MWCNTs through a simple process to obtain colloidal systems of interest for the design of functional conductive materials.