Dynamics of prolate spheroids in the vicinity of an air-water interface.

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.

Microspheres viscous drag at a deformed fluid interface: particle's weight and electrical charges effects.

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.

Multistable interaction between a spherical Brownian particle and an air-water interface.

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.

Janus Colloids Actively Rotating on the Surface of Water.

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 nonuniform 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.

Brownian diffusion of a partially wetted colloid.

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.

MHz Ultrasound Induced Roughness of Fluid Interfaces.

The interface between two fluids is never flat at the nanoscale, and this is important for transport across interfaces. In the absence of any external field, the surface roughness is due to thermally excited capillary waves possessing subnanometric amplitudes in the case of simple liquids. Here, we investigate the effect of ultrasound on the surface roughness of liquid-gas and liquid-liquid interfaces. Megahertz (MHz) frequency ultrasound was applied normal to the interface at relatively low ultrasonic pressures (<0.6 MPa), and the amplitudes of surface fluctuations have been measured by light reflectivity and ellipsometry. We found a dramatic enhancement of surface roughness, roughly linear with intensity, with vertical displacements of the interface as high as 50-100 nm. As a consequence, the effective contact area between two fluids can be increased by ultrasound. This result has a clear impact for enhancing interface based processes such as mass or heat transfer.

Wetting and orientation of catalytic Janus colloids at the surface of water.

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 the fuel and the catalytic surface as well as the efficiency of the transduction of the 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 for 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's 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 a kinetically driven system, where the contact line motion over the defects decorating the Janus faces rules the orientation and rotational diffusion of the particle.

Enhanced active motion of Janus colloids at the water surface.

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.

In situ assessment of the contact angles of nanoparticles adsorbed at fluid interfaces by multiple angle of incidence ellipsometry.

Here multiple angle of incidence ellipsometry was successfully applied to in situ assess the contact angle and surface coverage of gold nanoparticles as small as 18 nm, coated with stimuli-responsive polymers, at water-oil and water-air interfaces in the presence of NaCl and NaOH, respectively. The interfacial adsorption of the nanoparticles was found to be very slow and took days to reach a fairly low surface coverage. For water-oil interfaces, in situ nanoparticle contact angles agree with the macroscopic equilibrium contact angles of planar gold surfaces with the same polymer coatings, whilst for water-air interfaces, significant differences have been observed.

Capillary force on a micrometric sphere trapped at a fluid interface exhibiting arbitrary curvature gradients.

We report theoretical predictions and measurements of the capillary force acting on a spherical colloid smaller than the capillary length that is placed on a curved fluid interface of arbitrary shape. By coupling direct imaging and interferometry, we are able to measure the in situ colloid contact angle and to correlate its position with respect to the interface curvature. Extremely tiny capillary forces down to femtonewtons can be measured with this method. Measurements agree well with a theory relating the capillary force to the gradient of Gaussian curvature and to the mean curvature of the interface prior to colloidal deposition. Numerical calculations corroborate these results.

Microparticle Brownian motion near an air-water interface governed by direction-dependent boundary conditions.

HYPOTHESIS: Although 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. EXPERIMENTS: Using 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. FINDINGS: Intriguingly, 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.

Nematic-smectic transition in spherical shells.

We study the nematic-smectic phase transition of a thermotropic liquid crystal confined to a spherical shell. Far from the nematic-smectic phase transition temperature, T(NS), we observe a configuration with four +1/2 defects, as predicted by theory. Since in this case K(1)≈K(3), the four defects are confined at the thinnest part of the shell to minimize the energy associated with the defect cores. By contrast, near T(NS), where K(3)≫K(1), bend distortions become prohibited and the defects organize themselves along a great circle of the sphere, confirming recent theoretical and simulation results. During this structural change, the defects associate in two pairs that behave independently. In the smectic phase, we observe a new configuration displaying curvature walls.

Rotational solid friction of a nematic liquid crystal.

Liquid crystal defects are used as probes to study the local reorientation dynamics of the nematic surface director on SiO(x) alignment layers. The tracking of the defect's motion reveals the presence of solid friction forces, unexpected in this complex viscous fluid. We identify the director pinning due to a surface quenched disorder as a possible mechanism that gives rise to the measured solid friction.

Orientational quenched disorder of a nematic liquid crystal.

By means of direct imaging, we map the surface heterogeneities of the nematic director orientation on a SiOx anchoring layer. The spatial correlations of surface director orientations are well fitted with a compressed exponential with exponent of 1.5 and typical correlation length of few microns. To discuss these results a formal analogy is established between the equation governing the nematic surface torques and the Langevin equation. Based on this analogy we prove that the disorder is spatially correlated orientational quenched disorder. The measured correlation length is discussed in terms of substrate morphology and molecular adsorption.

Motion of micro- and nano- particles interacting with a fluid interface.

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.

Quasi-two-dimensional diffusion of single ellipsoids: aspect ratio and confinement effects.

We report on video-microscopy measurements of the translational and rotational Brownian motions of isolated ellipsoidal particles in quasi-two-dimensional sample cells of increasing thickness. The long-time diffusion coefficients were measured along the long (Da) and short (Db) ellipsoid axes, respectively, and the ratio, Da/Db, was determined as a function of wall confinement and particle aspect ratio. In three dimensions (3D), this ratio (Da/Db) cannot be larger than 2, but in quasi-two dimensions, wall confinement was found to substantially alter diffusion anisotropy and substantially slow particle diffusion along the short axis compared to 3D.

Rotational diffusion of partially wetted colloids at fluid interfaces.

HYPOTHESIS: Rotational Brownian diffusions of colloidal particles at a fluid interface play important roles in particle self-assembly and in surface microrheology. Recent experiments on translational Brownian motion of spherical particles at the air-water interface show a significant slowing down of the translational diffusion with respect to the hydrodynamic predictions (Boniello et al., 2015). For the rotational diffusions of partially wetted colloids, slowing down of the particle dynamics can be also expected. EXPERIMENTS: Here, the rotational dynamics of Janus colloids at the air-water interface have been experimentally investigated using optical microscopy. Bright field and fluorescent microscopies have been used to measure the in-plane and out-of-plane particle rotational diffusions exploiting the Janus geometry of the colloids we fabricated. FINDINGS: Our results show a severe slowing down of the rotational diffusion Dr,⊥ connected to the contact line motion and wetting-dewetting dynamics occurring on particle regions located at opposite liquid wedges. A slowing down of the particle rotational diffusion about an axis parallel to the interfacial normal Dr,|| was also observed. Contact line fluctuations due to partial wetting dynamics lead to a rotational line friction that we have modelled in order to describe our results.

Colloidal shape controlled by molecular adsorption at liquid crystal interfaces.

The ability to control finely the structure of materials remains a central issue in colloidal science. Due to their elastic properties, liquid crystals (LC) are increasingly used to organize matter at the micrometer scale in soft composites. Textures and shapes of LC droplets are currently controlled by the competition between elasticity and anchoring, hydrodynamic flows, or external fields. Molecules adsorbed specifically at LC interfaces are known to orient LC molecules (anchoring effect), but other induced effects have been poorly explored. Using specifically designed amphitropic surfactants, we demonstrate that large-shape transformations can be achieved in direct LC/water emulsions. In particular, we focus on unusual nematic filaments formed from spherical droplets. These results suggest new approaches to design new soft LC composite materials through the adsorption of molecules at liquid crystal interfaces.

A comparison between liquid drops and solid particles in partial wetting.

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.

Dispersions of Goethite Nanorods in Aprotic Polar Solvents.

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.