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Soft or rigid particles, suspended in a liquid melt, interact with an advancing solidification front in various industrial and natural processes, such as fabrication of particle-reinforced-composites, growth of crystals, cryopreservation, frost heave, and growth of sea ice. The particle dynamics relative to the front determine the microstructure as well as the functional properties of the solidified material. Previous studies have extensively investigated the interaction of foreign objects with a moving solid-liquid interface in pure melts while in most real-life systems, solutes or surface active impurities are almost always present. Here we study experimentally the interaction of spherical oil droplets with a moving planar ice-water interface, while systematically increasing the surfactant concentration in the bulk liquid, using

Hydrophobically modified polymers are good candidates for the stabilization of liquid interfaces thanks to the high anchoring energy of the hydrophobic parts. In this article we probe the interfacial anchoring of a series of home-made hydrophobically modified polymers with controlled degree of grafting by studying their behavior upon large area dilations and compressions. By comparing the measured interfacial tension to the one that we expect in the case of a constant number of adsorbed monomers, we are able to deduce whether desorption or adsorption occurs during area variations. We find that the polymer chains with the longest hydrophobic grafts desorb at larger compressions compared to the polymers with the shortest grafts, because of their larger desorption energy. Furthermore, for a given graft length, we observe more desorption for polymers with the highest grafting densities. We attribute this counter intuitive result to the fact that at high grafting densities, the length of the polymer loops is shorter, and hence the elastic penalty upon compression is larger for these layers, leading to a faster desorption. Comparing the elastic penalty to thermal energy, kBT, enables deducing a critical grafting density above which desorption of grafts is expected upon compression, which is consistent with our experimental results. In the case of large area dilations, the experiments reveal that the number of adsorbed anchors remains constant in the case of chains with a low grafting density while chains with the highest degree of grafting seem to show some degree of adsorption during the dilatation. Therefore, in these highly grafted chains there may be unadsorbed grafts remaining in the vicinity of the interface, which may adsorb quickly at the interface upon dilatation.

International audience; Encapsulation of chemicals using polymer membranes enables to control their transport and delivery for applications such as agrochemistry or detergency. To rationalize the design of polymer capsules, it is necessary to understand how the membranes' mechanical properties control the transport and release of the cargo. In this article, we use microfluidics to produce model polymer capsules and study in situ their behavior in controlled elongational flows. Our model capsules are obtained by assembling polymer mono and hydrogen-bonded bilayers at the surface of an oil droplet in water. We also use microfluidics to probe in situ the mechanical properties of the membranes in a controlled elongational flow generated by introducing the capsules through a constriction and then in a larger chamber. The deformation and relaxation of the capsules depend on their composition and especially on the molecular interactions between the polymer chains that form the membranes and the anchoring energy of the first layer. We develop a model and perform numerical simulations to extract the main interfacial properties of the capsules from the measurement of their deformations in the microchannels.

International audience; Molecular interactions in thin liquid films, such as the disjoining pressure, are involved in interfacial phenomena such as emulsion and foam stabilization. In this article we show that through light stimulation we can control remotely the disjoining pressure in a thin liquid film stabilized by a photosurfactant. We stabilize a horizontal thin liquid film using a cationic photosurfactant, AzoTAB, bearing an azobenzene moiety on the hydrophobic tail which can switch from a trans to a cis conformation upon light stimulation. As the film is illuminated at specific wavelengths the AzoTAB molecules switch continuously their conformation and consequently their interface affinity. The main consequence of stimulating the film with light is increasing the ratio of cis in the film. This provokes a desorption flux, and an increase in the concentration of free surfactants, as the CMC of the cis isomer is higher than that of the trans isomer. Therefore the electrostatic repulsion between the surfactant layers that stabilize the film decreases, inducing an instability in the film thickness. For films with a thickness between 20 nm and 60 nm, we observe the formation of spherical caps up to 100 μm wide, whose shape is controlled by the competition between surface tension and disjoining pressure. The motion of these caps in the film is restrained by the surface viscosity of the surfactant layers. In addition, for thicknesses below 40 nm and depending on light intensity, we can observe flat stratified islands up to 100 μm wide, with thickness steps corresponding to the size of a surfactant micelle. We suggest that this second instability is due to the oscillation of the disjoining pressure isotherm under light.

International audience; We investigate the stabilization of air-water interfaces by mixtures of negatively charged latex particles (sulfate polystyrene) and cationic surfactants (alkyl trimethylammonium bromides). First we report results concerning the binding of surfactant molecules to the latex particles. As the surfactant concentration increases, the charge of the particles reverses, from negative to positive, because C(n)TAB first binds electrostatically to the latex particles and then through hydrophobic interaction with the monolayer already adsorbed on the particles as well as directly with the hydrophobic surface of the latex. Over a large range of surfactant concentrations around the charge inversion, a strong flocculation is observed and 100 mu m large aggregates form in the suspension. Unlike previous studies published on mixtures of inorganic particles with oppositely charged surfactants, we show that we can vary the sign of the zeta potential of the particles without changing the contact angle of the particles over a large range of surfactant concentrations. Indeed, the latex particles that we study are more hydrophobic than inorganic particles, hence adding moderate concentrations of the surfactant results in a weak variation of the contact angle while the charge of the particles can be reversed. This enables decoupling of the effect of zeta potential and contact angle on the interfacial properties of the mixtures. Our study shows that the contact angle and the charge of the particles are not sufficient parameters to control the foam properties, and the key-parameters are the flocculation state and the shear energy applied to produce the foam. Indeed, flocculated samples, whatever the sign of the zeta potential, enable production of a stable armour at the interface. The large aggregates do not adsorb spontaneously at the interface because of their large size, however when a large shear energy is used to produce the foam very stable foam is obtained, where particles are trapped at interfaces. We suggest that the large aggregates may be broken during shear and may reform at the interface to form a solid armour. A simple calculation taking into account the adsorption dynamics of the aggregates as a function of their size is consistent with this hypothesis.

We study the wetting behaviour of water droplets on three types of thin water-soluble polymeric coatings spin-coated on a silicon wafer—(i) a neutral polymer, (ii) a cationic polyelectrolyte, and (iii) a zwitterionic polymer. We investigate the dynamical spreading of drops as well as their drying on these surfaces. The influence of a salt—added either in the water droplets or inside the coating—is investigated to better differentiate the three types of polymer. We find that the salt tends to promote spreading on charged polymeric coatings when it is added inside the coating, yet not when added in the drops. Salts inside a coating simply increase its surface tension, resulting in lower contact angles during the spreading motion of a drop. On the other hand, when drying starts on the neutral polymer, the drop starts receding (i.e. its radius decreases) immediately after its spreading motion has stopped, whereas for both charged polymers, it remains pinned on the substrate until the very end of drying. When an increasing amount of salt is incorporated inside the two types of charged coatings, the contact line tends to unpin faster. As charges and salts obviously have a strong impact on the pinning/receding of the contact line, we hypothesize that a contribution of osmotic nature drives the motion of the contact line of drying water drops on polymeric coatings. There is a competition between (i) the evaporation from the edge which favours recession and (ii) a flux of water resulting from the osmotic pressure imposed by the higher concentration of species at the edge of the drop. The osmotic pressure being high for charged polymers, water evaporating on a charged coating keeps flowing from the inside of a drop to the outer edge, and compensates for the fast evaporation in the edge: macroscopically, this keeps the apparent contact line at the same position so that it appears pinned as more and more dissolved polymer molecules are dragged toward the edge and accumulate in a ring like deposit. When a salt is added, the osmotic drive weakens and charged polymers eventually dry like neutral ones.

Thin layers made of water soluble polymers are good candidates to improve the wettability of surfaces as they are theoretically expected to be totally wettable. Yet, in this work we show that the wetting of a droplet of a volatile solvent, water, advancing on such films strongly depends on the degree of coating hydration beyond the contact line. The phenomenon originates not from the diffusion of water molecules through the coating, but from the evaporation from the advancing drop itself and re-condensation beyond the contact line. The faster a droplet is travelling on a coating, the less extensive the phenomenon of evaporation/recondensation beyond the contact line: macroscopically, the more hydrophobic the coating will appear. Expectedly, the contact angle θ strongly depends on the coating thickness e and the droplets advancing velocity U, two parameters that regulate the solvent uptake. More precisely, θ depends on product eU, which we predicted theoretically in a previous paper and verify here experimentally. The practical outcome of these findings is that hydrophilizing a substrate with a hydrophilic, fully water-soluble polymer will be more or less successful depending both on its thickness and on whether the substrate is static or moving during its use.

We study the adsorption dynamics of an azobenzene-based photo-responsive charged surfactant to investigate how photo-stimulation impacts the dynamics at an air–water interface. The hydrophobic tail of this photo-responsive surfactant photo-converts reversibly from a cis to a trans conformation when the wavelength switches from UV to blue. This change in conformation results in a decrease of the surface tension. Using a kinetically limited model of adsorption, including the electrostatics effects and the competition between the two photo-isomers, we reproduce the dynamics of adsorption of AzoTAB measured experimentally. We find that the cis isomer adsorbs 10 times faster than the trans isomer but the cis conformation also desorbs 300 times faster. As a result, within a few seconds a non-stimulated interface becomes composed of almost 100% trans isomers. We then focus on the competition between the photo-conversion and the adsorption at the interface. Indeed when the interface is stimulated, part of the adsorbed trans isomers rapidly convert to cis. As the latter quickly desorbs, the surface coverage decreases: the light induces a “pumping-out” of the interface. The photo-stimulated interface reaches a stationary state where a vertical gradient of composition is established below the surface. Finally, this study highlights a new way to stimulate a photo-responsive interface: for a solution prepared under blue light, instead of photo-converting the bulk composition by stimulating under UV (which can be quite slow for high absorbance solutions), one can tune and reach a stabilized value of the surface tension in few seconds by stimulating the interface with a blue light with high enough intensity.

By forcing a contact line to advance on the polymeric deposit left by a drying polymer droplet, we have been able to measure the contact angle of water on an hydrosoluble polymer substrate. Measuring simultaneously the contact angle and the hydration of the polymer, we show that a dry layer of hydrosoluble polymer is poorly wetting because of the hydrophobic backbone of the polymer, which segregates at the surface.