Cécile Monteux

Poly(N-isopropylacrylamide) microgels at the oil-water interface: interfacial properties as a function of temperature.

Highly monodisperse poly(N-isopropylacrylamide), PNiPAM, microgels were prepared by the conventional radical polymerization of NiPAM in the presence of dimethylamino ethyl methacrylate (DMAEMA) monomers at various concentrations. The effect of DMAEMA on the polymerization of PNiPAM microgels was examined at constant initiator (V50) and cross-linker (MBA) concentrations. The presence of DMAEMA in the synthesis batch allows for the preparation of PNiPAM microgels with controlled size and a narrow size distribution. The oil(dodecane)/water interfacial properties of the model PNiPAM microgels were then investigated. The pendant drop technique was used to measure the interfacial tensions as a function of temperature. Over the whole range of temperature (20-45 degrees C), the interfacial tension remains low (on the order of 17 mN/m) and goes through a minimum (12 mN/m) at a temperature of about 34 degrees C, which well matches the volume phase transition temperature (VPTT) of PNiPAM microgels. Below the VPTT, the decrease in the interfacial tension with temperature is likely to be due to the adsorption of dense layers because of the decrease of the excluded volume interactions. Above the VPTT, we suggest that the increase in the interfacial tension with temperature comes from the adsorption of loosely packed PNiPAM microgels. We also studied the effect of temperature on the stability of emulsions. Dodecane in water emulsions, which form at ambient temperature, are destabilized as the temperature exceeds the VPTT. In light of the interfacial tension results, we suggest that emulsion destabilization arises from the adsorption of aggregates above the VPTT and not from an important desorption of microgels. Aggregate adsorption would bring a sufficiently high number of dodecane molecules into contact with water to induce coalescence without changing the interfacial tension very much.

Packing and Sorting Colloids at the Contact Line of a Drying Drop

In this article, we study the drying kinetics of a sessile droplet containing latex particles. We find that a depletion film is left at the edge of the drops, whose width is controlled by two geometric parameters, the contact angle and the diameter of the particles. We show that this effect can be used to sort colloidal mixtures because nanometric colloids always segregate at the edge of the drop, whereas micrometric colloids are blocked further away from the edge. We also provide a simple method to measure the velocity of a micrometric latex as it flows toward the contact line. We find that the particles strongly accelerate at the end of the drying process. Using Deegans prediction for the rate of evaporation in the vicinity of the contact line, we quantitatively explain this phenomenon by the fact that the contact angle vanishes at the end of the drying process, therefore inducing a strong increase in the flux of water and particles close to the edge. The decrease in the contact angle also controls the width of the ringlike deposit.

Tracking the interfacial dynamics of PNiPAM soft microgels particles adsorbed at the air-water interface and in thin liquid films

We report the behavior of thermosensitive soft microgel particles adsorbed at the air–water interface. We study the effect of temperature on the adsorption, interfacial diffusion, and surface rheology of pure N-isopropylacrylamide (NiPAM) microgel particles at the air–water interface. We find that the surface tensions of the solutions are the same as those of polyNiPAM solution; hence, their adsorption properties are dominated by the surface activity of the NiPAM repeat units of the particles. Particle-tracking experiments show that the particles adsorb irreversibly at the interface and form stable clusters at very low concentrations, e.g., 5.10-3 wt%. We suggest that attractions between dangling arms or capillary interaction may be responsible for the formation of these clusters. For concentrations above 10-2 wt%, the interface is filled with particles, and their Brownian diffusivity is arrested. The compression elastic moduli—measured using the pendant drop method—are one or two orders of magnitude below those obtained for hard particles and NiPAM chains, and their value is probably dominated by the intrinsic compressibility of the particles. The thin liquid films made from microgels exhibit a symmetric drainage, consistent with a high surface viscosity, but their lifetime is surprisingly short, illustrating the fragility of the films. We observed the formation of a monolayer of microgels bridging the two interfaces of the film outside the dimple. This zone grows and thins over time to a point where the microgels are highly compressed and stretched, resulting in the rupture of the film.

Contact-line recession leaving a macroscopic polymer film in the drying droplets of water-poly(N,N-dimethylacrylamide) (PDMA) solution.

We found that the drying process of the droplet of water-poly(N,N-dimethylacrylamide) (PDMA) solution on a glass substrate shows a somewhat unusual behavior. In this system, the contact line starts to recede at an early stage of drying, and as it recedes, it leaves a macroscopic polymer film behind. The resulting film has a volcano-like profile, but the peak is not located at the edge of the film but in the middle of the film. We studied the drying process changing the polymer concentration and the wetting property of the substrate. We found that the onset of the contact-line recession depends upon the initial contact angle greatly, while the receding contact angle does not depend upon the initial contact angle. We conjecture that this phenomenon is caused by the Marangoni force, which acts to bring the surface of the solution inward because of the negative dependence of the surface tension on the polymer concentration.

Influence of surface rheology on dynamic wetting of droplets coated with insoluble surfactants

Wetting phenomena are crucial to many problems, including wetting and dewetting in the tear film in the eye. Here we introduce insoluble surfactants to the surface of advancing droplets of pure water. Three pure surfactants were chosen with similar Gibbs elasticity but distinct interfacial shear properties. The effects of arachidyl alcohol, a Newtonian monolayer, on dynamic wetting are compared to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesteryl myristate, which both form monolayers with shear elasticity. In addition, meibomian lipids, a natural mixture of lipids found in the tear film, were also studied. We show that while droplets covered with arachidyl alcohol follow classical hydrodynamics, surface shear elasticity introduces unique behavior including stick–release phenomena at low velocities and non-ideal behavior at higher velocities.

Advancing-drying droplets of polymer solutions: Local increase of the viscosity at the contact line

We present experimental results concerning the advancing motion of drops of polymer solutions in the presence of controlled evaporation. We find that at high advancing velocities the classical Cox-Voinov law is verified, i.e. the advancing contact angle varies linearly with the capillary number. Below a critical velocity the contact angle increases as the advancing velocity is reduced. These results can be explained by taking into account the divergence of the rate of evaporation close to the contact line leading to an accumulation of polymer close to the edge of the drop. The induced local increase of the viscosity explains the increase of the contact angle. We show that the accumulation of polymer over a few nanometers is sufficient to slow down the contact line.

Light induced flows opposing drainage in foams and thin-films using photosurfactants†

We study the influence of UV light on the drainage flows of foams and thin-liquid films stabilized by photoswitchable azobenzene surfactants, whose shape and hydrophobicity can be modified using UV illumination. This model system, the dynamics of which was well characterized in a previous study, enables us to trigger a controlled variation of the surface excess and surface tension. In both geometries we observe light-induced flows which are able to suppress the drainage flow induced by gravity. However, we show that the physical origin of the flows is different in both geometries. At the scale of a few films in the so-called ‘two-bubble’ experiment the comparisons of the physical length scales, i.e. the radius of the meniscus and the film thickness, to the chemical “reservoir length” (Γ/c) show that the flux of the surfactant at the interface in the presence of UV light is different in the films and in the meniscus, inducing a Marangoni flow from the meniscus to the film, which is stronger than gravity and capillary suction. The velocity of this flow can be tuned by the light intensity and the surfactant concentration. In the real foams, however, we show that the above mechanism is not relevant because the radii of curvature of the Plateau borders are orders of magnitude lower than in the two-bubble experiment, thus the capillary suction prevents such transfer between the films and the Plateau borders. Instead, the decrease of the drainage velocity is shown to be due to a gradient of the surface tension in the illuminated zone hence to a local variation of the capillary pressure. This study underlines the importance of characterizing the radius of the Plateau borders for the understanding of foams, as this key parameter sets the order of magnitude of capillary pressure, film thickness and amount of available surfactant. We also show that this photosurfactant is a new toolbox for the understanding of foam stability.

The evaluation of interdigitated array electrodes for measurement of catecholamines and indoleamines.

The use of Interdigitated Array (IDA) Microelectrodes for detection of low levels of biogenic amines has been demonstrated in stationary solutions and flow systems [M. Morita, et al., Electrochemica Acta 42 (20--21) (1997) 3177--3183]. This technique is highly sensitive. We have evaluated this technology as applied to High Pressure Liquid Chromatography with Electrochemical Detection (HPLC-EC) for analysis of microdialysate and tissue samples. With this new technology we demonstrated a x 10 fold increase in sensitivity in comparison to our existing technology. We are now able to detect dopamine at a level of 53 x 10(-18) moles on column and serotonin at 26 x 10(-18) moles on column. This technology now permits analysis of biogenic amines in samples from brain areas not previously amenable to this type of experiment.

One-Step Fabrication of pH-Responsive Membranes and Microcapsules through Interfacial H-Bond Polymer Complexation

Biocompatible microencapsulation is of widespread interest for the targeted delivery of active species in fields such as pharmaceuticals, cosmetics and agro-chemistry. Capsules obtained by the self-assembly of polymers at interfaces enable the combination of responsiveness to stimuli, biocompatibility and scaled up production. Here, we present a one-step method to produce in situ membranes at oil-water interfaces, based on the hydrogen bond complexation of polymers between H-bond acceptor and donor in the oil and aqueous phases, respectively. This robust process is realized through different methods, to obtain capsules of various sizes, from the micrometer scale using microfluidics or rotor-stator emulsification up to the centimeter scale using drop dripping. The polymer layer exhibits unique self-healing and pH-responsive properties. The membrane is viscoelastic at pH = 3, softens as pH is progressively raised, and eventually dissolves above pH = 6 to release the oil phase. This one-step method of preparation paves the way to the production of large quantities of functional capsules.

Giant Electrostrictive Response and Piezoresistivity of Emulsion Templated Nanocomposites

Using an emulsion road and optimizing the dispersion process, we prepare polymer carbone nanotubes (CNT) and polymer reduced graphene oxide (rGO) composites. The introduction of conductive nanoparticles into polymer matrices modifies the electronic properties of the material. We show that these materials exhibit giant electrostriction coefficients in the intermediate filler concentration (below 1 wt %). This makes them very promising for applications such as capacitive sensors and actuators. In addition, the values of the piezoresistivity measured in the high filler concentration situation are at least an order of magnitude greater than the one reported in the literature. This opens the way to use these materials for stress or strain sensor applications considering their giant responses to mechanical deformations.