Colloidal Particle Adsorption at Water-Water Interfaces with Ultralow Interfacial Tension

Using fluorescence confocal microscopy we study the adsorption of single latex microparticles at a water-water interface between demixing aqueous solutions of polymers, generally known as a water-in-water emulsion. Similar microparticles at the interface between molecular liquids have exhibited an extremely slow relaxation preventing the observation of expected equilibrium states. This phenomenon has been attributed to “long-lived” metastable states caused by significant energy barriers ΔF∼γAd≫kBT induced by high interfacial tension (γ∼10−2 N/m) and nanoscale surface defects with characteristic areas Ad≃10–30nm2. For the studied water-water interface with ultralow surface tension (γ∼10−4N/m) we are able to characterize the entire adsorption process and observe equilibrium states prescribed by a single equilibrium contact angle independent of the particle size. Notably, we observe crossovers from fast initial dynamics to slower kinetic regimes analytically predicted for large surface defects (Ad≃500 nm2). Moreover, particle trajectories reveal a position-independent damping coefficient that is unexpected given the large viscosity contrast between phases. These observations are attributed to the remarkably diffuse nature of the water-water interface and the adsorption and entanglement of polymer chains in the semidilute solutions. This work offers some first insights on the adsorption dynamics or kinetics of microparticles at water-water interfaces in biocolloidal systems

The evaluation of interdigitated array electrodes for the 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

Shape relaxation of gelling droplets

International audienceSolidification is a heterogeneous transformation from liquid-to-solid which usually combines transport, phase transition and mechanical strain: predict- ing the shapes resulting from such a complex problem is fascinating and has..

Remote control of the stability of bubbles, thin films and foams using photosensitive surfactants.

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

International audienceWe 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 fil