Etude de nanoménisques par AFM et MEB : hydrodynamique de la couche visqueuse, élasticité de l'interface et dynamique de la ligne de contact

The recent development of nanofluidic raises many issues about laws and characteristic lengths governing hydrodynamics and wetting at the nanometer scale. To address this issue, we used advanced microscopy techniques to probe the liquid/air interface with unconventional tips. The oscillating frequency modulation mode (FM-AFM) of the Atomic Force Microscope (AFM) gives independent access to the force applied by the liquid during an approach-withdrawal ramp, and to the conservative and dissipative components of the tip-meniscus interaction. Additional experiments conducted by electron microscopy (SEM) helped visualizing the shape of nanomeniscus to measure the resulting capillary force.The viscous layer set in motion by the oscillation of the tip is studied first. The friction coefficient and the added mass are measured by AFM-FM as a function of the viscosity of the liquid and of the excitation frequency. A model based on a classical description reflects quantitatively all experimental results enabling an evaluation of the velocity field caused by the nanoprobe.The developed methods also served to study the properties of the liquid interface. Nanomeniscus profile is modeled and validated through SEM observations. The stiffness measured experimentally by FM-AFM and described theoretically shows a logarithmic dependence with the lateral extension of the meniscus.Preliminary results are also obtained with carbon tips on which the contact line slides, giving access to the energy dissipation in the nanomeniscus and at the contact line, as well as to the anchoring of single defaults, an open issue of wetting physics.This study demonstrates that FM-AFM and SEM are relevant tools to probe quantitatively the properties of liquids at the nanoscale, opening the way for systematic studies on wetting at the nanoscaleLe développement récent de la nanofluidique pose de nombreuses questions concernant les lois et longueurs caractéristiques qui régissent l’hydrodynamique et le mouillage à l’échelle du nanomètre. Pour aborder ce sujet, nous avons utilisé des techniques de microscopies avancées en sondant l’interface liquide/air à l’aide de pointes non conventionnelles. L’AFM utilisé dans le mode oscillant modulation de fréquence (FM-AFM) donne accès, de manière indépendante, à la force exercée par le liquide pendant une approche-retrait, et aux composantes conservatives et dissipatives de l’interaction pointe-ménisque. Des expériences complémentaires menées en microscopie électronique (MEB) permettent de visualiser le nanoménisque créé et de mesurer la force capillaire résultante. La couche visqueuse entraînée par l’oscillation de la pointe est d’abord étudiée. Le coefficient de friction et la masse ajoutée sont mesurés par FM-AFM en fonction de la viscosité des liquides et de la fréquence d’excitation. Un modèle basé sur une description classique rend compte quantitativement de l’ensemble des résultats expérimentaux permettant ainsi une évaluation du champ de vitesse entraîné par la nanosonde.Les méthodes développées ont permis d’étudier les caractéristiques de l’interface liquide. Le profil du nanoménisque est modélisé et validé grâce aux observations MEB. La raideur du ménisque mesurée expérimentalement par FM-AFM et décrite théoriquement démontre une dépendance logarithmique avec l’extension latérale du ménisque.Des résultats préliminaires sont également obtenus avec des pointes de carbone sur lesquelles glisse la ligne de contact, donnant accès à la dissipation dans le nanoménisque et à la ligne de contact, ainsi qu’à l’ancrage sur des défauts uniques, un des problèmes ouverts de la physique du mouillage.Cette étude démontre que le FM-AFM et le MEB sont des outils pertinents pour sonder quantitativement les propriétés des liquides à l’échelle nanométrique, ouvrant la voie à des études systématiques sur le mouillage à l’échelle nanométriqu

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

Redox-Triggered Control of Cell Adhesion and Deadhesion on Poly(lysine)- g -poly(ethylene oxide) Adlayers

International audienceSpontaneous adsorption of poly(lysine)-g-poly(ethyleneglycol) comb-like copolymers (PLL-g-PEG) is a versatile mean to coat substrates with polymer layers that resist cell adhesion. We prepared redox cleavable PLL-g-PEG to switch adhesion on demand. Redox sensitivity was obtained by introducing disulfide linkers between the PLL backbone and PEG strands. This modification was done alone or in combination with an azide-end on the PEG strands that enabled in situ conjugations of adhesion peptides or fluorescent labels (by simple application of commercially available molecules for copper-free click chemistry compatible with cell survival). To balance the functional (adhesion-promoting) vs cell-repellent copolymers, mixed layers of adjusted compositions were obtained by coadsorption from mixed solutions of the cleavable copolymer with non-cleavable and repellant PLL-g-PEG. The deposition of copolymers and quantitative cleavage as triggered by reductive conditions (application of solutions of tris(carboxyethyl)phosphine, dithiothreitol or gluthatione) were characterized by QCM-D, XPS, and fluorescence microscopy. In cell culture conditions, redox-triggered cleavage was obtained by non toxic application of TCEP for a few minutes, enabling either to release cell attachment points (i.e. cleavage of RGD-presenting areas) or to "open" non-specific adherent areas (i.e. transition from PEG-presenting areas to adherent PLL-like coatings)

AFM study of hydrodynamics in boundary layers around micro- and nanofibers

International audienceThe description of hydrodynamic interactions between a particle and the surrounding liquid, down to the nanometer scale, is of primary importance since confined liquids are ubiquitous in many natural and technological situations. In this paper we combine three nonconventional atomic force microscopes to study hydrodynamics around micro-and nanocylinders. These complementary methods allow the independent measurement of the added mass and friction terms over a large range of probe sizes, fluid viscosities, and solicitation conditions. A theoretical model based on an analytical description of the velocity field around the probe shows that the friction force depends on a unique parameter, the ratio of the probe radius to the thickness of the viscous boundary layer. We demonstrate that the whole range of experimental data can be gathered in a master curve, which is well reproduced by the model. This validates the use of these atomic force microscopy modes for a quantitative study of hydrodynamics and opens the way to the investigation of other sources of dissipation in simple and complex fluids down to the submicron scale