P. Guenoun

How does dew form

Abstract Dew is a phenomenon that is experienced daily. It is formed of water droplets that imperfectly wet the substrate on which they condense. The formation of dew is connected to rich and numerous physical phenomena: heterogeneous nucleation, phase transitions, heat transfer, wetting. Although the growth of an isolated droplet still remains imperfectly understood, growth of an assembly of drops is better comprehended: the fact that drops coalesce during their growth, and that the substrate (plane, line, fractal …) is of lower dimensionality than that of the drops is at the origin of surprising properties (self-similarity, constant and universal value of the surface coverage, correlation of position). Experiments and numerical simulations of condensation of water vapor on planar or unidimensional solid substrates are presented. Condensation on a liquid substrate (oil) modifies the interactions between drops because the substrate is locally curved; the drops can then self-organize in two-dimensional (he...

New Hydrodynamic Mechanism for Drop Coarsening

We discuss a new mechanism of drop coarsening due to coalescence only, which describes the late stages of phase separation in fluids. Depending on the volume fraction of the minority phase, we identify two different regimes of growth, where the drops are interconnected and their characteristic size grows linearly with time, and where the spherical drops are disconnected and the growth follows (time) 1/3. The transition between the two regimes is sharp and occurs at a well defined volume fraction of order 30%.

Polymersome Shape Transformation at the Nanoscale

Polymer vesicles, also named polymersomes, are valuable candidates for drug delivery and micro- or nanoreactor applications. As far as drug delivery is concerned, the shape of the carrier is believed to have a strong influence on the biodistribution and cell internalization. Polymersomes can be submitted to an osmotic imbalance when injected in physiological media leading to morphological changes. To understand these osmotic stress-induced variations in membrane properties and shapes, several nanovesicles made of the graft polymer poly(dimethylsiloxane)-g-poly(ethylene oxide) (PDMS-g-PEO) or the triblock copolymer PEO-b-PDMS-b-PEO were osmotically stressed and observed by light scattering, neutron scattering (SANS), and cryo-transmission electron microscopy (cryo-TEM). Hypotonic shock leads to a swelling of the vesicles, comparable to optically observable giant polymersomes, and hypertonic shock leads to collapsed structures such as stomatocytes and original nested vesicles, the latter being only observed for bilayers classically formed by amphiphilic copolymers. Complementary SANS and cryo-TEM experiments are shown to be in quantitative agreement and highlight the importance of the membrane structure on the behavior of these nanopolymersomes under hypertonic conditions as the final morphology reached depends whether or not the copolymers assemble into a bilayer. The vesicle radius and membrane curvature are also shown to be critical parameters for such transformations: the shape evolution trajectory agrees with theoretical models only for large enough vesicle radii above a threshold value around 4 times the membrane thickness.

Multiple Emulsions Controlled by Stimuli‐Responsive Polymers

The phase inversion of water-toluene emulsions stabilized with a single thermo- and pH-sensitive copolymer occurs through the formation of multiple emulsions. At low pH and ambient temperature, oil in water emulsions are formed which transform into highly stable multiple emulsions at pHs immediately lower than the inversion border. At higher pHs, the emulsion turns into a water in oil one.

Monte Carlo simulations of star-branched polyelectrolyte micelles

Abstract:The concentration profiles of monomers and counterions in star-branched polyelectrolyte micelles are calculated through Monte Carlo simulations, using the freely jointed chain model. We have investigated the onset of different regimes corresponding to the spherical and Manning condensation of counterions as a function of the strength of the Coulomb coupling. The Monte Carlo results are in fair agreement with the predictions of Self-Consistent-Field analytical models. We have simulated a real system of diblock copolymer micelles of (sodium-polystyrene-sulfonate)(NaPSS)-(polyethylene-propylene)(PEP) with f = 54 hydrophilic branches of N = 251 monomers at room temperature in salt-free solution. The calculated form factor compares nicely with our neutron scattering data.

Crystalline amyloid structures at interfaces.

Laying the groundwork: The interfacial self-assembly properties of an amyloid peptide were used to develop crystalline nanostructures at air-water interfaces, which were studied by both AFM microscopy and X-ray diffraction (see image). These structures generate regular arrays of functional groups and pave the way to controlled deposition of inorganic materials like that observed in biomineralization.

Tailoring nanostructures using copolymer nanoimprint lithography.

The generation of defect-free polymer nanostructures by nanoimprinting methods is described. Long-range nanorheology and shorter-range surface energy effects can be efficiently combined to provide alignment of copolymer lamellae over several micrometers. As an example, a perpendicular organization with respect to circular tracks is shown, demonstrating the possibility of writing ordered radial nanostructures over large distances.

A critical look at surface force measurement using a commercial atomic force microscope in the noncontact mode

The use of commercial atomic force microscopes (AFM) operating in the noncontact mode for surface force measurements is critically reviewed. Approach curves (i.e., vibration amplitude versus tip–surface distance) using standard microfabricated tips are discussed with respect to the basic theory of an equivalent harmonic oscillator. Different artifacts are addressed. In particular, we show theoretically and experimentally that the force exerted by the layer of air confined between the cantilever and the surface is a major contribution to the force on the cantilever. However, by carefully choosing the parameters (essentially the vibration amplitude) for the measurement of the approach curve, and by taking into account the damping within the confined air layer, we succeeded in measuring reliable surface force profiles with commercial AFM in the air and in describing them quantitatively by dispersion force interactions.

Easy Orientation of Diblock Copolymers on Self-Assembled Monolayers Using UV Irradiation

A simple method based on UV/ozone treatment is proposed to control the surface energy of dense grafted silane layers for orientating block copolymer mesophases. Our method allows one to tune the surface energy down to a fraction of a mN/m. We show that related to the surface, perpendicular orientation of a lamellar phase of a PS-PMMA diblock copolymer (neutral surface) is obtained for a critical surface energy of 23.9-25.7 mN/m. Perpendicular cylinders are obtained for 24.6 mN/m and parallel cylinders for 26.8 mN/m.

Growth of droplets on a one-dimensional surface : experiments and simulation

Growth of water droplets (breath figures) undergoing one-dimensional interactions by coalescence is studied by experiments and numerical simulation. The experiments have been performed by condensing water vapor on a scratched silanized glass, and on a surface containing fibers (Iris leaves). Both experiments and simulations confirm the scaling theory which states that if an independent droplet grows as t?s, then droplets in interaction by coalescence grow as t?0 with ?0?=?(?d/(?d - ?s))?s. (?d and ?s are, respectively, the dimensionality of the droplets and substrate.) The behavior of the line coverage has been also investigated and compared to the parking limit.