Pascal Damman

In situ tuning the optical properties of a cavity by wrinkling

In this letter we propose an original, in situ, approach to tune the optical properties of an optical cavity, based on the wrinkling of compressed metal/polymer multilayer thin films. This phenomenon is conceptually described, simulated, and experimentally confirmed. The main idea is to use wrinkling to modulate the effective refractive index of the upper interface. This modulation induces a spectral shift of the cavity modes. The work presented here constitutes a first step to the development of stretchable and curved photonics.

Wrinkling Hierarchy in Constrained Thin Sheets from Suspended Graphene to Curtains

We show that thin sheets under boundary confinement spontaneously generate a universal self-similar hierarchy of wrinkles. From simple geometry arguments and energy scalings, we develop a formalism based on wrinklons, the localized transition zone in the merging of two wrinkles, as building blocks of the global pattern. Contrary to the case of crumpled paper where elastic energy is focused, this transition is described as smooth in agreement with a recent numerical work [R. D. Schroll, E. Katifori, and B. Davidovitch, Phys. Rev. Lett. 106, 074301 (2011)]. This formalism is validated from hundreds of nanometers for graphene sheets to meters for ordinary curtains, which shows the universality of our description. We finally describe the effect of an external tension to the distribution of the wrinkles.

Highly sensitive detection of molecular interactions with plasmonic optical fiber grating sensors.

Surface Plasmon resonance (SPR) optical fiber biosensors constitute a miniaturized counterpart to the bulky prism configuration and offer remote operation in very small volumes of analyte. They are a cost-effective and relatively straightforward technique to yield in situ (or even possibly in vivo) molecular detection. The biosensor configuration reported in this work uses nanometric-scale gold-coated tilted fiber Bragg gratings (TFBGs) interrogated by light polarized radially to the optical fiber outer surface, so as to maximize the optical coupling with the SPR. These gratings were recently associated to aptamers to assess their label-free biorecognition capability in buffer and serum solutions. In this work, using the well-acknowledged biotin-streptavidin pair as a benchmark, we go forward in the demonstration of their unique sensitivity. In addition to the monitoring of the self-assembled monolayer (SAM) in real time, we report an unprecedented limit of detection (LOD) as low as 2 pM. Finally, an immunosensing experiment is realized with human transferrin (dissociation constant Kd~10(-8) M(-1)). It allows to assess both the reversibility and the robustness of the SPR-TFBG biosensors and to confirm their high sensitivity.

Hierarchical wrinkling patterns

This paper reports a simple and flexible method for generating hierarchical patterns from wrinkling instability. Complex features with gradually changing topographies are generated by using the spontaneous wrinkling of a rigid membrane (titanium) on a soft foundation (polystyrene) compressed via the diffusion of a solvent. We show that the morphology of these unreported wrinkled patterns is directly related to the rheological properties of the polymer layer and the geometry of the diffusion front. Based on these ingredients, we rationalize the mechanism for the formation of hierarchical wrinkling patterns and quantify our experimental findings with a simple scaling theory. Finally, we illustrate the relevance of our structuration method by studying the mechanosensitivity of fibroblasts.

Wrinkle to fold transition: influence of the substrate response

Spatially confined rigid membranes reorganize their morphology in response to imposed constraints. Slight compression of a rigid membrane resting on a soft foundation creates a regular pattern of sinusoidal wrinkles with a broad spatial distribution of energy. For larger compression, the deformation energy is progressively localized in small regions which ultimately develop sharp folds. We review the influence of the substrate on this wrinkle to fold transition by considering two models based on purely viscous and purely elastic foundations. We analyze and contrast the physics and mathematics of both systems.

Superhydrophobic Aluminum Surfaces by Deposition of Micelles of Fluorinated Block Copolymers

Superhydrophobic surfaces are generated by chemisorption on aluminum substrates of fluorinated block copolymers synthesized by reversible addition-fragmentation chain transfer in supercritical carbon dioxide. In an appropriate solvent, those block copolymers can form micelles with a fluorinated corona, which are grafted on the aluminum substrate thanks to the presence of carboxylic acid groups in the corona. Water contact angle and drop impact analysis were used to characterize the wettability of the films at the macroscale, and atomic force microscopy measurements provided morphological information at the micro- and nanoscale. The simple solvent casting of the polymer solution on a hydroxylated aluminum surface results in a coating with multiscale roughness, which is fully superhydrophobic over areas up to 4 cm(2).

Static and quasielastic light scattering from solutions of poly(ethylene oxide) in methanol

Abstract Static and quasielastic light scattering measurements on dilute and semi-dilute solutions of monodisperse poly(ethylene oxide) in methanol have been carried out at 25°C. Four PEO fractions having weight average molecular weights equal to 31 500, 90 000, 230 000 and 904 000 and degrees of polydispersity lower than 1.1 were used. The angular dependence of Zimm plots shows no downturn at low angles. In addition, monomodal distribution curves were computed from the quasielastic measurements, confirming that PEO are molecularly dispersed in methanol at 25°C. Experimental scaling laws for the radius of gyration R G , the second virial coefficient A 2 and the hydrodynamic radius R H have been determined. The exponents characterizing these scaling laws confirm that methanol is a good solvent for this polymer. Finally, the overlap concentrations corresponding to the transition between the dilute and semi-dilute regimes were determined for the different PEO samples.

Characterization of molecular heterogeneities of LLDPE by multiple crystallization–dissolution steps

Abstract This article demonstrates that a fractionation technique, based on molecular segregation during the crystallization from the melt and dissolution of the crystals, allows a complete characterization of the molecular heterogeneity in the comonomer distribution of linear low density polyethylene. An ethylene–1-hexene copolymer with a bimodal short chain branching distribution has been fractionated by multiple step crystallizations followed by selective dissolutions in tetrachloroethylene. The relative amount of each component of the distribution in each fraction was determined from the melting curves of isothermally crystallized fractions.

Adhesion of soft viscoelastic adhesives on periodic rough surfaces

We experimentally study adhesion of soft viscoelastic materials on surfaces with periodic roughness. The materials tested are partially crosslinked PDMS layers with a low shear elastic modulus (below 0.1 MPa) and variable viscoelasticity. We use an elastic surface instability to produce wrinkled PDMS surfaces. The hard periodic surfaces are then molded in Norland resin from the master surfaces. The periodic surface roughness is typically of the order of 100 microns. For identical surface roughness, we can use two surface topologies: peaks and holes. Using our model material and the model surfaces, we study in detail the interplay between the elastic penalties needed to conform the adhesive to the rough surface and the enhanced viscous dissipation during debonding. In this way we show, that adhesion on these surfaces can either be enhanced or decreased as a function of the surface topology and the viscoelastic properties of the material.

Confined wrinkling: impact on pattern morphology and periodicity

Wrinkling instabilities are observed in compressed multilayers made of materials with contrasted properties. Here, we study the influence of confinement, i.e. using foundations with dimensions smaller than 50 nm, on the wrinkled patterns. We show that confinement generates a change in morphology from labyrinths to a dotted pattern with hexagonal symmetry. This transition occurs when the amplitude of the wrinkles becomes comparable to the thickness of the upper membrane, i.e. A ≃ h. For very thin foundations (h < 100 nm), a drastic deviation from the usual evolution of the wavelength with the foundation thickness is observed. This new regime was explained by considering the van der Waals interactions between the metal layer and the substrate. The VDW energy term was involved in a new scaling law model that agrees with the experimental data.