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Mixed monolayers of GM(1) glycolipid and stearoyl-oleoyl-phosphatidylcholine (SOPC) or dipalmitoyl-phosphatidycholine (DPPC) phospholipids were studied by surface pressure measurements. The effects induced by GM(1) on the mean molecular areas of mixtures and DPPC phase transition were followed for GM(1) concentrations ranging from 1 to 20 mol.%. Under our experimental conditions, one main parameter influencing the behavior of phospholipid-GM(1) monolayers is the ionic strength of the subphase. Mixed monolayers are in a more expanded state on buffer than on pure water. This could be due to a change of GM(1) orientation at the interface. The interaction of wheat germ agglutinin (WGA), a lectin recognizing specifically GM(1), with these monolayers was quantified in terms of the Gibbs equation. Specific WGA-GM(1) interactions are clearly reduced in the presence of DPPC as compared with SOPC, probably because of the higher packing density of these monolayers. Phospholipid-GM(1) monolayers could also undergo some rearrangements induced by WGA binding.

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In this paper, a dynamical force microscopy study of self-assembled monolayers of organosilanes, grafted onto a silica support, is reported. Organosilanes, terminated either with a functional group (ethylene glycol) or with a methyl group, were used. The influence of the reaction time and the solvent composition on the grafting was investigated to improve the homogeneity of the self-assembled monolayers. Numerical simulations of approach−retract curves, obtained in the tapping mode, were performed and compared to experimental ones. Information such as mechanical response and height of the grafted organic layers has been obtained.

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We study a simple model for an amphiphilic layer that may adsorb cosurfactant molecules from a reservoir. Upon varying the length of the cosurfactant, we obtain a discontinuous change of the adsorption rate and a corresponding discontinuity of the bending rigidity kappa. With realistic values for the chemical potential and the interaction energy of the hydrophobic tails, our model accounts quantitatively for the measured rigidity and the discontinuity observed for the ternary system AOT/water/oil and for SDS/alcohol bilayers.

Application of time-resolved femtosecond spectroscopy to the investigation of the ultrafast electron kinetics in metallic materials is reviewed. The main experimental techniques are presented and the results obtained on electron scattering processes discussed in bulk metals and nanoparticles, focusing on the energy redistribution processes (electron–electron and electron–phonon coupling) and electronic transport. Application of the femtosecond techniques to the investigation of the acoustic vibrations of metal films and nanoparticles is also presented.

When sand falling in the spacing between two plates goes past an obstacle, a dynamic dune with a parabolic shape and an inner triangular region of nonflowing or slowly creeping sand forms. The angle of the triangular zone increases with the height of the dune and saturates at a value determined by the geometry of the cell. The width of the dune, related to the radius of curvature at the tip, shows universal features versus its height rescaled by geometrical parameters. The velocity profile in the flowing part is determined and found to be nonlinear. The parabolic shape can be accounted for using a simple driven convection-diffusion equation for the interface.

We describe a new type of low-cost easy-to-use viscometer based on the temperature elevation in a liquid under shear flow. After calibration, this instrument can be used to measure the apparent steady state viscosity for both Newtonian and non-Newtonian liquids with no yield stress. We compute the rise in temperature due to viscous dissipation in a Couette cell and compare it to experimental results for different fluids. We show that the variation of the temperature with shear rate can be used to characterize the rheological behaviour of viscous fluids and to evaluate their viscosity in a large domain, from typically a few cP up to more than 10 P, with an accuracy of about ±5%. In contrast to simple viscometers, non-Newtonian fluids can be studied with this apparatus. We give experimental results for Newtonian and non-Newtonian liquids and show that they are very similar to those given in the literature by using much more sophisticated instruments.

International audience; no abstract

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International audience; no abstract

The Navier-Stokes equation is used to analyze the additional phase delay when an oscillating nanotip touches intermittently an entangled polymer melt. Even when the tip oscillates at frequencies of several hundred kilohertz, it is shown that the inertial contributions are negligible as long as the indentation depth is no more than a few ten nanometers. Consequently, a stationary solution can be used leading to the simple Stokes formula. Two simple geometries of the tip are investigated. A smooth tip apex with a spherical shape and an elongated tip apex that aims at mimicking a single asperity. The tip shape has a drastic influence on the measured viscosity at the local scale. A simple calculation indicates that the viscous force acting against the tip motion may exhibit several different behaviors as a function of the indentation depth. Using the variational principle of least action, we derive the corresponding phase variation of the oscillator as a function of the indentation depth. It is shown that there exist situations for which an absolute value of the local viscosity could be measured.

Dynamic light scattering experiments on lyotropic lamellar phases of brine and surfactant subjected to a flow field have been realized. The obtained results reveal that the relaxation times measured depend strongly on the velocity of the flow. This dependence is indicative of an increase of the effective elasticity modulus K and a decrease of the effective compressibility modulus (-)B of the lamellar phase with the flow velocity. This leads to the conclusion that the shear can induce a suppression of the undulation fluctuations of the bilayers of the lamellar phase. Our results show also that the rigidity of the membranes decreases as the salt concentration of the sample increases.

A virtual NC − AFM machine has been built using the Matlab language. The virtual NC − AFM is identical to a real hybrid machine built with Digital Instruments and Omicron blocks. The role of every subset is described in detail, and special attention is paid to the parameters of the Automatic Gain Control (AGC) which controls the amplitude of the oscillations. The virtual machine is a powerful tool, that allows to solve, without any approximation, non linear coupled differential equations describing the physics of the tip-surface interaction. The machine is then used to study the frequency shift and damping signal in the approach-retract mode. Two types of situations are analyzed: the first one corresponds to the case where no dissipative force is introduced in the tip-surface interaction, the second one deals with dissipative forces. These last ones are here introduced by assuming a mechanical deformation of the surface under the tip. Interesting results are then obtained, in particular the fact that unstability may occur under some particular conditions. Most of the results obtained by simulation are then compared with those of analytical models already published in the literature.

The abrupt increase of the damping coefficient of an oscillating microlever in the proximity of a surface is discussed as a direct consequence of a local relaxation. The power law found and the dependence of the damping coefficient on the oscillation amplitude are in good agreement with the experimental data recorded on a graphite surface. We show that a combined study of the resonance frequency shift and of the dissipation should bring new information on the intrinsic mechanical properties of a surface at a local scale.

International audience; no abstract

We report conductivity measurements on solutions of closed compact monodisperse multilamellar vesicles (the so-called "onion texture") formed by shearing an inverse lyotropic lamellar L(alpha) phase. The conductivity measured in different directions as a function of the applied shear rate reveals a small anisotropy of the onion structure due to the existence of free oriented membranes. The results are analyzed in terms of a simple model that allows one to deduce the conductivity tensor of the L(alpha) phase itself and the proportion of free oriented membranes. The variation of these two parameters is measured along a dilution line and discussed. The high value of the conductivity perpendicular to the layers with respect to that of solvent suggests the existence of a mechanism of ionic transport through the insulating solvent.

This paper is a theoretical and a numerical investigation of the stability of a tip-cantilever system used in noncontact atomic force microscopy (NC-AFM) when it oscillates close to a surface. No additional dissipative force is considered. The theoretical approach is based on a variational method exploiting a coarse grained operation that gives the temporal dependance of the non-linear coupled equations of motion in amplitude and phase of the oscillator. Stability criterions for the resonance peak are deduced and predict a stable behavior of the oscillator in the vicinity of the resonance. The numerical approach is based on the results obtained with a virtual NC-AFM developed by our group. The effect of the size of the stable domain in phase is investigated. These results are in particularly good agreement with the theoretical predictions. Also they show the influence of the phae shifter in the feedback loop and the way it can affect the damping signal.

We report the effect of shear flow on a phase-separated system composed of lyotropic lamellar (L(alpha)) and sponge (L3) phases in a mixture of brine, surfactant, and cosurfactant. Optical microscopy, small-angle light, and x-ray scattering measurements are consistent with the existence of a steady state made of multilamellar ribbon-like structures aligned in the flow direction. At high shear rates, these ribbon-like structures become unstable and break up into monodisperse droplets resulting in a shear-thickening transition.

International audience; no abstract

The generation of non-classical states of light is of fundamental scientific and technological interest. For example, 'squeezed' states enable measurements to be performed at lower noise levels than possible using classical light. Deterministic (or triggered) single-photon sources exhibit non-classical behaviour in that they emit, with a high degree of certainty, just one photon at a user-specified time. (In contrast, a classical source such as an attenuated pulsed laser emits photons according to Poisson statistics.) A deterministic source of single photons could find applications in quantum information processing, quantum cryptography and certain quantum computation problems. Here we realize a controllable source of single photons using optical pumping of a single molecule in a solid. Triggered single photons are produced at a high rate, whereas the probability of simultaneous emission of two photons is nearly zero--a useful property for secure quantum cryptography. Our approach is characterized by simplicity, room temperature operation and improved performance compared to other triggered sources of single photons.

We theoretically describe and experimentally explore the kinetics of holographic grating formation resulting from different laser-induced phase separation mechanisms. Our method makes use of two interfering c.w. laser waves to quench binary mixtures in composition, and to optically trap the nucleated domains on the fringes. Essentially, two different processes can lead to these variations in concentration: electrostriction and thermodiffusion. The former originates from induced dipolar couplings in a field gradient; as photopolymerization, this is a local process which is essentially sensitive to the ${q=q}_{0}$ Fourier mode forced by the fringe modulation. The latter corresponds to a variation in composition driven by a small thermal gradient; as solvent evaporation and thermal heating techniques, it is nonlocal and behaves as ${1/q}^{2}$ because of its dissipative origin. By making experiments in both cases, we show that this q dependence on excitation has a strong influence on the performance of holographic gratings. While in the first case reflectivity saturates because the phase transition is confined by the fringes which behave as separated optical boxes with ``soft walls'' which calibrate the droplet size, blurring is expected for fringe-trapped domains induced by a nonlocal phase transition because the transition is governed by the Gaussian shape of the pump beams, and nucleated domains can reach a much larger size than the fringe spacing. The good agreement observed with our general model clearly illustrates how to make the difference between local and nonlocal excitations, and offers a first step towards a unified description of holographic grating formation monitored by phase transitions.

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The penetratin peptide, a 16 amino acid sequence extracted from Antennapedia homeodomain, is able to translocate across a neural cell membrane through an unknown mechanism, most likely a non-specific interaction with membrane lipids. Beyond its potential application as vector targeting small hydrophilic molecules and enabling them to reach a cell nucleus, this observation raises intriguing questions concerning the physico-chemistry of peptide–lipid interactions. Here we present a study of the role of lipid surface pressure and head charge on the mechanism of interaction. This was performed using optical techniques: surface infrared spectroscopy and ellipsometry, applied to a monolayer of phospholipids deposited at the air–water interface. Determination of the structure and orientation of peptides and lipids (separately or together) evidenced that electrostatic rather than amphiphilic interactions determine the peptide adsorption and its action on lipids.

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We study the tail contributions to the elastic constants of an amphiphilic layer. For dense systems the terms arising from the attractive van der Waals interactions prevail, whereas the translational entropy of the liquid layer is essential at lower density. Both the membrane rigidity kappa and the Gaussian bending elastic constant kappa; strongly vary with the density of the hydrocarbon chains; for dense systems, they are of the order 100kT with a ratio kappa;/kappa = -2 / 3. At lower density, partial cancellation of interaction and entropic contributions leads to elastic constants of the order of kT, with a ratio close to the value -2 observed for microemulsions.

The experimental systems considered in this paper are isobaric binary mixed crystals of n-alkanes in the crystalline forms Op, Mdci and Mdep, which, on the temperature scale, precede mixed crystals in the rotator forms. The properties studied are the thermodynamic mixing properties, actually the excess enthalpy and excess entropy. For 14 binary systems in the range C17 to C28, quantitative thermodynamic data have been obtained by studying the transition to the rotator forms, for which the thermodynamic mixing properties had been determined earlier. The most important outcome of the investigation is that the complete set of systems is characterised by (i) a uniform ratio between excess enthalpy and excess entropy, having the value of 335 K, and (ii) a uniform dependence of the excess enthalpy on relative difference between number of carbon atoms in the molecules of the components of the system.

International audience; no abstract

International audience; no abstract

We have designed a simple mid-IR femtosecond OPA seeded by a white-light continuum and delivering very stable, largely tunable and energetic output pulses. This OPA requires fewer than 200 μJ of 120-fs Ti:sapphire pump pulses as energy. This system allowed us to experimentally compare the performances of potassium crystals (KTP, KTA, and KNbO3) for the generation of up to 100-fs Fourier-transform-limited pulses tunable in 1–4.6 μm mid-IR wavelengths.

The flow-structure relation of lamellar phases is studied using rheometry and cross-polarized microscopy under flow. The equilibrium phases show different defects. Low salinities lead to very viscous, "onion" phases, whereas at high salinity, a low viscosity plane lamellar phase is found. Under shear, the latter shows a sudden transition to a viscoelastic gel, with a texture and viscosity very similar to that of the onions. Gelation occurs after a certain delay time, increasing rapidly with salinity, by the nucleation of onions. This allows one to relate the delay time to the defect energy.

We have investigated a point mutant of the HIV-1 fusion peptide in a compressed monolayer at the air-water interface. A variety of surface sensitive techniques were applied to study structural features under conditions mimicking the hydrophobic/hydrophilic environment of a biomembrane. Possible partitioning into the aqueous bulk phase and molecular areas were examined by surface activity based mass conservation plots. This shows that the peptide is practically fully accumulated in the interface. Secondary structure and orientation was analyzed by means of polarized infrared reflectivity. Brewster angle microscopy and scanning force microscopy contributed nanostructural images. At low surface pressures the molecules form anti-parallel beta-sheets lying flat on the interface. Upon a moderate increase of the lateral pressure a flat beta-turn structure appears with inter- and intramolecular H-bonds. We also observed aggregates forming fingerprint-like structures with a diameter of approximately double the hydrophobic length of a beta-turn conformation. Beyond approximately 18 mN m(-1) the beta-turns straighten up. The lowest measured tilt angle was 45 degrees at 36 mN m(-1).

Single planar bilayers of DMPC free-standing in air (black films) have been studied by confocal micro Raman spectroscopy in the 600−4000 cm-1 spectral range. The DMPC vibrational bands are well-defined. This allows the derivation of two parameters associated with the packing and mobility and with the fluidity of the alkyl chains and to quantify the rigidity enhancement accompanying the Lα liquid crystalline to gel phase transition (Tm = 24 °C). The intensity of the water band allows to evaluate the thickness of the interstitial aqueous core which is 22 ± 4 A in the Lα phase. This water band exhibits a spectral change relative to bulk liquid water which is significant of the confinement of water molecules in the aqueous core.

International audience; no abstract

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We show the existence under shear flow of steady states in a two-phase region of a brine-surfactant system in which lyotropic dilute lamellar (non-Newtonian) and sponge (Newtonian) phases are coexisting. At high shear rates and low sponge phase-volume fractions, we report on the existence of a dynamic transition corresponding to the formation of a colloidal crystal of multilamellar vesicles (or "onions") immersed in the sponge matrix. As the sponge phase-volume fraction increases, this transition exhibits a hysteresis loop leading to a structural bistability of the two-phase flow. Contrary to single phase lamellar systems where it is always 100%, the onion volume fraction can be monitored continuously from 0 to 100 %.

Ultrathin and self-supported films of sodium dodecyl sulfate (SDS) constituted of two surfactant monolayers separated by an interstitial layer of water have been studied using a confocal micro-Raman spectrometer. It appears to be very convenient to follow the draining of water and later on to characterize the final equilibrium state of the film with thicknesses ranging from 100 to less than 50 A, depending on the experimental conditions. In spite of the very weak Raman signal emitted by these films (less than 1 count/s), it is shown that it is possible to obtain well defined spectra and consequently to draw direct and quantitative information on the residual amount of water and about the conformational and lateral order of the aliphatic surfactant chains.

Noncollinear frequency conversion of two femtosecond pulses is used to characterize the nonlinear profile induced in thermally poled fused silica. This technique permits one to deduce the nonlinear profile and the charge distribution induced by the poling treatment. These data and the current that passes through the sample during the poling are consistent with the occurrence of a built-in electric field associated with the diffusion of two ionic species.

A calibration procedure is presented to relate Fourier transform infrared reflection-absorption spectroscopy (IRRAS) spectra obtained by polarization modulation (PM) and conventional techniques. This procedure concerns only PM-IRRAS measurements obtained with a lock-in amplifier. At the first stage, it consists in measuring two calibrated spectra, by adding a linear polarizer after the sample. The corresponding corrected PM-IRRAS spectra display band intensities close to those observed on IRRAS spectra (at about 6%), but the value of the baseline is lower. The calibration procedure can be performed at the second stage, by normalizing the corrected PM-IRRAS spectra of the sample and the bare substrate. In this case, the band intensities and the baseline of the normalized corrected PM-IRRAS and IRRAS spectra are similar.

The various crystalline forms of an original bicyclic compound [ethyl (2‐chloromethyl‐2,3‐dihydro‐5 H ‐oxazolo[3,2‐ a ]pyrimidin‐5‐one)‐6‐carboxylate); EOC] have been obtained and characterized by powder and single‐crystal X‐ray diffraction, differential scanning calorimetry (DSC), and infrared (IR) and Raman spectroscopy. At 4°C in methanol, a monoclinic racemate (form II) crystallized from the racemic mixture, whereas at 20°C, an orthorhombic racemate (form I) was isolated in trichloroethylene. By increasing the temperature, a solid–solid transition from the stable form II to the stable form I was observed with a Guinier–Simon camera. A I → II transformation was observed at ambient temperature by DSC. © 2000 Wiley‐Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 89: 1496–1504, 2000

2-Chloronaphthalene−octafluoronaphthalene, 2-methylnaphthalene−octafluoronaphthalene binary systems and 2-chloronaphthalene−2-methylnaphthalene−octafluoronaphthalene ternary system have been assessed by differential scanning calorimetry and X-ray powder diffraction. Each binary system presents a 1:1 complex. The pseudobinary section of the ternary system shows a new phase which is probably ordered and may behave as a “complex of complexes”. Finally, ternary alloys show the turn over from ordered to random lattices. Thermodynamic properties of these systems are analyzed through the associated liquid model and the line-compound formalism.

Intense laser light is often represented as a classical Maxwell wave. At low intensities, however, light absorption leads to discrete detection events in photon-counting detectors, or to shot noise in the photodetector current. Although these and many such observations can be interpreted in a semiclassical frame where matter is quantized and waves are still classical, subtler experiments have shown that light, as well as matter, is a quantum object, and that photons have physical reality [1]. The quantum nature of light entails Heisenberg uncertainty relations between two conjugate variables in the harmonic oscillator Hamiltonian of each mode. These so-called quadratures can be the phase and the amplitude of the field. In normal light, e.g. laser light, the noise is equally distributed on the two quadratures. But if the noise on one quadrature is reduced — at the cost of increased noise on the other quadrature — one gets a new state of radiation, called squeezed light [2].

International audience; no abstract

A thermally controlled transport device was designed and tested. As hot food needs to be transported at temperatures between 60 and 70ºC in order to avoid contamination by microorganisms, the use of Molecular Alloy Phase Change Materials (MAPCM) can lead to improvements in this field of application. A heat transfer numerical simulation of the box used for transporting the food was conducted. Despite obvious simplifications, a good agreement between numerical simulation and experimental results was obtained. Furthermore, we compared our experimental results with those from other experiments related to the transport of hot food. Here, pizza is taken as the example, and it is shown that delivering time can be increased three-fold.Para evitar la proliferación de microorganismos, los alimentos cocinados deben ser transportados a temperaturas entre 60 y 70ºC. Los Materiales con Cambio de Fase a base de Aleaciones Moleculares (MAPCM) representan una solución en este tipo de aplicaciones. Para ello hemos diseñado y probado un contenedor que permite el transporte a temperatura controlada. Se ha realizado la experimentación y simulación numérica de la transferencia de calor en el dispositivo con el objetivo de determinar su rendimiento. A pesar de las necesarias simplificaciones, hemos obtenido un buen acuerdo entre resultados experimentales y de simulación. En este trabajo hemos tomado el transporte de pizzas como ejemplo, mostrando que el tiempo de protección térmica puede ser incrementado utilizando aleaciones moleculares.

By using a pairwise-interaction approach in which homomolecular energies depend on the composition, and heteromolecular energies on a long-range order parameter, a unified model able to explain the thermodynamic properties of random and ordered molecular alloys is obtained. The resulting Gibbs free energy describes both the asymmetry of mixing properties of random alloys and the coexistence of the immiscibility and long-range order through three independent parameters. The model is tested successfully in more than thirty binaries belonging to aromatic or aliphatic families.

In the Born–Oppenheimer approximation and considering a Debye nuclear motion, a theoretical computation of pump–probe two-beam coupling in liquids using femtosecond chirped pulses is proposed. This technique makes it possible to specifically evidence the non-instantaneous contribution to the third-order susceptibility χ (3) . Our model, which is an extension at the femtosecond scale of the one proposed by Dogariu et al., describes the temporal evolution of the probe signal as a function of different parameters such as the linear laser chirp, the ratio between the pulse duration and the nuclear response time. Experimentally, this method is applied to characterize the non-instantaneous χ (3) contribution in transparent liquids such as CS 2 , benzene and toluene. Time resolved pump–probe coupling data using parallel and perpendicular linear polarizations fit well with the model developed. The experimental ratio R between both fast and slow non-instantaneous χ (3) XXXX and χ (3) XYYX elements of the tensor is equal to 1.33±0.01 in all the liquids studied, and is in good agreement with the expected liquid nuclear symmetry.