Éric Freyssingeas

Excess Area in Fluctuating-Membrane Systems

Using neutron and high-resolution X-ray scattering, the repeating distances of dilute lyotropic lamellar phases have been measured as a function of the membrane volume fraction, for several ternary or quaternary systems. In the case of systems dominated by undulation forces, a systematic logarithmic deviation is observed. It is attributed to the excess area coming from the undulations. A method for measuring the membrane bending elastic constant κ is proposed.

The effect of water thickness on the bending rigidity of inverted bilayers

The effect of water thickness on the mean curvature modulus of an inverted bilayer has been investigated. Our system is a lamellar phase made up of a series of water films each surrounded by two ionic surfactant monolayers and separated with hydrophobic solvent. The elastic constant has been estimated using the excess-area method and it is shown to decrease when the thickness of the water layer increases. This result cannot be interpreted in terms of electrostatic effects - an increase of is expected instead - but may arise from a coupling between short-range surfactant interactions and curvature strains.

High-frequency rheological behaviour of a multiconnected lyotropic phase

High-frequency (up to ω = 6 104 rad/s) rheological measurements combined with light-scattering investigations show that an isotropic and multiconnected phase of surfactant micelles exhibits a terminal relaxation time of a few μs, much smaller than in solutions of entangled wormlike micelles. This result is explained in terms of the local hexagonal order of the microscopic structure and we discuss its relevance for the understanding of dynamic behaviour in related systems, such as wormlike micelles and sponge phases.

Dynamics of bulk fluctuations in a lamellar phase studied by coherent x-ray scattering

Using x-ray photon correlation spectroscopy, we studied the layer fluctuations in the lamellar phase of an ionic lyotropic system. We measured the relaxation rate of in-plane (undulation) fluctuations as a function of the wave vector. Static and dynamic results obtained during the same experiment were combined to yield the values of both elastic constants of the lamellar phase (compression and bending moduli) as well as that of the sliding viscosity. The results are in very good agreement with dynamic light-scattering data, validating the use of the technique in ordered phases.

Structure, thermodynamics and dynamics of the isotropic phase of spherical non-ionic surfactant micelles

We investigate a non-ionic surfactant (C(12)E(8))/water binary mixture, over a wide range of concentrations and temperatures (i.e. 1-35 wt.% of C(12)E(8) and 10-60 °C in temperature) by means of different experimental techniques: Small-Angle Neutron Scattering (SANS), Quasi Elastic Light Scattering (QELS) and High Frequency Rheology. The aims of this work are to provide information on structure, thermodynamics and dynamics of the isotropic phase of such a micellar system and, by combining these different types of information, to obtain a comprehensive image of the behaviour of this phase. Our results demonstrate that structural, thermodynamic and dynamic properties of these solutions are fully monitored by the temperature-induced changes in the ethylene-glycol chain hydration. They confirm that C(12)E(8) micelles are spherical and do not grow in the investigated range of concentrations and temperatures. They demonstrate that the interaction potential between C(12)E(8) micelles is more complicated than what was previously described, with an additional repulsive interaction. They allow us to put forward explanations for the Isotropic-Ordered phase transition as well as for the temperature behaviour of the viscosity of C(12)E(8) micellar solutions. Our investigation provides new and valuable information on the dynamics of these mixtures that reflect the complexity of the interaction potential between the C(12)E(8) micelles. It shows that concentrated solutions exhibit a viscoelastic behaviour that can be described by a simple Maxwell model.

Communications: Short-range dynamics of a nematic liquid-crystalline phase

Using x-ray photon correlation spectroscopy, we studied the dynamics in the nematic phase of a nanorod suspension. The collective diffusion coefficient in the plane perpendicular to the director varies sharply with the wave vector. Combining the structure factor and the diffusion coefficient, we find that the hydrodynamic function of the phase decreases by more than a factor of 10 when going from length scales comparable to the interparticle distance toward larger values. Thus, the collective dynamics of the nematic phase experiences strong and scale-dependent slowing down, in contrast with isotropic suspensions of slender rods or of spherical particles.

Dynamic light scattering as an investigating tool to study the global internal dynamics of a living cell nucleus

Recent progress in cellular biology has shown that the nucleus of a living cell is a structured integration of many functional domains with a complex spatial organization. This organization, as well as molecular and biochemical processes are time regulated. Despite a growing interest, the global internal dynamics of the nucleus is still unknown. In this letter, we show that this dynamics can be investigated successfully by means of dynamics light scattering experiments.

Structural transition in the isotropic phase of the C12EO6/H2O lyotropic mixture: A rheological investigation

We study the structural changes occurring in the isotropic phase of the C12EO6/H2O lyotropic mixture (up to 35% surfactant weight concentration) upon increasing the concentration and temperature, from small individual micelles to an entangled network which subsequently becomes connected. High-frequency (up to ω = 6 × 104 rad/s) rheological measurements give us access to the viscoelastic relaxation spectrum, which can be well described by the sum of two Maxwell models with very different temperature behaviors:  the slower one (τ1 ≈ 10-4 s) is probably due to reptation, and its associated viscosity first increases with temperature (micellar growth) and then decreases after reaching a maximum (appearance of connections). The fast mechanism (τ2 ≈ 10-6 s) remains practically unchanged in temperature and can be related to the relaxation of local micellar order, as observed at higher concentration in a previous investigation. This interpretation is confirmed by additional measurements in aqueous mixtures of the ...

Slow dynamics of a colloidal lamellar phase

We used x-ray photon correlation spectroscopy to study the dynamics in the lamellar phase of a platelet suspension as a function of the particle concentration. We measured the collective diffusion coefficient along the director of the phase, over length scales down to the interparticle distance, and quantified the hydrodynamic interaction between the particles. This interaction sets in with increasing concentration and can be described qualitatively by a simplified model. No change in the microscopic structure or dynamics is observed at the transition between the fluid and the gel-like lamellar phases.

Evolution of the Global Internal Dynamics of a Living Cell Nucleus during Interphase

Progress in cellular biology based on fluorescent microscopy techniques, shows that the spatial organization of the nucleus is dynamic. This dynamic is very complex and involves a multitude of phenomena that occur on very different time and size scales. Using an original light scattering experimental device, we investigated the global internal dynamics of the nucleus of a living cell according to the phases of the cell cycle. This dynamic presents two different and independent kinds of relaxation that are well separated in time and specific to the phase of the cell cycle.