arXiv subjects

Laurence Ramos

Publications and source records attributed to Laurence Ramos.

42 records · Page 3Linked to original sources

arXiv2006

Collective rearrangement at the onset of flow of a polycrystalline hexagonal columnar phase

Creep experiments on polycrystalline surfactant hexagonal columnar phases show a power law regime, followed by a drastic fluidization before reaching a final stationary flow. The scaling of the fluidization time with the shear modulus of the sample and stress applied suggests that the onset of flow involves a bulk reorganization of the material. This is confirmed by X-ray scattering under stress coupled to \textit{in situ} rheology experiments, which show a collective reorientation of all crystallites at the onset of flow. The analogy with the fracture of heterogeneous materials is discussed.

cond-mat.soft↗

arXiv2005

Slow dynamics in glassy soft matter

Measuring, characterizing and modelling the slow dynamics of glassy soft matter is a great challenge, with an impact that ranges from industrial applications to fundamental issues in modern statistical physics, such as the glass transition and the description of out-of-equilibrium systems. Although our understanding of these phenomena is still far from complete, recent simulations and novel theoretical approaches and experimental methods have shed new light on the dynamics of soft glassy materials. In this paper, we review the work of the last few years, with an emphasis on experiments in four distinct and yet related areas: the existence of two different glass states (attractive and repulsive), the dynamics of systems very far from equilibrium, the effect of an external perturbation on glassy materials, and dynamical heterogeneity.

cond-mat.soft↗

arXiv2005

Polyelectrolyte-induced peeling of charged multilamellar vesicles

We study mixtures of charged surfactants, which alone in solution form uni- and multilamellar vesicles, and oppositely charged polyelectrolytes (PEs). The phase behavior is investigated at fixed surfactant concentration as a function of the PE-to-surfactant charge ratio $x$. We find that, for $x>0$, aggregates form. Light microscopy and X-ray scattering experiments show that the isoelectric point plays a crucial role since the morphology and the microscopic structure of the aggregates are different before ($x\leq1$) and after the isoelectric point ($x>1$). To better understand the dynamics for the formation of PE/surfactant complexes, we perform light microscopy experiments where we follow in real-time the effect of a PE solution on one multilamellar vesicle (MLV). We find that the PE induces a peeling of the bilayers of the MLV one by one. The peeling is accompanied by strong shape fluctuations of the MLV and leads ultimately to a pile of small aggregates. This novel phenomenon is analyzed in detail and discussed in terms of PE-induced tension, and pore formation and growth in a surfactant bilayer.

cond-mat.soft↗

arXiv2004

Intrinsic aging and effective viscosity in the slow dynamics of a soft glass with tunable elasticity

We investigate by rheology and light scattering the influence of the elastic modulus, $G_0$, on the slow dynamics and the aging of a soft glass. We show that the slow dynamics and the aging can be entirely described by the evolution of an effective viscosity, $η_{eff}$, defined as the characteristic time measured in a stress relaxation experiment times $G_0$. At all time, $η_{eff}$ is found to be independent of $G_0$, of elastic perturbations, and of the rate at which the sample is quenched in the glassy phase. We propose a simple model that links $η_{eff}$ to the internal stress built up at the fluid-to-solid transition.

cond-mat.soft↗

arXiv2001

Ultraslow dynamics and stress relaxation in the aging of a soft glassy system

We use linear rheology and multispeckle dynamic light scattering (MDLS) to investigate the aging of a gel composed of multilamellar vesicles. Light scattering data indicate rearrangement of the gel through an unusual ultraslow ballistic motion. A dramatic slowdown of the dynamics with sample age $t_{w}$ is observed for both rheology and MDLS, the characteristic relaxation time scaling as $t_{w}^μ$. We find the same aging exponent $μ=0.78$ for both techniques, suggesting that they probe similar physical processes, that is the relaxation of applied or internal stresses for rheology or MDLS, respectively. A simple phenomenological model is developed to account for the observed dynamics.

cond-mat.soft↗

arXiv1999

Electrostatic Repulsion of Positively Charged Vesicles and Negatively Charged Objects

A positively charged, mixed bilayer vesicle in the presence of negatively charged surfaces (for example, colloidal particles) can spontaneously partition into an adhesion zone of definite area, and another zone that repels additional negative objects. Although the membrane itself has nonnegative charge in the repulsive zone, negative counterions on the interior of the vesicle spontaneously aggregate there, and present a net negative charge to the exterior. Beyond the fundamental result that oppositely charged objects can repel, our mechanism helps explain recent experiments on surfactant vesicles.

cond-mat.soft↗