Lionel Foret

Electric-Field Induced Capillary Interaction of Charged Particles at a Polar Interface

We study the electric-field induced capillary interaction of charged particles at a polar interface. The algebraic tail of the electrostatic pressure of each charge results in a deformation of the interface u approximately r(-4), where r is the lateral distance. The capillary interaction of nearby particles is repulsive and varies as rho(-6) with their distance rho. As a consequence, electric-field induced capillary forces cannot be at the origin of the secondary minimum observed recently for charged poly(methyl methacrylate) particles at an oil-water interface.

The physics of lipid droplet nucleation, growth and budding

Lipid droplets (LDs) are intracellular oil-in-water emulsion droplets, covered by a phospholipid monolayer and mainly present in the cytosol. Despite their important role in cellular metabolism and growing number of newly identified functions, LD formation mechanism from the endoplasmic reticulum remains poorly understood. To form a LD, the oil molecules synthesized in the ER accumulate between the monolayer leaflets and induce deformation of the membrane. This formation process works through three steps: nucleation, growth and budding, exactly as in phase separation and dewetting phenomena. These steps involve sequential biophysical membrane remodeling mechanisms for which we present basic tools of statistical physics, membrane biophysics, and soft matter science underlying them. We aim to highlight relevant factors that could control LD formation size, site and number through this physics description. An emphasis will be given to a currently underestimated contribution of the molecular interactions between lipids to favor an energetically costless mechanism of LD formation.

Thermodynamics of nanocluster phases: a unifying theory

We propose a unifying, analytical theory accounting for the self-organization of colloidal systems in nanocluster or microcluster phases. We predict the distribution of cluster sizes with respect to interaction parameters and colloid concentration. In particular, we anticipate a proportionality regime where the mean cluster size grows proportionally to the concentration, as observed in several experiments. We emphasize the interest in a predictive theory in soft matter, nanotechnologies, and biophysics.

Lipid Droplets Can Spontaneously Bud Off from a Symmetric Bilayer

Lipid droplets (LDs) are cytosolic organelles that protrude from the endoplasmic reticulum membrane under energy-rich conditions. How an LD buds off from the endoplasmic reticulum bilayer is still elusive. By using a continuous media description, we computed the morphology of a lipid droplet embedded in between two identical monolayers of a bilayer. We found that beyond a critical volume, the droplet morphology abruptly transits from a symmetrical elongated lens to a spherical protrusion. This budding transition does not require any energy-consuming machinery, or curvature-inducing agent, or intrinsic asymmetry of the bilayer; it is solely driven by the large interfacial energy of the LD, as opposed to the bilayer surface tension. This spontaneous budding mechanism gives key insights on cellular LD formation.

Theory of Cargo and Membrane Trafficking

Endocytosis underlies many cellular functions including signaling and nutrient uptake. The endocytosed cargo gets redistributed across a dynamic network of endosomes undergoing fusion and fission. Here, a theoretical approach is reviewed which can explain how the microscopic properties of endosome interactions cause the emergent macroscopic properties of cargo trafficking in the endosomal network. Predictions by the theory have been tested experimentally and include the inference of dependencies and parameter values of the microscopic processes. This theory could also be used to infer mechanisms of signal-trafficking crosstalk. It is applicable to in vivo systems since fixed samples at few time points suffice as input data.

Modeling the Kinetics of Open Self-Assembly

In this work, we explore theoretically the kinetics of molecular self-assembly in the presence of constant monomer flux as an input, and a maximal size. The proposed model is supposed to reproduce the dynamics of viral self-assembly for enveloped virus. It turns out that the kinetics of open self-assembly is rather quantitatively different from the kinetics of similar closed assembly. In particular, our results show that the convergence toward the stationary state is reached through assembly waves. Interestingly, we show that the production of complete clusters is much more efficient in the presence of a constant input flux, rather than providing all monomers at the beginning of the self-assembly.

Entropy production of a Rayleigh piston separating gases of different temperature

For thermodiffusion of polymers, it was shown that the transport coefficient has two contributions of different origins: first, dispersion forces between solvent and solute that grow it to cold areas, and secondly, a gradient of chemical potential of the solvent that acts as generalized force on the solute and promotes to hot areas. The competition of these opposite effects explains the variation with the molecular weight of polymer diffusion coefficient, which can change sign. As a toy model for this problem, we looked at the example of the Rayleigh piston: in a cylinder, a piston, assumed adiabatic (representing the solute as a macro particle without internal structure) fluctuates because of collisions with the two gases which it separates. Even if the pressures in the two semiinfinite reservoirs are equal, i.e., even if there is macroscopic equilibrium, when the temperatures are different, the system is out of equilibrium and acquires a nonzero average velocity, proportional to temperature gradient. The piston thus acts as a rectifier of fluctuations of Brownian motion. Therefore, there is a heat transfer between the two gases, causing a flow of entropy that we have calculated, together with the work of the ”generalized force”.