Journée scientifique

Abstracts of the different presentations

Phd defense S. Sheikh:

Understanding soft interactions is important for engineering new devices that should interact with biological systems, like in the biomedical and soft robotic fields. More particularly in biology, endocytosis is one of the most important processes, where species uptake by a living cell occurs through its plasma membrane, governed by complex biophysical and biochemical mechanisms. Tremendous research efforts have emerged in the last decade to develop drug carriers to vehicle medicines to targeted cells, where the carrier, a hard (gold or silica) or a soft colloid (i.e. liposomes, polymer micelles…), should succeed to approach the cell membrane and cross it.

An exhaustive description of the interaction of the drug carrier with the membrane cell should include the dynamics along the interaction pathway. To simplify the problem, we considered a passive interaction of mechanical origin (where electrical and chemical contributions are negligible). A large part of this PhD work was dedicated to investigate transport properties of a soft nanoparticle (polymer micelle of size ~10-20 nm) near a model biological membrane (lipid bilayer) separated by various distances. Mesoscopic simulations using a dissipative particle dynamics model were carried to examine the impact of the micelle’s distance from the membrane on the micelle Brownian motion along parallel and perpendicular directions for different micelle sizes. When the micelle is sufficiently close to the bilayer, the numerical measurements have shown that during a time range smaller than the diffusion time scale, so that the micelle remains fairly close to its initial position, its mean-square displacement (MSD) exhibits different behavior along parallel and perpendicular directions. On the one hand, the micelle motion becomes quickly subdiffusive in the normal direction and its MSD excess in that direction follows that of a nanoparticle near an elastic membrane. This subdiffusive behavior starts around the characteristic (bending) time at which the particle suspended in a viscous fluid starts to “feel” the spontaneous fluctuations from the membrane. On the other hand, in the parallel direction, the measured MSD excess is comparable to that obtained for Brownian motion near a non-deformable liquid-liquid interface.

Additionally in this work, we have established a methodology to measure the membrane mechanical properties from numerical simulations. We applied this methodology to characterize the mechanical properties of hybrid bilayers (membranes with mixed lipids and polymers) as this situation might follow the fusion of polymer micelles with a lipid bilayer. Relatively short polymers were considered, the polymer-to-lipid length ratio being around two. In those conditions, we evidenced that the presence of polymers enhances yielding of the interface when stretched, before its rupture, and that phase separation occurs as soon as the chemical compatibility between the hydrophobic chains of lipids and polymers is slightly reduced.

Seminar Thomas Bickel (Théorie de la matière condensée, LOMA)

When a liquid interface is exposed to a temperature gradient, the local variation of its surface tension induces shear stresses that set the fluid in motion. The resulting thermocapillary (or Marangoni) flows are ubiquitous in everyday life as well as in industrial processes, and their structure and stability have been thoroughly documented. Current challenges focus on microscopic scales. I will first show that light absorption from a focused laser beam is an efficient mechanism for the actuation micron-sized particles at the liquid-air interface. Then I will discuss the azimuthal instabilities that are observed at low Reynolds number. In particular, I will show that the presence of a minute quantity of surface-active impurities may explain the complex structure of the flows.

Seminar Fabrice Thalmann (Physique des membranes et matière molle, ICS)

The Physics of Membrane and Soft Matter team (M3) at Institut Charles Sadron has a broad interest in model lipid bilayer systems. I will discuss here two numerical approaches that we have proposed recently. The first one aims at obtaining the interleaflet friction coefficient b from unbiased simulations (Benazieb et al., J.of Phys.Comm. 2023, https://doi.org/10.1088/2399-6528/acb313, Gromacs/Martini CG approach). The second aims at determining the value of the « Flory-Huggins » interaction parameter of a DPPC (saturated) – DLiPC (2 insaturations) mixture (PhD L. Berezovska, manuscript under writing, Lammps/Spica CG approach). Pros and cons of driven vs equilibrium approaches will be discussed as a conclusion.