The European Physical Journal E

The impact of selected metallophthalocyanines, featuring diverse molecular structure, upon the fluidity of liposome membranes was studied using the spin label EPR technique. The “mono”-type MPc's (M = Zn, Sn; Pc = C32H16N8 is the phthalocyanine ligand) and sandwich LnPc2 complexes (Ln = Nd, Sm, Gd) were explored. Liposomes were obtained in a sonication process, from egg yolk lecithin (EYL) in water. TEMPO and 16-DOXYL spin labels were used to monitor the peripheral and central part of the lipid double layer, respectively, which allowed to localize the phthalocyanine additive within the bilayer, as well as to perform independent measurements of changes in fluidity upon addition thereof. All the complexes tested were found to increase the fluidity in the middle of the lipid bilayer. However, at the water-lipid interface the LnPc2 compounds showed a relative small effect upon the phospholipids' arrangement, whereas in the case of ZnPc and SnPc it was found much more pronounced. EPR results were supplemented by measurements of static electrical charge, the investigated phthalocyanines may potentially feed into the membrane thus affecting its stability.

The shape as well as tension and pressure inside an uncharged vesicle are understood to be determined by the reduced volume of a vesicle. These parameters are important for a vesicle or a biological cell, since they can affect bio-physical processes such as osmosis and permeation, interaction with external agents such as bio-macromolecules as well as thermal fluctuations in a bilayer membrane of a vesicle. Charged membranes are ubiquitous in nature, most biological cell bio-membranes are charged, and therefore the knowledge of shape, tension and pressure of charged vesicles is critical. Additionally, the distribution of charges in the inner and outer leaflets is also important as it can affect the spatial interaction of a bilayer membrane with proteins and other micro and macromolecular species. This work addresses these issues in the low-charge and low-curvature limit. Our analysis indicates that despite a very strong two-way coupling between the charge and the curvature, the shapes of charged vesicles remain similar to that of uncharged vesicles at comparable reduced volumes, even for reasonable values of total charge. However, the tension and pressure values are higher, and are accurately estimated in our analysis. The charge distribution on the outer and inner leaflet which is strongly affected by the curvature is calculated. The value of spontaneous curvature due to charge redistribution is also estimated. The insensitivity of the shape to charges persists even when only the outer leaflet is charged instead of charged inner and outer leaflets.

When a microparticle is trapped at a fluid interface, particle’s electrical charge and weight combine to deform the interface. Such deformation is expected to affect the particle diffusion via hydrodynamics boundary conditions. Using available models of particle-induced electrostatic deformation of the interface and particle dynamics at the interface, we are able to analytically predict particle diffusion coefficient values in a large range of particle’s contact angle and size. This might offer a solid background of numerical values to compare with for future experimental studies in the field of particle diffusion at a fluid interface.

The aim of the paper is to study the deviation of magnetic properties of the magnetic fluids prepared for this study, from ideal (Langevin) behaviour, i.e. to estimate particle interaction influence and dimensions and influence of particle aggregates, as well as to explain the related effects observed. We also determine the particle coupling parameter, the particle nonmagnetic layer thickness, and the particle distribution, which are fundamental for sample characterization. A comparison of the studied magnetic fluids with each other, with respect to microstructure formation and particle interaction strength is finally done. For these purposes, a concentration dependence study, following the proposed “dilution series approach”, is performed. Three series of dilutions of three types of magnetic fluids were prepared and analyzed.

It is now more than 50 years since the first observation of ferrofluid spin-up by a rotating magnetic field by the author and R. Moskowitz. The ferrofluid rotated in direction opposite to that of the applied field. The intervening years have seen multiple unsuccessful attempts to explain this phenomenon. A clue was experimentally discovered by J. Popplewell and the author in 1987; rotation direction depends on shape of the ferrofluid meniscus. That notion is studied in further detail in the present work which describes tangential stress generated on the meniscus surface and an analytical expression for torque is developed. Increasing spin rate in smaller diameter vessels is rationalized. In addition, a physical picture of spin-up is introduced.

The behaviour of dense assemblies of dry grains submitted to continuous shear deformation has been the subject of many experiments and discrete particle simulations. This paper is a collective work carried out among the French research group Groupement de Recherche Milieux Divisés (GDR MiDi). It proceeds from the collection of results on steady uniform granular flows obtained by different groups in six different geometries both in experiments and numerical works. The goal is to achieve a coherent presentation of the relevant quantities to be measured i.e. flowing thresholds, kinematic profiles, effective friction, etc. First, a quantitative comparison between data coming from different experiments in the same geometry identifies the robust features in each case. Second, a transverse analysis of the data across the different configurations, allows us to identify the relevant dimensionless parameters, the different flow regimes and to propose simple interpretations. The present work, more than a simple juxtaposition of results, demonstrates the richness of granular flows and underlines the open problem of defining a single rheology.