Hydrodynamic lift on bound vesicles

Bound vesicles subject to lateral forces such as arising from shear flow are investigated theoretically by combining a lubrication analysis of the bound part with a scaling approach to the global motion. A minor inclination of the bound part leads to significant lift due to the additive effects of lateral and tank-treading motions. With increasing shear rate, the vesicle unbinds from the substrate at a critical value. Estimates are in agreement with recent experimental data.Comment: 9 pages, one figur

Geometry of lipid vesicle adhesion

The adhesion of a lipid membrane vesicle to a fixed substrate is examined from a geometrical point of view. This vesicle is described by the Helfrich hamiltonian quadratic in mean curvature; it interacts by contact with the substrate, with an interaction energy proportional to the area of contact. We identify the constraints on the geometry at the boundary of the shared surface. The result is interpreted in terms of the balance of the force normal to this boundary. No assumptions are made either on the symmetry of the vesicle or on that of the substrate. The strong bonding limit as well as the effect of curvature asymmetry on the boundary are discussed.Comment: 7 pages, some major changes in sections III and IV, version published in Physical Review

Fluctuation spectrum of fluid membranes coupled to an elastic meshwork: jump of the effective surface tension at the mesh size

We identify a class of composite membranes: fluid bilayers coupled to an elastic meshwork, that are such that the meshwork's energy is a function $F_\mathrm{el}[A_\xi]$ \textit{not} of the real microscopic membrane area $A$, but of a \textit{smoothed} membrane's area $A_\xi$, which corresponds to the area of the membrane coarse-grained at the mesh size $\xi$. We show that the meshwork modifies the membrane tension $\sigma$ both below and above the scale $\xi$, inducing a tension-jump $\Delta\sigma=dF_\mathrm{el}/dA_\xi$. The predictions of our model account for the fluctuation spectrum of red blood cells membranes coupled to their cytoskeleton. Our results indicate that the cytoskeleton might be under extensional stress, which would provide a means to regulate available membrane area. We also predict an observable tension jump for membranes decorated with polymer "brushes"

Giant membranes of swollen phosphatidylethanolamines and glycolipids

The swelling of phosphatidylethanolamines and biological glycolipids is studied by phase contrast microscopy. In their fluid phase the bilayers of all the lipids separate in water and, with one proven exception, in NaCl solutions. The very rare mutual adhesion of giant membranes which normally separate is apparently always induced by lateral tensions. Its average contact angles are larger than in the case of egg phosphatidylcholine but, as there, do not depend on tension within the one or two powers of ten covered by the observations. Certain cerebrosides form needles and fibres when the bilayers are in the solid phase

Bilayer bending rigidity of some synthetic lecithins

The curvature elastic modulus of fluid membranes of DMPC, DPPC, and DSPC in water was measured. From the equilibrium distribution of the thermal bending fluctuations of tubular vesicles we obtained values of 2.4, 2.0, and 1.8 × 10-12 erg, respectively, with errors of about 20 %. We also observed some deformations of the fluctuating tubes such as sharp knee-like bends and pronounced short-waved wriggles which apparent elastic energies seem too high for thermal excitation.Nous avons mesuré le module d'élasticité de courbure des membranes fluides de DMPC, DPPC et DSPC dans l'eau. De la distribution d'équilibre des fluctuations thermiques de flexion de vésicules tubulaires, nous avons obtenu les valeurs de 2,4,2,0 et 1,8 x 10-12 erg avec une erreur d'environ 20 %. Nous avons également observé quelques déformations des tubules fluctuantes telles que des « coudes » bien marqués et des ondulations prononcées de courtes longueurs d'onde dont l'énergie élastique fictive nous a paru trop grande pour être excités thermiquement