Showing papers by "Udo Seifert published in 1993"

Viscous Modes of Fluid Bilayer Membranes

Abstract: We determine the dispersion relation for a fluid bilayer membrane, taking into account the coupling between bending and the local density of the two monolayers. Apart from important corrections to the conventional bending mode, we obtain a second slow mode which is essentially a fluctuation in the density difference of the two monolayers, damped by inter-monolayer friction. Estimates for a stack of membranes show reasonable agreement with a recent spin-echo study of membrane undulations.

Curvature-induced lateral phase segregation in two-component vesicles

Abstract: Shape transformations of two-compoent vesicles in which the spontaneous curvature depends on the local compositions in the two monolayers are investigated theoretically. Even if the two components do not exhibit lateral phase separation in spherical vesicles, temperature-induced shape transformations cause lateral phase segregation. Vesiculation of smaller buds is favored in the two-component system. In two-component vesicles with intermonolayer phase separation, thermal budding is dominated by the different thermal expansion coefficients of the two monolayers.

Negative Poisson ratio in two-dimensional networks under tension

Abstract: The elastic properties of two-dimensional networks under tension are studied by the mean-Aeld approximation and Monte Carlo simulation. The networks are characterized by Axed (polymerized) connectivity and either a square-well or a Hooke s-law interaction among their components. Both selfavoiding and phantom networks are examined. The elastic properties of Hooke's-law networks at large equilibrium length are found to be well represented by a mean-field model. All the networks investigated show a negative Poisson ratio over a range of tension. At finite tension, the phantom networks exhibit a phase transition to a collapsed state.

Conformal degeneracy and conformal diffusion of vesicles.

Abstract: The shape of vesicles with genus g=2, i.e., with two holes or two handles, is studied in the framework of curvature models. These vesicles exhibit a new phase which also persists for higher genus gg2. In this phase, the ground state of the vesicle is conformally degenerate even when the volume, the area, and the total mean curvature of the vesicle are kept constant. It is predicted that such vesicles undergo a new type of diffusive motion, termed conformal diffusion, which should be observable in experiments as pronounced shape fluctuations.

Phase diagrams and shape transformations of toroidal vesicles

Abstract: Shapes of vesicles with toroidal topology are studied in the context of curvature models for the membrane. For two simplified curvature models, the spontaneous-curvature (SC) model and the bilayer-couple (BC) model, the structure of energy diagrams, sheets of stationary shapes and phase diagrams are obtained by solving shape equations for axisymmetric shapes. Three different sheets of axisymmetric shapes are investigated systematically: I) discoid tori; it) sickle-shaped tori and iii) toroidal stomatocytes. A stability analysis of axisymmetric shapes with respect to symmetry breaking conformal transformations reveals that large regions of the phase diagrams of toroidal vesicles are non-axisymmetric. Non-axisymmetric shapes are determined approxim~tely using conformal transformations. To compare the theory with experiments, a generalization of the SC and BC model, the area-difference-elasticity-model (ADlGmodel), which is a more realistic curvature model for lipid bilayers, is discussed. Shapes of toroidal vesicles which have been observed recently can be located in the phase diagram of the ADlGmodel. We predict the effect oftemperature changes on the observed shapes-The new class of shapes, the toroidal stomatocytes, have not yet been observed.