Universität Stuttgart

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Subject: search.filters.subject.530Institute: search.filters.UBSInstitut.Sonstige EinrichtungStart date: 1990End date: 1994Author: search.filters.author.Seifert, Udo

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Now showing 1 - 10 of 25
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    Adhesion of vesicles and membranes
    (1991) Lipowsky, Reinhard; Seifert, Udo
    In the presence of an attractive surface, a vesicle can undergo shape transformations between two different free states, between a free and a bound state, and between two different bound states. Adhesion can also lead to topological changes such as vesicle rupture and vesicle fusion. The interaction between the vesicle membrane and the surface is renormalized by thermally excited shape fluctuations. This renormalization leads to unbinding phenomena both for fluid and for polymerized (or solid-like) membranes.
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    Conformal transformations of vesicle shapes [Letter to the editor]
    (1991) Seifert, Udo
    Conformal transformations are used to derive an exact geometrical relation for equilibrium vesicle shapes within the spontaneous curvature and bilayer coupling models. Stability criteria with respect to these transformations efficiently detect instabilities related to the breaking of reflection symmetry.
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    Conformal degeneracy and conformal diffusion of vesicles
    (1993) Jülicher, Frank; Seifert, Udo; Lipowsky, Reinhard
    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 g>2. 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.
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    Budding transitions of fluid-bilayer vesicles: the effect of area-difference elasticity
    (1994) Miao, Ling; Seifert, Udo; Wortis, Michael; Döbereiner, Hans-Günther
    Budding and vesiculation are prominent shape transformations of fluid lipid-bilayer vesicles. We discuss these transitions within the context of a curvature model which contains two types of bending energy. In addition to the usual local curvature elasticity κ, we include the effect of a relative areal stretching of the two monolayers. This area-difference elasticity leads to an effective nonlocal curvature energy characterized by another parameter κ¯. We argue that the two contributions to the curvature energy are typically comparable in magnitude. The model interpolates smoothly between the spontaneous-curvature model (κ¯=0) and the bilayer-couple model (κ¯→∞), discussed previously in the literature. Conceptually, this model is not new; however, neither its consequences nor its relation to experiment has previously been explored in detail. In particular, budding is discontinuous (first order) for small κ¯ but changes via a tricritical point to continuous (second order) for large κ¯. The order of the budding transition depends on both the ratio κ¯/κ (which is a material parameter) and the initial area difference between the inner and outer monolayers (which can be modified by appropriate treatment of the vesicle). Estimates suggest that, under typical laboratory conditions, the budding process should be discontinuous, in apparent disagreement with some recent experiments. Possible reasons for this discrepancy are discussed. We propose, in particular, that hysteretic effects are important and that the observed behavior may reflect a spinodal instability.
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    Dual network model for red blood cell membranes
    (1992) Boal, David H.; Seifert, Udo; Zilker, Andreas
    A two-component network is studied by Monte Carlo simulation to model the lipid/spectrin membrane of red blood cells. The model predicts that the shear modulus decreases rapidly with the maximum length of the model spectrin and should be in the 10-7 J/m2 range for human red blood cells. A simplified model for the isolated spectrin network shows a negative Lamé coefficient λ. Transverse fluctuations of the dual membrane are found to be fluidlike over the range of wavelengths investigated.
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    Vesicles of toroidal topology
    (1991) Seifert, Udo
    We consider fluid vesicles of toroidal topology. Minimization of the curvature energy at fixed volume and area leads to three different branches of axisymmetric shapes. By using conformal transformations, we identify a large region of nonaxisymmetric shapes in the phase diagram. For vanishing spontaneous curvature, the ground state is twofold degenerate in this region and corresponds to zero pressure difference across the membrane. The relation of these results to the recent observation of toroidal shapes for partially polymerized vesicles is discussed.
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    Viscous modes of fluid bilayer membranes
    (1993) Seifert, Udo; Langer, Stephen A.
    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.
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    Dynamics of a bound membrane
    (1994) Seifert, Udo
    The dispersion relation for the overdamped bending modes of a membrane bound to a substrate by an attractive potential is determined. The damping rate γ as a function of the wave vector q behaves, for small q, like γ∼q2 arising from the interplay between the hydrodynamic damping by the surrounding liquid and the restoring force in the binding potential. With increasing wave vector q, various crossovers can occur, leading to the possibility of nonmonotonic damping where γ decreases with q as ∼1/q.
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    Phase diagrams and shape transformations of toroidal vesicles
    (1993) Jülicher, Frank; Seifert, Udo; Lipowsky, Reinhard
    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; ii) 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 approximately using conformal transformations. To compare the theory with experiments, a generalization of the SC and BC model, the area-difference-elasticity-model (ADE-model), 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 ADE-model. We predict the effect of temperature changes on the observed shapes. The new class of shapes, the toroidal stomatocytes, have not yet been observed.
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    Adhesion of vesicles in two dimensions
    (1991) Seifert, Udo
    The adhesion of vesicles in two dimensions is studied by solving the shape equations that determine the state of lowest energy. Two ensembles are considered where for a fixed circumference of the vesicle either a pressure difference between the exterior and the interior is applied or the enclosed area is prescribed. First, a short discussion of the shape of free vesicles is given. Then, vesicles confined to a wall by an attractive potential are considered for two cases: (i) For a contact potential, a universal boundary condition determines the contact curvature as a function of the potential strength and the bending rigidity. Bound shapes are calculated, and an adhesion transition between bound and free states is found, which arises from the competition between bending and adhesion energy. (ii) For adhesion in a potential with finite range, the crossover from the long-ranged to the short-ranged case is studied. For a short-ranged potential, a decrease in the strength of the potential can lead to a shape transition between a bound state and a "pinned" state, where the vesicle acquires its free shape but remains pinned by the potential. In such a potential, fluctuations lead to unbinding for which two different cases are found. Small vesicles unbind via fluctuations of their position, while large vesicles unbind via shape fluctuations.