15 Fakultätsübergreifend / Sonstige Einrichtung
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/16
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Item Open Access Adhesion of vesicles and membranes(1991) Lipowsky, Reinhard; Seifert, UdoIn 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.Item Open Access Wetting by critical layers(1985) Lipowsky, Reinhard; Seifert, UdoInterface depinning transitions such as wetting are described where the surface layer becomes critical as the interface unbinds from the wall. As a consequence, there are two different types of fluctuations, namely, capillary waves and critical fluctuations within the surface layer, which are shown to be governed by two different length scales.Item Open Access Fluid membranes - theory of vesicle conformations(Jülich : Forschungszentrum Jülich GmbH, Zentralbibliothek, 1994) Seifert, UdoThe theory of conformations of fluid membranes and vesicles presented in this treatise started from a continuum description based on two relevant degrees of freedom: the local shape, and the local density difference in the two monolayers. Energy minimization, taking into account the geometrical constraints, yields a variety of shapes of various topologies. These shapes are arranged into phase diagrams which separate phases of different symmetry. For vesicles of higher genus, this approach culminates in the prediction of conformal diffusion which amounts to a one-fold continuous degeneracy of the ground state. For bound vesicles and vesicles consisting of two components, an additional energy enters the minimization, which leads to the prediction of a curvature-driven adhesion transition and curvature-induced lateral phase segregation, respectively.Item Open Access Conformal transformations of vesicle shapes [Letter to the editor](1991) Seifert, UdoConformal 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.Item Open Access Conformal degeneracy and conformal diffusion of vesicles(1993) Jülicher, Frank; Seifert, Udo; Lipowsky, ReinhardThe 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.Item Open Access Non-Markovian description of the Hedvall effect(1987) Seifert, Udo; Dietrich, SiegfriedActivated processes at surfaces - like desorption or oxidation - exhibit thermal anomalies at phase transitions or the underlying substrate. Inter alia, such singularities in the case of a continuous transition are caused by the critical slowing down in the substrate, which leads to pronounced memory effects in the viscosity coefficient. Therefore, we apply a non-Markovian analog of Kramers' classical rate theory. As a result, the anomalies can be expressed in terms of critical exponents associated with the critical surface behaviour.Item Open Access Budding transitions of fluid-bilayer vesicles: the effect of area-difference elasticity(1994) Miao, Ling; Seifert, Udo; Wortis, Michael; Döbereiner, Hans-GüntherBudding 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.Item Open Access Dual network model for red blood cell membranes(1992) Boal, David H.; Seifert, Udo; Zilker, AndreasA 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.Item Open Access Vesicles of toroidal topology(1991) Seifert, UdoWe 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.Item Open Access 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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