Browsing by Author "Seifert, Udo"

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Open Access
Adhesion of membranes : a theoretical perspective
(1991) Lipowsky, Reinhard; Seifert, Udo
We theoretically study (i) a large membrane segment and (ii) a closed membrane surface or vesicle that adhere to another surface. The membrane segment can undergo unbinding transitions as a result of thermally excited shape fluctuations. These transitions are studied by renormalization group methods and by Monte Carlo simulations. The shape of a bound vesicle is determined by the interplay of bending and adhesion energies. This interplay leads to adhesion transitions from a free to a bound vesicle state even in the absence of shape fluctuations. Our theory helps to clarify the notion of a contact angle for membranes.
Open Access
Adhesion of vesicles
(1990) Seifert, Udo; Lipowsky, Reinhard
A simple model for the adhesion of vesicles to interfaces and membranes is introduced and theoretically studied. It is shown that adhering (or bound) vesicles can exhibit a large variety of different shapes. The notion of a contact angle governed by tension is found to be applicable only for a restricted subset of these shapes. Furthermore, the vesicle undergoes a nontrivial adhesion transition from a free to a bound state. This transition is governed by the balance between the overall bending and adhesion energies, and occurs even in the absence of shape fluctuations.
Open Access
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.
Open Access
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.
Open Access
Budding transition for bilayer fluid vesicles with area-difference elasticity
(1992) Seifert, Udo; Miao, Ling; Döbereiner, Hans-Günther; Wortis, Michael
We consider a curvature model for bilayer vesicles with an area-difference elasticity or non-local bending-energy term. Such a model interpolates between the bilayer-couple and spontaneous-curvature models. We report preliminary results for the budding transition. The shape transformation between the dumbbell and the pear phases can be continuous or discontinuous depending on the ratio of the non-local to the local bending rigidities.
Open Access
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.
Open Access
Coherence-enhanced efficiency of feedback-driven quantum engines
(2015) Brandner, Kay; Bauer, Michael; Schmid, Michael T.; Seifert, Udo
A genuine feature of projective quantum measurements is that they inevitably alter the mean energy of the observed system if the measured quantity does not commute with the Hamiltonian. Compared to the classical case, Jacobs proved that this additional energetic cost leads to a stronger bound on the work extractable after a single measurement from a system initially in thermal equilibrium (2009 Phys. Rev. A 80 012322). Here, we extend this bound to a large class of feedback-driven quantum engines operating periodically and in finite time. The bound thus implies a natural definition for the efficiency of information to work conversion in such devices. For a simple model consisting of a laser-driven two-level system, we maximize the efficiency with respect to the observable whose measurement is used to control the feedback operations. We find that the optimal observable typically does not commute with the Hamiltonian and hence would not be available in a classical two level system. This result reveals that periodic feedback engines operating in the quantum realm can exploit quantum coherences to enhance efficiency.
Open Access
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.
Open Access
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.
Open Access
Critical magnetic field dependence of thermally activated surface processes
(1989) Seifert, Udo; Wagner, Herbert
Activated processes at surfaces such as desorption or sublimation may exhibit a thermal anomaly at the Curie point of a magnetic substrate. We propose to measure this anomaly with an applied magnetic field H, and we predict a decrease in the reaction rate proportional to Hx with x(T > Tc)=2, x(T < Tc)=1, and x(T = Tc)=2/δ1, where δ1≃1.9 is an exponent for ordinary phase transitions. In the case of a surface transition, or if the substrate is a film with two-dimensional Ising behavior, the anomaly is significantly enhanced.
Open Access
Curvature-induced lateral phase segregation in two-component vesicles
(1993) Seifert, Udo
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.
Open Access
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.
Open Access
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.
Open Access
Field-theoretic thermodynamic uncertainty relation : general formulation exemplified with the Kardar-Parisi-Zhang equation
(2020) Niggemann, Oliver; Seifert, Udo
We propose a field-theoretic thermodynamic uncertainty relation as an extension of the one derived so far for a Markovian dynamics on a discrete set of states and for overdamped Langevin equations. We first formulate a framework which describes quantities like current, entropy production and diffusivity in the case of a generic field theory. We will then apply this general setting to the one-dimensional Kardar-Parisi-Zhang equation, a paradigmatic example of a non-linear field-theoretic Langevin equation. In particular, we will treat the dimensionless Kardar-Parisi-Zhang equation with an effective coupling parameter measuring the strength of the non-linearity. It will be shown that a field-theoretic thermodynamic uncertainty relation holds up to second order in a perturbation expansion with respect to a small effective coupling constant. The calculations show that the field-theoretic variant of the thermodynamic uncertainty relation is not saturated for the case of the Kardar-Parisi-Zhang equation due to an excess term stemming from its non-linearity.
Open Access
Fluctuating entropy production on the coarse-grained : inference and localization of irreversibility
(2024) Degünther, Julius; Meer, Jann van der; Seifert, Udo
Open Access
Fluid membranes - theory of vesicle conformations
(Jülich : Forschungszentrum Jülich GmbH, Zentralbibliothek, 1994) Seifert, Udo
The 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.
Open Access
Fluide Membranen
(1994) Seifert, Udo
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Open Access
Hydrodynamics of a membrane bound to a substrate
(1994) Seifert, Udo
We briefly review recent theoretical work on the dynamical equilibrium shape fluctuations of membrane interaction, with a substrate. Solving the hydrodynamical equations we determine the damping rate γ(q) as a function of the wave-vector q. For small q, we find universally γ=q². In an intermediate q-range, the damping rate behaves like either γ-q 6 (monotonic damping) or γ~1/q (non-monotonic damping) depending on the relative sizes of the mean separation and the parallel correlation length, which are both determined by the adhesion potential. For large q, one recovers the behavior γ~q³ of a free membrane. A numerical solution of the dispersion relation is presented for a membrane subject to electrostatic repulsion in a linear attractive potential.
Open Access
Hydrodynamics of membranes: the bilayer aspect and adhesion
(1994) Seifert, Udo; Langer, Stephen A.
We review recent work on the dynamics of membranes. First, we discuss the viscous modes of a free 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 intermonolayer friction. Estimates for a stack of membranes show reasonable agreement with a recent spin-echo study of membrane undulations. Second, we discuss the dispersion relation for both fluid and polymerized membranes bound to a substrate by an attractive potential. The interplay between the hydrodynamic damping by the surrounding liquid and the restoring force in the binding potential leads to a q2 dependence of the damping rate γ at small wavevector q. With increasing q, various crossovers can occur, including the possibility of nonmonotonic damping in which γ decreases with q as ∼ 1/q.
Open Access
Inferring kinetics and entropy production from observable transitions in partially accessible, periodically driven Markov networks
(2024) Maier, Alexander M.; Degünther, Julius; Meer, Jann van der; Seifert, Udo
For a network of discrete states with a periodically driven Markovian dynamics, we develop an inference scheme for an external observer who has access to some transitions. Based on waiting-time distributions between these transitions, the periodic probabilities of states connected by these observed transitions and their time-dependent transition rates can be inferred. Moreover, the smallest number of hidden transitions between accessible ones and some of their transition rates can be extracted. We prove and conjecture lower bounds on the total entropy production for such periodic stationary states. Even though our techniques are based on generalizations of known methods for steady states, we obtain original results for those as well.