Showing papers by "Udo Seifert published in 2011"

Extracting work from a single heat bath through feedback

Abstract: Work can be extracted from a single heat bath if additional information is available. For the paradigmatic case of a Brownian particle in a harmonic potential, whose position has been measured with finite precision, we determine the optimal protocol for manipulating the center and stiffness of the potential in order to maximize this work in a finite-time process. The bound on this work imposed by a generalized-second-law inequality involving information can be reached only if both position and stiffness of the potential are controlled and the process is quasistatic. Estimates on the power delivered by such an "information machine" operating cyclically follow from our analytical results.

Stochastic thermodynamics of single enzymes and molecular motors

Abstract: For a single enzyme or molecular motor operating in an aqueous solution of non-equilibrated solute concentrations, a thermodynamic description is developed on the level of an individual trajectory of transitions between states. The concept of internal energy, intrinsic entropy and free energy for states follows from a microscopic description using one assumption on time scale separation. A first-law energy balance then allows the unique identification of the heat dissipated in one transition. Consistency with the second law on the ensemble level enforces both stochastic entropy as third contribution to the entropy change involved in one transition and the local detailed balance condition for the ratio between forward and backward rates for any transition. These results follow without assuming weak coupling between the enzyme and the solutes, ideal solution behavior or mass action law kinetics. The present approach highlights both the crucial role of the intrinsic entropy of each state and the physically questionable role of chemiostats for deriving the first law for molecular motors subject to an external force under realistic conditions.

Efficiency of autonomous soft nanomachines at maximum power.

Abstract: We consider nanosized artificial or biological machines working in steady state enforced by imposing nonequilibrium concentrations of solutes or by applying external forces, torques, or electric fields. For unicyclic and strongly coupled multicyclic machines, efficiency at maximum power is not bounded by the linear response value 1/2. For strong driving, it can even approach the thermodynamic limit 1. Quite generally, such machines fall into three different classes characterized, respectively, as "strong and efficient," "strong and inefficient," and "balanced." For weakly coupled multicyclic machines, efficiency at maximum power has lost any universality even in the linear response regime.

Micro-capsules in shear flow

Abstract: This paper deals with flow-induced shape changes of elastic capsules. The state of the art concerning both theory and experiments is briefly reviewed starting with dynamically induced small deformation of initially spherical capsules and the formation of wrinkles on polymerized membranes. Initially non-spherical capsules show tumbling and tank-treading motion in shear flow. Theoretical descriptions of the transition between these two types of motion assuming a fixed shape are at odds with the full capsule dynamics obtained numerically. To resolve the discrepancy, we expand the exact equations of motion for small deformations and find that shape changes play a dominant role. We classify the dynamical phase transitions and obtain numerical and analytical results for the phase boundaries as a function of viscosity contrast, shear and elongational flow rate. We conclude with perspectives on time-dependent flow, on shear-induced unbinding from surfaces, on the role of thermal fluctuations and on applying the concepts of stochastic thermodynamics to these systems.

Switching from Ultraweak to Strong Adhesion

Abstract: Nature switches from weak to strong adhesion at the cellular level to spectacular effect – for example, for incredibly sensitive recognition and decisive action during immune response. [ 1 ] If realized in an artifi cial system, such a switching could one day be harnessed as a powerful tool to manipulate weakly interacting objects. The fi rst step towards realizing such a system involves understanding how to create and detect ultraweak adhesion and how to then switch-on a strong interaction. So far, in the context of model membranes, weak adhesion has been achieved only with a ligand-receptor of intrinsically low binding affi nity. [ 2 ] Whatever, the intrinsic strength of the bonds, so far they were usually found to be arranged in compact stable domains. [ 3 ] Here, we present experiments and simulations that indicate how to create and detect ultraweak adhesion in the context of fl uid two dimensional membranes interacting via specifi c ligand/receptor bonds. Thus, specifi c adhesion is mediated by transient domains consisting of sparsely distributed bonds. Amazingly, we demonstrate that the avidin/biotin pair – famous for forming the strongest receptor/ligand bond known in nature, mediates ultraweak adhesion under suitable circumstances. This choice of binders allows us to switch on strong binding once sensitive detection is achieved – without resorting to a second binding pair – something not possible with intrinsically weak binders. However, this goal necessitates an appropriate design strategy elaborated below. The in vitro system consists of two membranes: a solid supported lipid bilayer (SLB) and the freely fl uctuating membrane of a giant unillamelar vesicle (GUV). A GUV is a two

Approximate thermodynamic structure for driven lattice gases in contact

Abstract: For a class of nonequilibrium systems, called driven lattice gases, we study what happens when two systems are kept in contact and allowed to exchange particles with the total number of particles conserved. For both attractive and repulsive nearest-neighbor interactions among particles and for a wide range of parameter values, we find that, to a good approximation, one could define an intensive thermodynamic variable, such as the equilibrium chemical potential, that determines the final steady state for two initially separated driven lattice gases brought into contact. However, due to nontrivial contact dynamics, there are also observable deviations from this simple thermodynamic law. To illustrate the role of the contact dynamics, we study a variant of the zero-range process and discuss how the deviations could be explained by a modified large-deviation principle. We identify an additional contribution to the large-deviation function, which we call the excess chemical potential, for the variant of the zero-range process as well as the driven lattice gases. The excess chemical potential depends on the specifics of the contact dynamics and is in general a priori unknown. A contact dependence implies that, even though an intensive variable may equalize, the zeroth law could still be violated.

Two intertwined facets of adherent membranes: membrane roughness and correlations between ligand–receptors bonds

Abstract: We study equilibrium fluctuations of adherent membranes by means of Langevin simulations in the case when the interaction of the membrane with the substrate is twofold: a non-specific homogeneous harmonic potential is placed at large distances, whereas discrete ligand–receptor interactions occur at short distances from the flat substrate. We analyze the correlations between neighboring ligand–receptor bonds in a regime of relatively strong membrane fluctuations. By comparison with the random distribution of bonds, we find that the correlations between the bonds are always positive, suggesting spontaneous formation of domains. The equilibrium roughness of the membrane is then determined by fluctuations in the number density of bonds within the domains. Furthermore, we show that the excess number of bonds arising due to correlations and the instantaneous roughness of the membrane both follow master curves that depend only on the instantaneous bond density and not on the intrinsic binding strength of the ligand–receptor pair. The master curves show identical trends, further corroborating the link between membrane roughness and bond correlations.

Stochastic thermodynamics: An introduction

Abstract: These seminar notes contain a brief introduction into the principles of stochastic thermodynamics and some of its recent ramifications from a personal perspective. Thermodynamic concepts like work, exchanged heat and entropy production can consistently be defined on the level of individual fluctuating trajectories taken from either a time‐dependent or a non‐equilibrium steady state ensemble. Fluctuation theorems constrain the probability distributions for these thermody‐namic quantities. For systems containing fast internal degrees of freedom the crucial distinction between internal and free energy for a correct identification of both dissipated heat and system entropy is emphasized. For non‐equilibrium steady states, a generalized fluctuation‐dissipation theorem relates the response to a small perturbation to correlation functions in the steady state involving observables expressing the various contributions to entropy production along the trajectory.

Dynamics and efficiency of a self-propelled, diffusiophoretic swimmer

Abstract: Active diffusiophoresis - swimming through interaction with a self-generated, neutral, solute gradient - is a paradigm for autonomous motion at the micrometer scale. We study this propulsion mechanism within a linear response theory. Firstly, we consider several aspects relating to the dynamics of the swimming particle. We extend established analytical formulae to describe small swimmers, which interact with their environment on a finite lengthscale. Solute convection is also taken into account. Modeling of the chemical reaction reveals a coupling between the angular distribution of reactivity on the swimmer and the concentration field. This effect, which we term "reaction induced concentration distortion", strongly influences the particle speed. Building on these insights, we employ irreversible, linear thermodynamics to formulate an energy balance. This approach highlights the importance of solute convection for a consistent treatment of the energetics. The efficiency of swimming is calculated numerically and approximated analytically. Finally, we define an efficiency of transport for swimmers which are moving in random directions. It is shown that this efficiency scales as the inverse of the macroscopic distance over which transport is to occur.

Extracting work from a single heat bath through feedback

Abstract: Work can be extracted from a single heat bath if additional information is available. For the paradigmatic case of a Brownian particle in a harmonic potential, whose position has been measured with finite precision, we determine the optimal protocol for manipulating the center and stiffness of the potential in order to maximize this work in a finite-time process. The bound on this work imposed by a generalized second law inequality involving information can be reached only if both position and stiffness of the potential are controlled and the process is quasistatic. Estimates on the power delivered by such an "information machine" operating cyclically follow from our analytical results.

Thermodynamic theory of phase transitions in driven lattice gases.

Abstract: We formulate an approximate thermodynamic theory of the phase transition in driven lattice gases with attractive nearest-neighbor interactions. We construct the van der Waals equation of state for a driven system where a nonequilibrium chemical potential can be expressed as a function of density and driving field. A Maxwell's construction leads to the phase transition from a homogeneous fluid phase to the coexisting phases of gas and liquid.