Inst. Theoretische Physik

212 Items

Recent Submissions
Collective dynamics in a monolayer of squirmers confined to a boundary by gravity

Kuhr, Jan-Timm ; Rühle, Felix ; Stark, Holger (2019-06-21)

We present a hydrodynamic study of a monolayer of squirmer model microswimmers confined to a boundary by strong gravity using the simulation method of multi-particle collision dynamics. The squirmers interact with each other via their self-generated hydrodynamic flow fields and thereby form a variety of fascinating dynamic states when density and squirmer type are varied. Weak pushers, neutral ...

Second Law and Non-Equilibrium Entropy of Schottky Systems—Doubts and Verification−

Muschik, Wolfgang (2018-09-28)

Meixner’s historical remark in 1969 “... it can be shown that the concept of entropy in the absence of equilibrium is in fact not only questionable but that it cannot even be defined....” is investigated from today’s insight. Several statements—such as the three laws of phenomenological thermodynamics, the embedding theorem and the adiabatical uniqueness—are used to get rid of non-equilibrium e...

Mode locking in systems of globally-coupled phase oscillators

Eydam, Richard Sebastian (2019)

In systems of globally coupled phase oscillators with sufficiently structured natural frequencies, a new type of collective behavior is discovered that exists below the synchronization threshold. The solution type is distinguished by the appearance of sharp pulses in the mean field amplitude which imply a temporary high coherence among the phases. This is similar to a process known in lase...

Synchronization and learning in networks of chemical oscillators

Fengler, Enrico (2019)

In this thesis, the collective behavior of chemical oscillators in adaptive networks with time-dependent coupling weights is studied theoretically and experimentally. About 100-150 micro meter in size catalytic beads based on the Belousov-Zhabotinsky (BZ) oscillatory chemical reaction are fixed in an array of conical slots covered by a gel layer, and are placed into contact with BZ solution in ...

Shaping of membranes by arc-like particles

Bonazzi, Francesco (2019)

This dissertation analyzes the interaction of biomembranes with arc-like particles, which gives rise to complex morphologies of organelles inside cells. The shaping of membranes is investigated with coarse-grained modeling and Monte Carlo simulations. In the coarse-grained model, the membrane is described as a discretized elastic surface, and the particles as segmented arcs. Concave arc-like p...

Delayed feedback control of active particles: a controlled journey towards the destination

Khadem, Seyed Mohsen Jebreiil ; Klapp, Sabine H. L. (2019-06-07)

We explore theoretically the navigation of an active particle based on delayed feedback control. The delayed feedback enters in our expression for the particle orientation which, for an active particle, determines (up to noise) the direction of motion in the next time step. Here we estimate the orientation by comparing the delayed position of the particle with the actual one. This method does n...

Chimera States on a Ring of Strongly Coupled Relaxation Oscillators

Rode, Julian ; Totz, Jan Frederik ; Fengler, Enrico ; Engel, Harald (2019-06-19)

Weakly coupled oscillators can exhibit seemingly incongruous synchronization patterns comprised of coherent and incoherent spatial domains known as chimera states. However, the weak coupling approximation is invalid when the characteristic phase response curve of an oscillator does not scale linearly with the coupling strength and instead changes its shape. In chemical experiments with photo-co...

Anomalous transport of magnetic colloids in a liquid crystal–magnetic colloid mixture

Shrivastav, Gaurav P. ; Klapp, Sabine H. L. (2019-01-03)

We report an extensive molecular dynamics study on the translational dynamics of a hybrid system composed of dipolar soft spheres (DSS), representing ferromagnetic particles, suspended in a liquid crystal (LC) matrix. We observe that the LC matrix strongly modifies the dynamics of the DSS. In the isotropic regime, the DSS show a crossover from subdiffusive to normal diffusive behavior at long t...

A flowing pair of particles in inertial microfluidics

Schaaf, Christian ; Rühle, Felix ; Stark, Holger (2019-01-23)

A flowing pair of particles in inertial microfluidics gives important insights into understanding and controlling the collective dynamics of particles like cells or droplets in microfluidic devices. They are applied in medical cell analysis and engineering. We study the dynamics of a pair of solid particles flowing through a rectangular microchannel using lattice Boltzmann simulations. We deter...

Chimeras in Multiplex Networks: Interplay of Inter- and Intra-Layer Delays

Sawicki, Jakub ; Ghosh, Saptarshi ; Jalan, Sarika ; Zakharova, Anna (2019-04-24)

Time delay in complex networks with multiple interacting layers gives rise to special dynamics. We study the scenarios of time delay induced patterns in a three-layer network of FitzHugh-Nagumo oscillators. The topology of each layer is given by a nonlocally coupled ring. For appropriate values of the time delay in the couplings between the nodes, we find chimera states, i.e., hybrid spatio-tem...

Symmetry-Breaking bifurcations and reservoir computing in regular oscillator networks

Röhm, André (2019)

The focus of this thesis is the investigation of regular oscillator networks with numerical and analytical tools. Such systems are ubiquitous in nature and can emerge in the shape of vastly different processes. Among others, they can be mathematically derived as discrete approximations of continuous wave equations or as a local approximation of coupled dynamical systems. In the most basic meani...

Dynamics of collective attention

Lorenz-Spreen, Philipp Gert Josef (2019)

This dissertation aims to capture and understand the macroscopic ebbs and flows of public interest and popular topics. Operationalized as e.g. usage volume of hashtags, movie ticket sales or the counts of comments on online forums, we measure the dynamics of `public attention' for various cultural items in large online data sets. These trajectories of public attention became accessible since th...

Computational and analytical approaches towards epidemic spread containment of temporal animal trade networks

Bassett, George Jason (2019)

In this thesis a threefold approach to assess the risk of epidemic spread on animal trade networks and pinpoint strategies to contain it is presented. Firstly, an exhaustive description of a stochastic, event-driven, hierarchical agent-based model designed to reproduce the infectious state of the cattle disease called Bovine Viral Diarrhea (BVD) on the German trade network is outlined. It takes...

Heat flow due to time-delayed feedback

Loos, Sarah A. M. ; Klapp, Sabine H. L. (2019-02-21)

Many stochastic systems in biology, physics and technology involve discrete time delays in the underlying equations of motion, stemming, e. g., from finite signal transmission times, or a time lag between signal detection and adaption of an apparatus. From a mathematical perspective, delayed systems represent a special class of non-Markovian processes with delta-peaked memory kernels. It is wel...

Control of Chimera States in Multilayer Networks

Omelchenko, Iryna ; Hülser, Tobias ; Zakharova, Anna ; Schöll, Eckehard (2019-01-30)

Chimera states are intriguing complex spatio-temporal patterns of coexisting coherent and incoherent domains. They can often be observed in networks with non-local coupling topology, where each element interacts with its neighbors within a fixed range. In small-size non-locally coupled networks, chimera states usually exhibit short lifetimes and erratic drifting of the spatial position of the i...

Out-of-equilibrium dynamics of open quantum many-body systems

Droenner, Leon Janek (2019)

Many-body localization, which prevents a many-body quantum systems from reaching thermal equilibrium has opened the possibility to study out-of-equilibrium dynamics of isolated quantum systems, leading to new phases of matter such as the discrete time crystal. However, little is known about these effects when the quantum many-body system is coupled to an external environment. An intuitive assum...

Is Spreading Depolarization Characterized by an Abrupt, Massive Release of Gibbs Free Energy from the Human Brain Cortex?

Dreier, Jens P. ; Isele, Thomas ; Reiffurth, Clemens ; Offenhauser, Nikolas ; Kirov, Sergei A. ; Dahlem, Markus A. ; Herreras, Oscar (2013)

In the evolution of the cerebral cortex, the sophisticated organization in a steady state far away from thermodynamic equilibrium has produced the side effect of two fundamental pathological network events: ictal epileptic activity and spreading depolarization. Ictal epileptic activity describes the partial disruption, and spreading depolarization describes the near-complete disruption of the p...

The Divergence of the Viscosity of a Fluid of Hard Spheres as an Indicator for the Fluid-Solid Phase Transition

Koo, Hyearn-Maw ; Hess, Siegfried (1987)

Solution of the Kirkwood-Smoluchowski equation for a hard sphere fluid yields an expression for the viscosity which shows a dramatic pretransitional increase and a divergence at a number density close to that one observed in computer simulations and in colloidal dispersions. The value for the transition density stems from a boundary condition at the surface of the hard sphere in the configurati...

The Viscosity Coefficients of Oriented Nematic and Nematic Discotic Liquid Crystals; Affine Transformation Model

Baalss, D. ; Hess, Siegfried (1988)

General expressions are derived for the anisotropy of the viscosity and for the Leslie coefficients governing the flow alignment subject to the assumption that the equipotential surfaces for the interaction of oriented nonspherical molecules in the nematic or nematic discotic phase of a liquid crystal are related to a spherical interaction potential by an affine transformation. In particular, f...

Exciton-phonon coupling in monolayers of transition metal dichalcogenides

Selig, Malte (2018)

Monolayers of transition metal dichalcogenides (TMDs) attracted much attention in recent research due to their promising optical and electronic properties for future technological applications. In this thesis, an excitonic description within the Heisenberg equation of motion formalism for the optical response of these semiconducting, ultra-thin materials is developed. Hereby the main focus lies...