## FG Statistische Physik weicher Materie und biologischer Systeme

22 Items

**Steering droplets on substrates using moving steps in wettability**

*Grawitter, Josua ; Stark, Holger* (2021-01-07)

Droplets move on substrates with a spatio-temporal wettability pattern as generated, for example, on light-switchable surfaces. To study such cases, we implement the boundary-element method to solve the governing Stokes equations for the fluid flow field inside and on the surface of a droplet and supplement it by the Cox–Voinov law for the dynamics of the contact line. Our approach reproduces t...

**Particle pairs and trains in inertial microfluidics**

*Schaaf, Christian ; Stark, Holger* (2020-08-04)

Staggered and linear multi-particle trains constitute characteristic structures in inertial microfluidics. Using lattice-Boltzmann simulations, we investigate their properties and stability, when flowing through microfluidic channels. We confirm the stability of cross-streamline pairs by showing how they contract or expand to their equilibrium axial distance. In contrast, same-streamline pairs ...

**Active open-loop control of elastic turbulence**

*van Buel, Reinier ; Stark, Holger* (2020-09-24)

We demonstrate through numerical solutions of the Oldroyd-B model in a two-dimensional Taylor–Couette geometry that the onset of elastic turbulence in a viscoelastic fluid can be controlled by imposed shear-rate modulations, one form of active open-loop control. Slow modulations display rich and complex behavior where elastic turbulence is still present, while it vanishes for fast modulations a...

**Emergent collective dynamics of bottom-heavy squirmers under gravity**

*Rühle, Felix ; Stark, Holger* (2020-05-25)

We present the results of hydrodynamic simulations using the method of multi-particle collision dynamics for a system of squirmer microswimmers moving under the influence of gravity at low Reynolds numbers. In addition, the squirmers are bottom-heavy so that they experience a torque which aligns them along the vertical. The squirmers interact hydrodynamically by the flow fields of a stokeslet a...

**Multi-particle collision dynamics with a non-ideal equation of state. I**

*Zantop, Arne W. ; Stark, Holger* (2021-01-08)

The method of multi-particle collision dynamics (MPCD) and its different implementations are commonly used in the field of soft matter physics to simulate fluid flow at the micron scale. Typically, the coarse-grained fluid particles are described by the equation of state of an ideal gas, and the fluid is rather compressible. This is in contrast to conventional fluids, which are incompressible f...

**Inferring the Chemotactic Strategy of P. putida and E. coli Using Modified Kramers-Moyal Coefficients**

*Pohl, Oliver ; Hintsche, Marius ; Alirezaeizanjani, Zahra ; Seyrich, Maximilian ; Beta, Carsten ; Stark, Holger* (2017-01-23)

Many bacteria perform a run-and-tumble random walk to explore their surrounding and to perform chemotaxis. In this article we present a novel method to infer the relevant parameters of bacterial motion from experimental trajectories including the tumbling events. We introduce a stochastic model for the orientation angle, where a shot-noise process initiates tumbles, and analytically calculate c...

**Optimal Control of Colloidal Trajectories in Inertial Microfluidics Using the Saffman Effect**

*Rühle, Felix ; Schaaf, Christian ; Stark, Holger* (2020-06-15)

In inertial microfluidics colloidal particles in a Poiseuille flow experience the Segré-Silberberg lift force, which drives them to specific positions in the channel cross section. An external force applied along the microchannel induces a cross-streamline migration to a new equilibrium position because of the Saffman effect. We apply optimal control theory to design the time protocol of the ax...

**Modeling, Simulation, and Reconstruction of 2-Reservoir Heat-to-Power Processes in Finite-Time Thermodynamics**

*Muschik, Wolfgang ; Hoffmann, Karl Heinz* (2020-09-07)

The connection between endoreversible models of Finite-Time Thermodynamics and the corresponding real running irreversible processes is investigated by introducing two concepts which complement each other: Simulation and Reconstruction. In that context, the importance of particular machine diagrams for Simulation and (reconstruction) parameter diagrams for Reconstruction is emphasized. Addition...

**Squirmer rods as elongated microswimmers: flow fields and confinement**

*Zantop, Arne W. ; Stark, Holger* (2020-06-25)

Microswimmers or active elements, such as bacteria and active filaments, have an elongated shape, which determines their individual and collective dynamics. There is still a need to identify what role long-range hydrodynamic interactions play in their fascinating dynamic structure formation. We construct rods of different aspect ratios using several spherical squirmer model swimmers. With the h...

**Statistical physics of bacterial chemotaxis**

*Seyrich, Maximilian* (2020)

Das Darmbakterium Escherichia coli (E.coli ) ist in den Bereichen der Molekularbiologie, der Zellbiologie und des Forschungsfeldes der Mikroschwimmer eines der am besten erforschten Lebewesen. Die vorliegende Arbeit leistet einen Beitrag zur Grundlagenforschung der Chemotaxis von E.coli (gerichtete Bewegung entlang eines externen chemischen Stimulus). Insbesondere werden neue theoretische Model...

**Lamellipodial dynamics in mesenchymal cell motility**

*Dolati, Setareh* (2019)

Biological processes that require cell migration include a variety of complex and critical processes such as embryonic development, the immune response, and cancer metastasis. Attempts to understand and control the underlying mechanisms of cell movement, especially in cancer research, have recently increased. Although most of the biochemical processes in cell movement are known, some biomechani...

**Traveling concentration pulses of bacteria in a generalized Keller–Segel model**

*Seyrich, Maximilian ; Palugniok, Andrzej ; Stark, Holger* (2019-10-01)

We formulate a Markovian response theory for the tumble rate of a bacterium moving in a chemical field and use it in the Smoluchowski equation. Based on a multipole expansion for the one-particle distribution function and a reaction-diffusion equation for the chemoattractant field, we derive a polarization extended model, which also includes the recently discovered angle bias. In the adiabatic ...

**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 ...

**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...

**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...

**Dissipative systems with nonlocal delayed feedback control**

*Grawitter, Josua ; van Buel, Reinier ; Schaaf, Christian ; Stark, Holger* (2018-11-07)

We present a linear model, which mimics the response of a spatially extended dissipative medium to a distant perturbation, and investigate its dynamics under delayed feedback control. The time a perturbation needs to propagate to a measurement point is captured by an inherent delay time (or latency). A detailed linear stability analysis demonstrates that a nonzero system delay acts to destabili...

**Simulating the complex cell design of Trypanosoma brucei and its motility**

*Alizadehrad, Davod ; Krüger, Timothy ; Engstler, Markus ; Stark, Holger* (2015)

The flagellate Trypanosoma brucei, which causes the sleeping sickness when infecting a mammalian host, goes through an intricate life cycle. It has a rather complex propulsion mechanism and swims in diverse microenvironments. These continuously exert selective pressure, to which the trypanosome adjusts with its architecture and behavior. As a result, the trypanosome assumes a diversity of compl...

**Gravity-induced dynamics of a squirmer microswimmer in wall proximity**

*Rühle, Felix ; Blaschke, Johannes ; Kuhr, Jan-Timm ; Stark, Holger* (2018-02-07)

We perform hydrodynamic simulations using the method of multi-particle collision dynamics and a theoretical analysis to study a single squirmer microswimmer at high Péclet number, which moves in a low Reynolds number fluid and under gravity. The relevant parameters are the ratio α of swimming to bulk sedimentation velocity and the squirmer type β. The combination of self-propulsion, gravitation...

**Physical minimal models of amoeboid cell motility**

*Kulawiak, Dirk Alexander* (2017)

Cell locomotion plays an important role in many biological processes such as the immune system, embryonic development, or cancer metastasis. In these examples, cells interact with their environments or coordinate their movements with other cells, creating collective behavior. In this thesis, we utilize minimal modeling to investigate single and collective cell motility in three different settin...

**Dynamics of a bacterial flagellum under reverse rotation**

*Adhyapak, Tapan Chandra ; Stark, Holger* (2016)

To initiate tumbling of an E. coli, one of the helical flagella reverses its sense of rotation. It then transforms from its normal form first to the transient semicoiled state and subsequently to the curly-I state. The dynamics of polymorphism is effectively modeled by describing flagellar elasticity through an extended Kirchhoff free energy. However, the complete landscape of the free energy r...