Iman Abdoli: „Odd Diffusivity”
Institute Theory of Polymers, Leibniz Institute for Polymer Research, Dresden - Seminar@HHUD: 9.1.23 14:30 s.t., Seminarroom 25.32 O2.51

Odd diffusion systems, such as colloidal systems subjected to Lorentz forces and chiral active particles are characterized by a diffusion tensor which has both a symmetric part and an antisymmetric part. The antisymmetric or odd part is a consequence of broken time-reversal symmetry. In the same way that odd viscosity generates normal stresses in the direction perpendicular to shear flow, odd diffusivity generates additional rotational fluxes perpendicular to typical diffusive fluxes. In this presentation, I will be talking about oddity and the diffusion systems performing odd behavior.

Ion Santra: „Activity driven transport in harmonic chains”
Raman Research Institute, Bangalore, India - Seminar@HHUD: 21.12.22 11:00 s.t., Seminarroom 25.32 O2.51

The transport properties of an extended system driven by active reservoirs is an issue of paramount importance, which remains virtually unexplored. Here we address this issue, for the first time, in the context of energy transport between two active reservoirs connected by a chain of harmonic oscillators. The couplings to the active reservoirs, which exert correlated stochastic forces on the boundary oscillators, lead to fascinating behavior of the energy current and kinetic temperature profile even for this linear system. We analytically show that the stationary active current (i) changes non-monotonically as the activity of the reservoirs are changed, leading to a negative differential conductivity (NDC), and (ii) exhibits an unexpected direction reversal at some finite value of the activity drive.We also show that despite an apparent similarity of the kinetic temperature profile to the thermally driven scenario, no effective thermal picture can be consistently built in general. However, such a picture emerges in the small activity limit.

Simon K. Schnyder: „Cell colony dynamics and competition in a hybrid mechanochemical model”
Institute of Industrial Science, The University of Tokyo, Japan - Seminar@HHUD: 14.12.22 15:30 s.t., Seminarroom 25.32 O2.51

Mechanics-based models for cellular tissues often assume that cell growth and division directly depend on physical variables such as pressure. However, biologists understand these processes as being organized by the cell cycle. While both approaches have had successes, they have so far not been properly unified. In a first step, we developed a hybrid model for tissue growth that combines a regulatory cell cycle with a physical model. We studied this model in particle-based simulations and continuum analysis, investigating the expansion of non-motile colonies in quasi-1D channels and on 2D substrates [1]. The model reproduces a range of experimental observations, including faithful pressure, volume and speed regulation. We then studied the competition of two colonies with different rates of programmed cell death (apoptosis) rates and characteristic cell-cycle control pressures [2]. Synchronisation of cell division/apoptosis events can emerge, causing oscillations in cell number, pressure and cell-cycle activity.

References

1. Li, J., Schnyder, S. K., Turner, M. S. & Yamamoto, R., Role of the Cell Cycle in Collective Cell Dynamics, Phys. Rev. X 11, 031025 (2021)
2. Li, J., Schnyder, S. K., Turner, M. S. & Yamamoto, R., Competition between cell types under cell cycle regulation with apoptosis, Phys. Rev. Res. 4, 033156 (2022)

Felix Höfling: „Theory and simulation of liquid samples with open boundaries”
Department of Mathematics and Computer Science, FU Berlin - Seminar@HHUD: 13.10.2022 14:30 s.t., Seminarroom 25.32 O2.51

The presence of a confining boundary can change the local structure of a liquid markedly. However, also open boundaries can modify the outcomes of observations made on a sample that is virtually cut out from a large piece of homogeneous liquid. I will show that local den- sity fluctuations pick up finite-size corrections in the presence of a surface with free bound- ary conditions [1]. The analytic expressions, in excellent agreement with simulation data, show that the effect is particularly pronounced if the size of the open sub-volume decreases below the correlation length. This dependence can be used to establish a finite-size scaling analysis based on open subsystems for the calculation of critical points in the phase diagram of, e.g., a binary liquid [2]. To this end, it is necessary to account for two competing length scales: the extent of the sub-volume and the size of the simulation box. Eventually, I will discuss molecular dynamics simulations far from equilibrium by coupling an open system to thermodynamically distinct reservoirs [3]. We have used this approach to study steady heat and mass flows in liquid samples.

References

1. F. Höfling and S. Dietrich, J. Chem. Phys. 153, 054119 (2020)
2. Y. Pathania, D. Chakraborty, and F. Höfling, Adv. Theory Simul. 4, 2000235 (2021)
3. R. Ebrahimi Viand, F. Höfling, R. Klein, and L. Delle Site, J. Chem. Phys. 153, 101102 (2020)

Marco G. Mazza: „Emergent probability fluxes in confined microbial navigation: finding order from chaos”
Loughborough University, United Kingdom & MPI Göttingen - Seminar@HHUD: 30.8.22 14:30 s.t., Seminarroom 25.32 O2.51

In recent years, biological motile cells like bacteria and microalgae have attracted considerable interest not only among biologists but also in the physics community and related fields. Understanding their motion has immense biological and ecological implications. The possibility to harness their motion to power microdevices is a topic of exceptional importance for modern microtechnology. When the motion of a motile cell is observed closely, it appears erratic, and yet the combination of nonequilibrium forces and surfaces can produce striking examples of organization in microbial systems. While our current understanding is based on very simple environments, it remains elusive how, and, at which length scale self-organization emerges in complex geometries. Combining experiments, analytical and numerical calculations [1,2] we study the motion of motile cells and demonstrate that intricate patterns can be observed even at the level of a single cell exploring an isolated habitat. We can explain how curvature guides the motion of the cell. We theoretically predict a universal relation between probability fluxes and global geometric properties that is directly confirmed by experiments [2]. Our results represent a general description of the structure of such nonequilibrium fluxes down at the single cell level. This might open the possibility of designing devices that are able to guide the motion of such microbial cells.

References

1. J. Cammann, et al., Proc. Natl. Acad. Sci. 118, e2024752118 (2021)
2. T. Ostapenko, et al, Phys. Rev. Lett. 120, 068002 (2018)

Laura Alvarez: „Soft and Reconfigurable Active Systems: towards bioinspired microdevices”
ETH Zurich,Switzerland & Université de Bordeaux, France - Seminar@HHUD: 17.8.22 14:30 s.t., Seminarroom 25.32 O2.51

Motile microorganisms, such as bacteria, have developed sophisticated mechanisms to regulate their dynamics based on environmental cues [1]. Synthetic microswimmers mimicking this self-regulated motion serve as candidates towards the development of smart micromachines. However, existing systems based on self-propelled microparticles lack the intrinsic adaptive mechanisms present in their biological counterparts, mostly relying on external actuation. How can we push the experimental limits and realize bioinspired microscale devices?

Here I will demonstrate that the incorporation of soft responsive materials is the key towards a new generation of artificial microswimmers. First, I will present adaptive active assemblies comprising thermo-responsive PNIPAM microgels. The active clusters, actuated by AC electric fields, reconfigure their shape and dielectric properties via light-induce temperature variations [2]. Thus, we observe intrinsic modulation in their persistence length, direction of motion, and chirality thanks to the presence if an internal temperature 'sensor'. Secondly, I will present our recent advances on the fabrication of cell-like active assemblies using giant unilamellar vesicles (GUVs). In contrast to the traditional active colloids, active GUVs present an excellent cell-model system, thanks to their membrane properties and ability to enclose nano and micro-objects [3]. Both of the presented assemblies reveal exciting opportunities for the development of soft and adaptive microdevices with in-built feedback via bottom-up approaches.

References

1. Son, K., Brumley, D. & Stocker, R. Live from under the lens: exploring microbial motility with dynamic imaging and microfluidics. Nat Rev Microbiol 13, 761 (2015).
2. L. Alvarez, M.A. Fernandez-Rodriguez, A. Alegria, S. Arrese-Igor, K. Zhao, M. Kröger and L. Isa Reconfigurable Active Colloids with Internal Feedback, Nature Communications, 12, 4762 (2021)
3. V. Willems, Eric Dufresne, L. Alvarez. Active motion and division of phase separated soft biocomparments. In preparation.

Suvendu Mandal: „Optimal spreading of active polymers in porous media”
Universität Darmstadt - Seminar@HHUD: 5.5.22 12:30 s.t., Seminarroom 25.32 O3.51

We perform Brownian dynamics simulations of active stiff polymers undergoing run-reverse dynamics, and so mimic bacterial swimming, in porous media. In accord with recent experiments of Escherichia coli, the polymer dynamics are characterized by trapping phases interrupted by directed hopping motion through the pores. We find that the effective translational diffusivities of run-reverse agents can be enhanced up to two orders in magnitude, compared to their non-reversing counterparts, and exhibit a non-monotonic behavior as a function of the reversal rate, which we rationalize using a coarse-grained model. We discover a geometric criterion for the optimal spreading, which emerges when their run lengths are comparable to the longest straight path available in the porous medium. More significantly, our criterion unifies results for porous media with disparate pore sizes and shapes and thus provides a fundamental principle for optimal transport of microorganisms and cargo-carriers in densely-packed biological .nd environmental settings.

Prof. Dr. Martin Müser: „Contact mechanics and its relation to empirical friction laws”
Universität des Saarlandes, Saarbrücken - Seminar@HHUD: 02.02.2023 16:30 s.t., Lecture hall 25.31 O0.5J

Empirical laws describing the friction between solids are simple, however, the microscopic origin of solid friction is generally quite complex, involving the concerted action of many degrees of freedom. Over the centuries, many different processes have been identified causing solid friction to occur, however, for most processes, it is generally difficult to argue why they would lead to the frequently observed (a) proportionality between solid friction and load and (b) irrelevance of the apparent contact area. Moreover, when several mechanisms act in parallel, as is usually the case, it is difficult to predict simple trends like whether more roughness increases or decreases friction: More roughness generally leads to more plastic and viscoelastic deformation and thus to higher friction but roughness also reduces true, microscopic contact and adhesive or capillary forces thereby lowering friction. Given the diversity of mechanisms, it is clear that there can be no universal reason for the empirical laws describing solid friction. Nonetheless, it turns out that the surface topography of the solids in contact and their change with load and sliding turns out crucial.

In my talk, I will present recent advances in contact mechanics, which describes contact-induced deformations and stresses, and explore how the newly gained insights benefit the explanation of the validity of empirical friction laws as well as reasons for their breakdown.

Prof. Dr. Roel Dullens: „Emergence of interparticle friction in attractive colloidal matter”
Radboud University Nijmegen - Seminar@HHUD: 26.01.2023 16:30 s.t., Lecture hall 25.31 O0.5J

Interparticle friction plays a governing role in the mechanics of particulate materials. However, virtually all experimental studies to date rely on measuring macroscopic responses, and as such it remains largely unknown how frictional effects emerge at the microscopic level. This is particularly challenging in systems subject to thermal fluctuations due to the transient nature of interparticle contacts. Here, we directly relate particle-level frictional arrest to local coordination in an attractive colloidal model system. We reveal that the orientational dynamics of particles slows down exponentially with increasing coordination number due to the emergence of frictional interactions, the strength of which can be tuned simply by varying the attraction strength. Using a simple computer simulation model, we uncover how the interparti-cle interactions govern the formation of frictional contacts between particles. Our results establish quantitative relations between friction, coordination and interparticle interactions. This is a key step towards using interparticle friction to tune the mechan-ical properties of particulate materials.

Prof. Dr. Ulf Wiedwald: „Magnetic Nanoparticles: From Physical Design to Medical Applications”
Universität Duisburg-Essen - Seminar@HHUD: 19.01.2023 16:30 s.t., Lecture hall 25.31 O0.5J

The application of magnetic nanoparticles (MNPs) in biomedicine and theranostics is one of the most dynamic and promising fields of nanoparticles research. In my presentation, several examples for the use of multifunctional hybrid MNPs in theranostics are discussed. We designed, synthesized and tested various MNPs like ferrites [1], core-shell architectures [2], and magnetite-gold (Fe₃O₄-Au) hybrids [3] for optimized performance, e.g. in magnetic resonance imaging (MRI), magnetic particle hyperthermia (MPH) or DNA splitting [4]. The multidisciplinary approach allows us to address the entire research chain from the chemical synthesis, over the physical properties to cancer treatment methodologies [5] and toxicity screening [6].

References

1. E. Myrovali et al., Sci. Rep. 6, 37934 (2016).
2. S. Liébana-Viñas et al., RSC Advances 6, 72918 (2016).
3. M. V. Efremova et al., Sci. Rep. 8, 11295 (2018).
4. A. Nikitin et al., ACS Applied Materials & Interfaces 13, 14458–14469 (2021).
5. A. Garanina et al., Nanomedicine: Nanotechnol., Biol. and Med. 25, 102171 (2020).
6. A. Garanina et al., Nanomaterials 12, 38 (2022).

Dr. Francesco Buccheri: „Transport in Topological materials”
Institut für Theoretische Physik IV, Heinrich-Heine-Universität Düsseldorf - Seminar@HHUD: 12.1.23 16:30 s.t., Lecture hall 25.31 O0.5J

The notion of topology applied to the band theory of crystalline solids has become a paradigm to complement our understanding of phases of matter and opens the possibility of important technological advances in the fields of quantum computation and electronic control. In this talk, I will introduce two paradigmatic models, a quasi-one-dimensional supercon-ductor and a three-dimensional semimetal, and review some of their recent experimental realization. I will characterize the associated topologically non-trivial states and their robustness against electron-phonon interaction, a key step toward their detection. I will also investigate the signatures in thermoelectric transport, highlighting the contribution of the edge or surface states.

Prof. Dr. Reinhard Werner: „Geschichten über Quantenkorrelationen”
Leibniz Universität Hannover - Seminar@HHUD: 1.12.2022 16:30 s.t., Lecture hall 25.31 O0.5J

Verschränkung ist ein Schlüsselbegriff der Quantentheorie und allermodernen Quantentechnologien. In diesem Vortrag betrachte ich die Meilensteine der Entwicklung und was die jeweiligen Forscher umgetrieben hat. Von Einstein-Podolsky-Rosen über Bell und Tsirelson läuft eine Theorie-Entwicklung, für deren experimentelle Bestätigung es den dies-jährigen Physik-Nobelpreis für Clauser, Aspect und Zeilinger gab. Dies wird wieder eine Flut von wohlmeinenden Erklärungen auslösen, die Verschränkung zu einem mysteriösen Spuk vernebeln. Diesen Nebel ein wenig transparenter zu machen, ist das Hauptanliegen meines Vortrags.

Julia Sammet: „Physik-Lernzentrum: Von Studierenden für Studierende”
Goethe-Universität Frankfurt am Main - Seminar@HHUD: 20.10.22 16:30 s.t., Lecture hall 25.31 O0.5J

Außercurriculare Unterstützungsstrukturen sind in den letzten Jahren immer populärer geworden. Bei der Einrichtung solcher Strukturen stellt sich die Frage der organisatorischen und finanziellen Ausgestaltung. Das Physik-Lernzentrum in Frankfurt wurde von fünf Fachschaften aus verschiedenen Fachbereichen für mehrere Preise nominiert und wurde neben dem universitätseigenen 1822-Preis für exzellente Lehre auch mit dem Ars Legendi Fakultäten Preis 2022 ausgezeichnet. Im Vortrag stellen wir das Konzept unseres Lernzentrums und der Physik Vorkurse vor und diskutieren die Notwendigkeit/Sinnhaftigkeit solcher außercurricularen Lernstrukturen.

Prof. Dr. Smarajit Karmakar: „Glass Transition in Active and Passive Disordered Systems”
Tata Institute of Fundamental Research, Hyderabad, India - Seminar@HHUD: 7.7.22 16:30 s.t., Lecture hall 25.31 O0.5J

The physics of the glass transition is an age-old problem and the 2021 Physics Nobel Prize was awarded to Prof. Giorgio Parisi for his seminal work on disordered systems. Recently glassy behaviour in systems with active self-propelled particles has added a new dimension to this problem. Although it is not clear whether activity can help us in having a better understanding of the equilibrium glass transition problem, it definitely is generating a plethora of new phenomena.

In this colloquium, I want to talk about some of our works on glasses, but to celebrate the 2021 Physics Nobel Prize, I will start by giving a brief introduction to the works of my former postdoctoral mentor, Prof. Giorgio Parisi. I intend to talk about some of his important works like replica symmetry breaking in the context of spin glasses, and Monte Carlo techniques, in particular, the swap Monte Carlo method for understanding deep supercooled glassy states along with some of my own works in these directions. Then I will briefly discuss his seminal work on bird flocking which led to new insights in the field of active matter and active glasses. Finally, I will conclude by showing some of our recent results on active glasses.

Prof. Dr. Tanja Weil: „Chemical Bonding and Valence”
Max-Planck-Institut für Polymerforschung, Mainz - Seminar@HHUD: 8.6.22 16:30 s.t., Lecture hall 26.41 O0.6G

Wouldn´t it be amazing if we could design soft materials that could actively integrate into cells or tissues and stimulate cellular responses? Can we envision materials instructing cells to grow, proliferate or induce apoptosis? How would such materials look like and could we learn the language of cells and translate it into communicating materials?

In my presentation, I will first discuss the identification of bioactive peptide nanostructures that stimulate neuron outgrowth or enrich virions at the cell membrane, which is attractive for applications in regenerative medicine and gene therapy. The controlled formation of peptide nanostructures within the cytoplasm through chemical cascade reactions opens new avenues as metabolic inhibitors of cancer cells.

Ultimately, our field requires a materials revolution to design soft materials that resemble certain features of living matter so that they could communicate and stimulate desired cellular processes, such as those required for regeneration or cancer therapy, among others.

Prof. Dr. Mathias Kläui: „Antiferromagnetic Insulatronics: Spintronics without magnetic fields”
Johannes Gutenberg-University Mainz - Seminar@HHUD: 02.06.2022 16:30 s.t., Lecture hall 25.31 O0.5J

While known for a long time, antiferromagnetically ordered systems have previously been considered, as "interesting but useless". However, since antiferromagnets potentially promises faster operation, enhanced stability and higher integration densities, they could potentially become a game changer for new spintronic devices. Here I will show how antiferromagnets can be used as active spintronics devices by demonstrating the key operations of "reading"[1], "writing"[2], and "transporting information"[3] in antiferromagnets.

References

1. S. Bodnar et al., Nature Commun. 9, 348 (2018); L. Baldrati et al., Phys. Rev. Lett. 125, 077201 (2020)
2. L. Baldrati et al., Phys. Rev. Lett. 123, 177201 (2019); H. Meer et al., Nano Lett. 21, 114 (2020); S. P. Bommanaboyena et al., Nature Commun. 12, 6539 (2021);
3. R. Lebrun et al., Nature 561, 222 (2018). R. Lebrun et al., Nature Commun. 11, 6332 (2020)

Dr. Selym Villalba-Chavez: „Gravitational waves and binary black holes”
Institut für Theoretische Physik I, Heinrich-Heine-Universität Düsseldorf - Seminar@HHUD: 19.5.2022 16:30 s.t., Lecture hall 25.31 O0.5J

Gravitational waves are space-time ripples propagating with the speed of light somewhat similar to electromagnetic waves. In 2015, the gravitational-wave observatory LIGO detected, for the first time, the passing of gravitational waves through Earth. This event was triggered by waves produced by two black holes that were orbiting on quasi-circular orbits. The talk focuses first on verifying that the existence of gravitational waves is unavoidable within the framework of General Relativity. The final part of the presentation is oriented to show how one can estimate some physical parameters of the binary black hole system, from the properties of the detected signal.

The left panel sketches two orbiting black holes while losing energy in the form of gravitational waves. The signal detected at LIGO on September 14, 2015 is shown in the right.

Dr. Selym Villalba-Chávez: „Probing quantum vacuum-like scenarios with high-intensity laser pulses”
Institut für Theoretische Physik I, Heinrich-Heine-Universität Düsseldorf - Seminar@HHUD: 9.7.2020 16:30 s.t., Seminarroom Online

Gaining comprehensive insights into the properties of the quantum vacuum is central in our continuous quest for obtaining an in-depth and clear understanding of the fundamental laws of our Universe. While the vacuum in classical physics simply describes a region devoid of matter, the framework of quantum field theory reveals that it is characterized by fluctuations of all plausible and realizable fields in Nature. The talk focuses first on the hitherto unobserved effect of the vacuum decay into electron-positron pairs, paying particular attention to a novel way of amplifying this phenomenon. It will be shown that the principle on which this enhancement relies can be tested via gapped graphene monolayers, and revealed that – although this material resembles the QED vacuum – in some processes its two dimensional structure causes some striking differences between both scenarios. The final part of the talk is oriented to show how experiments involving high-intensity lasers can become a powerful tool to limit the parameter spaces of some dark matter candidates such as axions, hidden-photons and minicharged particles.

The left panel depicts the electron/positron spectrum produced by superimposing a strong time dependent electric field and a fast oscillating wave. A scheme for testing the Breit-Wheeler-like production of massive Dirac pairs in a gapped graphene monolayer is shown in the right panel.