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.

Tanja Schilling: „The interplay between memory and the potential of mean force”
Universität Freiburg - Seminar@HHUD: 24.3.2022 14:30 s.t., Lecture hall 25.31 5J

Active matter, responsive materials and materials under time-dependent load are systems out of thermal equilibrium. To construct coarse-grained models for such systems, one needs to integrate out a distribution of microstates that evolves in time. This is a challenging task. In this talk, we review recent developments in theoretical approaches to the non-equilibrium coarse-graining problem, in particular, projection operator formalisms. We discuss the role of the potential of mean force in non-linear versions of the Langevin equation and its relation to the second fluctuation dissipation theorem.

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.