Superdiffusion, large-scale synchronization and topological defects

We study an ensemble of random walkers carrying internal noisy phase oscillators which are synchronized among the walkers by local interactions. Due to individual mobility, the interaction partners of every walker change randomly, hereby introducing an additional, independent source of fluctuations, thus constituting the intrinsic nonequilibrium nature of the temporal dynamics. We employ this paradigmatic model system to discuss how the emergence of order is affected by motion of individual entities. In particular, we consider both, normal diffusive motion and superdiffusion. A non-Hamiltonian field theory including multiplicative noise terms is derived which describes the nonequilibrium dynamics at the macroscale. This theory reveals a defect-mediated transition from incoherence to quasi long-range order for normal diffusion of oscillators in two dimensions, implying a power-law dependence of all synchronization properties on system size. In contrast, superdiffusive transport suppresses the emergence of topological defects, thereby inducing a continuous synchronization transition to long-range order in two dimensions. These results are consistent with particle-based simulations.Comment: 7 pages, 5 figures, submitted to Phys. Rev.

A particle-field representation unifies paradigms in active matter

Active matter research focuses on the emergent behavior among interacting self-propelled particles. Unification of seemingly disconnected paradigms -- active phase-separation of repulsive discs and collective motion of self-propelled rods -- is a major challenge in contemporary active matter. Inspired by the quanto-mechanical wave-particle duality, we develop an approach based on the representation of active particles by smoothed continuum fields. On the basis of the collision kinetics, we demonstrate analytically and numerically how nonequilibrium stresses acting among self-driven, anisotropic objects hinder the formation of phase-separated states as observed for self-propelled discs and facilitate the emergence of orientational order. Besides particle shape, the rigidity of self-propelled objects controlling the symmetry of emergent ordered states is as a crucial parameter: impenetrable, anisotropic rods are found to form polar, moving clusters, whereas large-scale nematic structures emerge for soft rods, notably separated by a bistable coexistence regime. These results indicate that the symmetry of the ordered state is not dictated by the symmetry of the interaction potential but is rather a dynamical, emergent property of active systems. This unifying theoretical framework can represent a variety of active systems: living cell tissues, bacterial colonies, cytoskeletal extracts as well as shaken granular media.Comment: 14 pages, 8 figure

Cargo size limits and forces of cell-driven microtransport

The integration of motile cells into biohybrid microrobots offers unique properties such as sensitive responses to external stimuli, resilience, and intrinsic energy supply. Here we study biohybrid microtransporters that are driven by amoeboid Dictyostelium discoideum cells and explore how the speed of transport and the resulting viscous drag force scales with increasing radius of the spherical cargo particle. Using a simplified geometrical model of the cell-cargo interaction, we extrapolate our findings towards larger cargo sizes that are not accessible with our experimental setup and predict a maximal cargo size beyond which active cell-driven transport will stall. The active forces exerted by the cells to move a cargo show mechanoresponsive adaptation and increase dramatically when challenged by an external pulling force, a mechanism that may become relevant when navigating cargo through complex heterogeneous environments

From single to collective motion of social amoebae: a computational study of interacting cells

The coupling of the internal mechanisms of cell polarization to cell shape deformations and subsequent cell crawling poses many interdisciplinary scientific challenges. Several mathematical approaches have been proposed to model the coupling of both processes, where one of the most successful methods relies on a phase field that encodes the morphology of the cell, together with the integration of partial differential equations that account for the polarization mechanism inside the cell domain as defined by the phase field. This approach has been previously employed to model the motion of single cells of the social amoeba Dictyostelium discoideum, a widely used model organism to study actin-driven motility and chemotaxis of eukaryotic cells. Besides single cell motility, Dictyostelium discoideum is also well-known for its collective behavior. Here, we extend the previously introduced model for single cell motility to describe the collective motion of large populations of interacting amoebae by including repulsive interactions between the cells. We performed numerical simulations of this model, first characterizing the motion of single cells in terms of their polarity and velocity vectors. We then systematically studied the collisions between two cells that provided the basic interaction scenarios also observed in larger ensembles of interacting amoebae. Finally, the relevance of the cell density was analyzed, revealing a systematic decrease of the motility with density, associated with the formation of transient cell clusters that emerge in this system even though our model does not include any attractive interactions between cells. This model is a prototypical active matter system for the investigation of the emergent collective dynamics of deformable, self-driven cells with a highly complex, nonlinear coupling of cell shape deformations, self-propulsion and repulsive cell-cell interactions. Understanding these self-organization processes of cells like their autonomous aggregation is of high relevance as collective amoeboid motility is part of wound healing, embryonic morphogenesis or pathological processes like the spreading of metastatic cancer cells.Postprint (published version

Erfahrungsbericht Segregationsanalysen, Teil 1: Die Hürden der Datenbeschaffung

Teil 1 des Erfahrungsberichts zur Analyse von Segregationstrends in Thüringer Klein- und Mittelstädten berichtet von den Hürden bei der Akquise kleinräumiger statistischer Daten, die Aufschluss über sozioökonomische, ethnische oder demographische Trends geben könnten. Die Städtestatistik ist in der Verwaltung meist mit so geringen Ressourcen ausgestattet, dass die Chancen, einen Zugang zu nutzbaren Daten zu erhalten, sinken, je kleiner die Städte sind. Auch die Sparzwänge schrumpfender Städte tragen dazu bei.Part 1 of the report analysing segregation trends in small and medium-sized towns in Thuringia discusses barriers to the acquisition of small-scale statistical data that could provide information about socio-economic, ethnic or demographic trends. The limited resources provided for urban statistics within local administrations mean that the chances of obtaining access to useable data decline with the size of the town. The austerity measures necessary in shrinking towns also contribute to this problem

Erfahrungsbericht Segregationsanalysen, Teil 2: Empirische Befunde

Teil 2 des Erfahrungsberichts zur Analyse von Segregationstrends in Thüringer Kleinund Mittelstädten dokumentiert die Ergebnisse der Analyse der tatsächlich zusammengetragenen Daten. Während aufgrund der Hürden in der Datenakquise keine Ergebnisse für die sozioökonomische Segregation vorhanden sind, können in der ethnischen und demographischen Dimension Trends gezeigt werden. So steigen die Werte für den Segregationsindex von Einwohnern/Einwohnerinnen ohne deutschen Pass und für den Isolationsindex von Senioren/Seniorinnen. In diesem Beitrag wird der Schluss gezogen, dass residentielle Segregation nicht nur ein Thema auch für Klein- und Mittelstädte ist, sondern dass es Hinweise auf ihre Zunahme gibt.Part 2 of the report analysing segregation trends in small and medium-sized towns in Thuringia documents the results of the analysis of the data that were actually gathered. While barriers to data acquisition mean that there are no results on socio-economic segregation, it is possible to show trends in the ethnic and demographic dimensions. Thus, the values increased for the segregation index of residents without a German passport and for the isolation index of older people. The article concludes that not only is residential segregation an issue for small and medium-sized towns but there are also indications that it is increasing

Small town research in Germany - status quo and recommendations

Urban studies in Germany are traditionally oriented towards large cities. The structures, meanings and functions of small towns are not sufficiently perceived and differentiated in scientific or political debates. Adequate research on small towns requires systematic, comparative, inter- and transdisciplinary approaches. Traditional attributions should be questioned critically and small towns should be examined empirically in their diversity and differentiation. This involves paying attention to external influences and heterogeneous internal structures as well as to regional functions and interdependencies. The availability and generation of statistical data, which also make small-scale analyses possible, are just as necessary as more comprehensive studies, which go beyond limited case studies. Finally, also research funding and academic teaching should address small towns more systematically than it has been the case in the past. This position paper presents recommendations for research, university teaching, official statistics and research funding in the field of small town research. The Ad-hoc Working Group focused on small town research in Germany and German-language literature, respectively

The use of standards for identifying, codifying and transmitting expert ergonomic knowledge

Formal technical standards based on ergonomic principles can ensure that products, systems and services are fit for purpose, accessible and useable. The application of these standards should be used to ensure that items of technology meet political requirements for equality by enabling the full range of end users to participate in the digital society. Ergonomists and representatives of consumers participate in the specification and creation of these standards to ensure that their content is relevant, correct and up-to-date. They work to ensure that the standards accurately represent the needs and requirements of end users including amongst others people with disabilities, older people and people with different language and cultural backgrounds. A number of these standards are referenced in law and in procurement contracts. They are not often not used in higher education resulting in knowledge deficit for young technical professionals. The paper is based on the authors experience including working in the area of accessibility standardization and at a University which prides itself on the diversity of its staff and has students from more than 150 nations. The paper ends with a consideration of the way in which more effective use can be made of these standards