arXiv subjects

Alexander Mietke

Publications and source records attributed to Alexander Mietke.

14 recordsLinked to original sources

arXiv2026

Orientational order on non-orientable domains

We study the statistical properties of passive and active many-body systems with orientational degrees of freedom on non-orientable domains. By rephrasing topological constraints as non-local symmetry relations on an orientable double-cover, we show that non-orientability eliminates global rotational soft modes without acting like an external field. In a passive XY model, this results in topological caging, where orientational fluctuations that exhibit conventional diffusive behavior on a torus saturate on a Klein bottle to a finite value that we compute exactly in the thermodynamic limit. In models of active self-propelled particles with orientational degrees of freedom, topological caging persists despite continuously changing interaction neighborhoods. In an active Ising spin model, non-orientability enforces the coexistence of ordered anti-parallel domains with vanishing global polar order, a state that is absent on orientable domains.

cond-mat.stat-mech↗

arXiv2026

Resonances in Overdamped Odd Materials

Odd viscoelasticity arises in parity-violating nonequilibrium materials, where it leads to unconventional mechanical responses and oscillatory relaxation even in overdamped systems. While many living and active chiral materials present promising candidates to exhibit odd viscoelasticity, there is currently no approach that allows for a rheological inference of the large number of elastic and viscous moduli that even a minimal isotropic odd viscoelastic material can depend on. Generalizing the century-old Papkovich-Neuber ansatz to active materials, our work introduces an odd Papkovich-Neuber (OPN) solution -- an analytic solution for any isotropic linear odd fluid or solid, each described by up to 6 independent moduli -- that enable us to study the boundary-driven response in geometries that mimic common rheology methods. OPN solutions reveal three physically distinct resonances in odd viscoelastic solids that are characteristic of the underlying material moduli and can all be interpreted within a single geometric framework. Underlying this unification is an equivalent description of overdamped odd viscoelastic materials in terms of damped harmonic oscillators. Resonances appear as the effective damping coefficients of these oscillators vanish, which is facilitated by the activity that powers odd material properties.

cond-mat.soft↗

arXiv2026

Active Fluid Patterning in Inhomogeneous Environments

Active stresses in biological cells and tissues drive many developmental processes. However, increasing experimental evidence suggests that additional mechanical interactions with surrounding material can play a crucial role in guiding these processes. We introduce a minimal model of this scenario and investigate how pattern formation in an active material can be controlled by an inhomogeneous environment. Specifically, we consider an active fluid in which a chemical species regulates local active stresses and is redistributed by the resulting flows. We show that active stress patterns within such a fluid exhibit frictiotaxis and systematically characterize how inhomogeneous external friction affects mechanochemical pattern formation instabilities. We find that hydrodynamic screening plays a crucial role in mediating the cross-talk between friction patterns and active fluid self-organization and identify a mechanochemical frustration mechanism that gives rise to pattern oscillations caused by inhomogeneous friction.

cond-mat.soft↗

arXiv2026

Confinement-Induced Symmetry Breaking of Active Surfaces

The actomyosin cortex, a thin layer of a cross-linked polymer network near the cell surface, generates active forces that are responsible for cell shape changes. Many developmental processes that involve such cell shape changes, most prominently embryonic cell division, are spatially confined by eggshells. To investigate the potential role of confinement in redirecting active stresses and enabling symmetry breaking phenomena during cell shape transformations, we study a hydrodynamic minimal model in which the cell cortex is represented as an active fluid surface that undergoes symmetric division in the absence of confinement. When enclosed by an ellipsoidal shell, a spontaneous symmetry-breaking transition emerges at a critical degree of confinement, where symmetrically dividing surfaces become unstable and polarized geometries appear. We show that this transition is controlled by the tightness of the confinement and analyze the solution space of stationary surfaces to identify the mechanisms underlying confinement-induced symmetry breaking.

cond-mat.soft↗

arXiv2025

Odd electrical circuits

Non-reciprocal interactions in elastic media give rise to rich non-equilibrium behaviors, but controllable experimental realizations of such odd elastic phenomena remain scarce. Building on recent breakthroughs in electrical analogs of non-Hermitian solid-state systems, we design and analyze scalable odd electrical circuits (OECs) as exact analogs of an odd solid. We show that electrical work can be extracted from OECs via cyclic excitations and trace the apparent energy gain back to active circuit elements. We show that OECs host oscillatory modes that resemble recent experimental observations in living chiral crystals and identify active resonances that reveal a perspective on odd elasticity as a mechanism for mechanical amplification.

cond-mat.soft↗

arXiv2025

Self-organized dynamics and emergent shape spaces of active isotropic fluid surfaces

Theories of self-organized active fluid surfaces have emerged as an important class of minimal models for the shape dynamics of biological membranes, cells and tissues. However, due to their inherent geometric nonlinearities and the absence of general minimization principles in active systems, it remains a major challenge to systematically study the emergent shape spaces such theories give rise to. Here, we introduce a novel variational approach that allows for a direct computation of stationary surface geometries and flows, which enables the classification of non-equilibrium phase transitions in shape spaces described by active surface theories. To achieve this, we construct a dissipation functional systematically from the entropy production in active surfaces and show how generic symmetries imposed by Onsager relations can be exploited to also account for reactive non-dissipative terms in constitutive laws. This functional is supplemented by Lagrange multipliers that relax nonlinear geometric constraints, which leads to a tractable variational problem suitable for implicit dynamic simulations and explicit calculations of non-trivial steady state geometries and flows. We apply this framework to study the dynamics of open fluid membranes and closed active fluid surfaces, and characterize the space of stationary solutions that corresponding surfaces and flows occupy. These analyses rationalize the interplay of first-order shape transitions of internally and externally forced fluid membranes, reveal degenerate regions in stationary shape spaces of mechanochemically active surfaces and identify a mechanism by which hydrodynamic screening controls the geometry of active surfaces undergoing cell division-like shape transformations.

cond-mat.soft↗

arXiv2023

Dynamics, scaling behavior, and control of nuclear wrinkling

The cell nucleus is enveloped by a complex membrane, whose wrinkling has been implicated in disease and cellular aging. The biophysical dynamics and spectral evolution of nuclear wrinkling during multicellular development remain poorly understood due to a lack of direct quantitative measurements. Here, we combine live-imaging experiments, theory, and simulations to characterize the onset and dynamics of nuclear wrinkling during egg development in the fruit fly, Drosophila melanogaster, when nurse cell nuclei increase in size and display stereotypical wrinkling behavior. A spectral analysis of three-dimensional high-resolution data from several hundred nuclei reveals a robust asymptotic power-law scaling of angular fluctuations consistent with renormalization and scaling predictions from a nonlinear elastic shell model. We further demonstrate that nuclear wrinkling can be reversed through osmotic shock and suppressed by microtubule disruption, providing tunable physical and biological control parameters for probing mechanical properties of the nuclear envelope. Our findings advance the biophysical understanding of nuclear membrane fluctuations during early multicellular development.

physics.bio-ph↗

arXiv2023

Rheology of suspensions of flat elastic particles

We consider a suspension of non-interacting flat elastic particles in a Newtonian fluid. We model a flat shape as three beads, carried along by the flow according to Stokes' law, and connected by nonlinear springs, chosen such that the energy is quadratic in the area. In analogy with common dumbbell models involving two beads connected by linear springs, we solve the stochastic equations of motion exactly to compute the constitutive law for the stress tensor of a flat elastic particle suspension. A lower convected time derivative naturally arises as part of the constitutive law, but surprisingly the rheological response in strong extensional and strong contracting flows is similar to that of the classical Oldroyd-B model associated with dumbbell suspensions.

physics.flu-dyn↗

arXiv2022

Odd dynamics of living chiral crystals

Active crystals are highly ordered structures that emerge from the self-organization of motile objects, and have been widely studied in synthetic and bacterial active matter. Whether collective crystallization phenomena can occur in groups of autonomously developing multicellular organisms is currently unknown. Here, we show that swimming starfish embryos spontaneously assemble into chiral crystals that span thousands of spinning organisms and persist for tens of hours. Combining experiments, theory, and simulations, we demonstrate that the formation, dynamics, and dissolution of these living crystals are controlled by the hydrodynamic properties and natural development of embryos. Remarkably, living chiral crystals exhibit self-sustained chiral oscillations as well as various unconventional deformation response behaviors recently predicted for odd elastic materials. Our results provide direct experimental evidence for how nonreciprocal interactions between autonomous multicellular components may facilitate novel nonequilibrium phases of chiral active matter.

cond-mat.soft↗

arXiv2021

Learning developmental mode dynamics from single-cell trajectories

Embryogenesis is a multiscale process during which developmental symmetry breaking transitions give rise to complex multicellular organisms. Recent advances in high-resolution live-cell microscopy provide unprecedented insights into the collective cell dynamics at various stages of embryonic development. This rapid experimental progress poses the theoretical challenge of translating high-dimensional imaging data into predictive low-dimensional models that capture the essential ordering principles governing developmental cell migration in complex geometries. Here, we combine mode decomposition ideas that have proved successful in condensed matter physics and turbulence theory with recent advances in sparse dynamical systems inference to realize a computational framework for learning quantitative continuum models from single-cell imaging data. Considering pan-embryo cell migration during early gastrulation in zebrafish as a widely studied example, we show how cell trajectory data on a curved surface can be coarse-grained and compressed with suitable harmonic basis functions. The resulting low-dimensional representation of the collective cell dynamics enables a compact characterization of developmental symmetry breaking and the direct inference of an interpretable hydrodynamic model, which reveals similarities between pan-embryo cell migration and active Brownian particle dynamics on curved surfaces. Due to its generic conceptual foundation, we expect that mode-based model learning can help advance the quantitative biophysical understanding of a wide range of developmental structure formation processes.

physics.bio-ph↗

arXiv2021

Learning hydrodynamic equations for active matter from particle simulations and experiments

Recent advances in high-resolution imaging techniques and particle-based simulation methods have enabled the precise microscopic characterization of collective dynamics in various biological and engineered active matter systems. In parallel, data-driven algorithms for learning interpretable continuum models have shown promising potential for the recovery of underlying partial differential equations (PDEs) from continuum simulation data. By contrast, learning macroscopic hydrodynamic equations for active matter directly from experiments or particle simulations remains a major challenge. Here, we present a framework that leverages spectral basis representations and sparse regression algorithms to discover PDE models from microscopic simulation and experimental data, while incorporating the relevant physical symmetries. We illustrate the practical potential through applications to a chiral active particle model mimicking swimming cells and to recent microroller experiments. In both cases, our scheme learns hydrodynamic equations that reproduce quantitatively the self-organized collective dynamics observed in the simulations and experiments. This inference framework makes it possible to measure a large number of hydrodynamic parameters in parallel and directly from video data.

cond-mat.soft↗

arXiv2021

Anyonic defect braiding and spontaneous chiral symmetry breaking in dihedral liquid crystals

Dihedral ('$k$-atic') liquid crystals (DLCs) are assemblies of microscopic constituent particles that exhibit $k$-fold discrete rotational and reflection symmetries. Generalizing the half-integer defects in nematic liquid crystals, two-dimensional $k$-atic DLCs can host point defects of fractional topological charge $\pm m/k$. Starting from a generic microscopic model, we derive a unified hydrodynamic description of DLCs with aligning or anti-aligning short-range interactions in terms of Ginzburg-Landau and Landau-Brazovskii-Swift-Hohenberg theories for a universal complex order-parameter field. Building on this framework, we demonstrate in both particle and continuum simulations how adiabatic braiding protocols, implemented through suitable boundary conditions, can emulate anyonic exchange behavior in a classical system. Analytic solutions and simulations of the mean-field theory further predict a novel spontaneous chiral symmetry breaking transition in anti-aligning DLCs, in quantitative agreement with the patterns observed in particle simulations.

cond-mat.soft↗

arXiv2019

Minimal model of cellular symmetry breaking

The cell cortex, a thin film of active material assembled below the cell membrane, plays a key role in cellular symmetry breaking processes such as cell polarity establishment and cell division. Here, we present a minimal model of the self-organization of the cell cortex that is based on a hydrodynamic theory of curved active surfaces. Active stresses on this surface are regulated by a diffusing molecular species. We show that coupling of the active surface to a passive bulk fluid enables spontaneous polarization and the formation of a contractile ring on the surface via mechano-chemical instabilities. We discuss the role of external fields in guiding such pattern formation. Our work reveals that key features of cellular symmetry breaking and cell division can emerge in a minimal model via general dynamic instabilities.

physics.bio-ph↗

arXiv2012

Lessons from Oxypnictide Thin Films

First experiments on the growth of oxypnictide F-doped LaFeAsO thin films indicated an incomplete normal-to-superconducting transition and offered a work programme challenging to overcome possible difficulties in their fabrication. In this regard the possibility of an all in-situ epitaxial growth appeared to be a matter of time and growth parameters. The following review clarifies that F-doped oxypnictide thin films are extremely difficult to grow by in-situ PLD due to the formation of very stable impurity phases such as oxyfluorides (LaOF) and oxides (La2O3) and the loss of stoichiometry possibly due to incongruent evaporation of the target or re-evaporation of volatile elements at the substrate surface. However, the review also demonstrates that the employed two-step fabrication process for oxypnictide thin films has been successfully applied in the preparation of clean polycrystalline as well as of epitaxial thin films. Fundamental investigations on the upper critical field, its temperature dependence and its anisotropy contributed to an understanding of multiband superconductivity in oxypnictides.

cond-mat.supr-con↗