arXiv subjects

Klaus Kroy

Publications and source records attributed to Klaus Kroy.

At least 19 recordsLinked to original sources

arXiv2026

Reaction Delays Boost, Rectify, and Reverse Motility in Activity Landscapes

Local detailed balance restricts transport by self-propulsion in static activity landscapes. We show that a delayed speed adaptation (``delayed motility/kinesis'') generically breaks this symme try, inducing directed transport in asymmetric periodic motility profiles and enhancing diffusion in symmetric ones. Both effects occur for run-and-tumble, active Brownian, and inertial active particles, without requiring potential interactions, walls, imposed gradients, higher dimensions, or translational diffusion. Their magnitudes, and even current reversals, are conveniently tuned via the delay time, which establishes delayed motility as a versatile generic and experimentally accessible mechanism for autonomous self-steering and transport control in motile-active-matter circuity.

cond-mat.soft↗

arXiv2026

Probing scale-dependent liveliness with nonequilibrium thermospectroscopy

Probing the spatially heterogeneous activity across scales is a major challenge in living matter. Energy injection at diverse length scales leads to mode coupling, inter-modal energy transfer, and entropy production. We demonstrate the emergence of multiple effective (``active'') temperatures in nonequilibrium molecular- and Brownian-dynamics simulations of an active polymer. Via a generalised Langevin equation for a labelled monomer we identify spectral noise temperatures and their relation to the underlying activity landscape. A harmonic trap of variable stiffness can serve as a minimally invasive prototypical spectroscopic device to selectively scan through the emergent effective temperatures and thereby resolve the scale-dependent activity.

cond-mat.soft↗

arXiv2025

Coevolving aerodynamic and impact ripples on Earth

Windblown sand creates multiscale bedforms on Earth, Mars, and other planetary bodies. According to conventional wisdom, decameter-scale dunes and decimeter-scale ripples emerge via distinct mechanisms on Earth: a hydrodynamic instability related to a phase shift between the turbulent flow and the topography, and a granular instability related to a synchronization of hopping grains with the topography. Here, we report the reproducible creation of coevolving centimeter and decimeter-scale ripples on fine-grained monodisperse sand beds in ambient-air and low-pressure wind-tunnels, revealing two adjacent mesoscale growth instabilities. Their morphological traits and our quantitative grain-scale numerical simulations authenticate the smaller structures as impact ripples but point at a hydrodynamic origin for the larger ones. This suggests that the aeolian transport layer would have to partially respond to the topography on a scale comparable to the average hop length, hence faster than previously thought, but consistent with the phase lag of the inferred aeolian sand flux relative to the wind. Hydrodynamic modelling supports the existence of hydrodynamic aerodynamic ripples on Earth, connecting them mechanistically to megaripples and to the debated Martian ripples. We thereby propose a unified framework for mesoscale granular bedforms found across the Solar System.

physics.geo-ph↗

arXiv2024

Active Ornstein-Uhlenbeck Model for Bacterial Heat Engines

We use Brownian dynamics simulations to study a model of a cyclic bacterial heat engine based on a harmonically confined colloidal probe particle in a bath formed by active Brownian particles. For intermediate activities, active noise experienced by large enough probes becomes Gaussian with exponential autocorrelation function. We show that, in this experimentally pertinent regime, the probability densities for stochastic work, heat, and efficiency are well represented by those of an effective active Ornstein-Uhlenbeck particle (AOUP). Due to the probe's fast relaxation in the potential in typical experimental implementations, good agreement can prevail even when the noise autocorrelation function develops non-exponential tails. Our results show that the AOUP provides a convenient and accurate, analytically tractable effective model to mimic and analyze experimental bacterial heat engines, especially when operating with comparatively large probes and stiff traps.

cond-mat.soft↗

arXiv2024

Modeling the Efficiency and Effective Temperature of Bacterial Heat Engines

We present a Brownian dynamics simulation of the bacterial Stirling engine studied by Krishnamurthy et al., Nat. Phys. 12, 1134 (2016). In their experimental setup, an overdamped colloid in an optical trap with time-modulated stiffness interacts with a bacterial bath that we represent by an ensemble of overdamped active particles. In the parameter regime of the experiment the thermodynamic performance is governed by an effective temperature and can be parametrized analytically by active Brownian particle models. We quantitatively reproduce the efficiencies reported for the experiments under the assumption that energy exchange with the bath during isochores of the cycle is entirely recuperated and in agreement with the second law.

cond-mat.soft↗

arXiv2023

Fluid-Glass-Jamming Rheology of Soft Active Brownian Particles

We numerically study the shear rheology of a binary mixture of soft Active Brownian Particles, from the fluid to the disordered solid regime. At low shear rates, we find a Newtonian regime, where a Green-Kubo relation with an effective temperature provides the linear viscosity. It is followed by a shear-thinning regime at larger shear rates. At high densities, solidification is signalled by the emergence of a finite yield stress. We construct a "fluid-glass-jamming" phase diagram with activity replacing temperature. While both parameters gauge fluctuations, activity also changes the exponent characterizing the decay of the diffusivity close to the glass transition and the shape of the yield stress surface. The dense disordered active solid appears to be mostly dominated by athermal jamming rather than glass rheology.

cond-mat.soft↗

arXiv2023

Irreversible mesoscale fluctuations herald the emergence of dynamical phases

We study fluctuating field models with spontaneously emerging dynamical phases. We consider two typical transition scenarios associated with parity-time symmetry breaking: oscillatory instabilities and critical exceptional points. An analytical investigation of the low-noise regime reveals a drastic increase of the mesoscopic entropy production toward the transitions. For an illustrative model of two nonreciprocally coupled Cahn-Hilliard fields, we find physical interpretations in terms of actively propelled interfaces and a coupling of modes near the critical exceptional point.

cond-mat.stat-mech↗

arXiv2023

Entropy production in the nonreciprocal Cahn-Hilliard model

We study the nonreciprocal Cahn-Hilliard model with thermal noise as a prototypical example of a generic class of non-Hermitian stochastic field theories, analyzed in two companion papers [Suchanek, Kroy, Loos, ArXiv:2303.16701 (2023); Suchanek, Kroy, Loos, ArXiv:2305.05633 (2023)]. Due to the nonreciprocal coupling between two field components, the model is inherently out of equilibrium and can be regarded as an active field theory. Beyond the conventional homogeneous and static-demixed phases, it exhibits a traveling-wave phase, which can be entered via either an oscillatory instability or a critical exceptional point. By means of a Fourier decomposition of the entropy production rate, we quantify the associated scale-resolved time-reversal symmetry breaking, in all phases and across the transitions, in the low-noise regime. Our perturbative calculation reveals its dependence on the strength of the nonreciprocal coupling. Surging entropy production near the static-dynamic transitions can be attributed to entropy-generating fluctuations in the longest wavelength mode and heralds the emerging traveling wave. Its translational dynamics can be mapped on the dissipative ballistic motion of an active (quasi)particle.

cond-mat.soft↗

arXiv2023

Time-reversal and PT symmetry breaking in non-Hermitian field theories

We study time-reversal symmetry breaking in non-Hermitian fluctuating field theories with conserved dynamics, comprising the mesoscopic descriptions of a wide range of nonequilibrium phenomena. They exhibit continuous parity-time ($\mathcal{PT}$) symmetry breaking phase transitions to dynamical phases. For two concrete transition scenarios, exclusive to non-Hermitian dynamics, namely oscillatory instabilities and critical exceptional points, a low-noise expansion exposes a pre-transitional surge of the mesoscale (informatic) entropy production rate, inside the static phases. Its scaling in the susceptibility contrasts conventional critical points (such as second-order phase transitions), where the susceptibility also diverges, but the entropy production generally remains finite. The difference can be attributed to active fluctuations in the wavelengths that become unstable. For critical exceptional points, we identify the coupling of eigenmodes as the entropy-generating mechanism, causing a drastic noise amplification in the Goldstone mode.

cond-mat.stat-mech↗

arXiv2023

Force-Free and Autonomous Active Brownian Ratchets

Autonomous active Brownian ratchets rectify active Brownian particle motion solely by means of a spatially modulated but stationary activity, without external forces. We argue that such ratcheting requires at least a two-dimensional geometry. The underlying principle is similar to the ratcheting induced by steric obstacles in microswimmer baths: suitably polarized swimmers get channeled, while the others get trapped in low-activity regions until they loose direction. The maximum current is generally reached in the limit of large propulsion speeds, in which the rectification efficiency vanishes. Maximum efficiency is attained at intermediate activities and numerically found to be on the order of a few percent, for ratchets with simple wedge-shaped low-activity regions.

cond-mat.soft↗

arXiv2023

Active particles with delayed attractions form quaking crystallites

Perception-reaction delays have experimentally been found to cause a spontaneous circling of microswimmers around a targeted center. Here we investigate the many-body version of this experiment with Brownian-dynamics simulations of active particles in a plane. For short delays, the soft spherical discs form a hexagonal colloidal crystallite around a fixed target particle. Upon increasing the delay time, we observe a bifurcation to a chiral dynamical state that we can map onto that found for a single active particle. The different angular velocities at different distances from the target induce shear stresses that grow with increasing delay. As a result, tangential and, later, also radial shear bands intermittently break the rotating crystallite. Eventually, for long delays, the discs detach from the target particle to circle around it near the preferred single-particle orbit, while spinning and trembling from tidal quakes

cond-mat.soft↗

arXiv2023

Time-reversal symmetries and equilibrium-like Langevin equations

Graham has shown in Z. Physik B 26, 397-405 (1977) that a fluctuation-dissipation relation can be imposed on a class of non-equilibrium Markovian Langevin equations that admit a stationary solution of the corresponding Fokker-Planck equation. The resulting equilibrium form of the Langevin equation is associated with a nonequilibrium Hamiltonian. Here we provide some explicit insight into how this Hamiltonian may loose its time reversal invariance and how the "reactive" and "dissipative" fluxes loose their distinct time reversal symmetries. The antisymmetric coupling matrix between forces and fluxes no longer originates from Poisson brackets and the "reactive" fluxes contribute to the ("housekeeping") entropy production, in the steady state. The time-reversal even and odd parts of the nonequilibrium Hamiltonian contribute in qualitatively different but physically instructive ways to the entropy. We find instances where fluctuations due to noise are solely responsible for the dissipation. Finally, this structure gives rise to a new, physically pertinent instance of frenesy.

cond-mat.stat-mech↗

arXiv2023

Superspreading and Heterogeneity in Epidemics

Epidemic disease spreading is conventionally often modelled and analyzed by means of rate and diffusion equations, following the paradigms of well-controlled chemical reactions and diffusive dynamics in a test tube. Yet, serious worries that this suggestive and appealing similarity might be a false friend were already voiced by the pioneers of mathematical epidemiology. A century later, we can draw on cross-fertilizations from network and game theory and the emerging field of eco-evolutionary dynamics to substantiate them. Epidemiological spreading is thereby revealed as a fundamentally heterogeneous and erratic process that shares certain properties with more unwieldy phenomena, such as earthquakes, hurricanes, traffic jams, and stock crashes. They are all characterised by high tail risks that materialize very rarely but fatally. That they arise from bursts of unlikely chains of localized random "superspreader" events, by which micro-scale fluctuations and uncertainties may get heftily magnified, makes their accurate prediction and control intrinsically and notoriously hard. That epidemic disease spreading is moreover closely intertwined with equally heterogeneous genetic drift and information feedback adds new challenges -- and chances.

physics.soc-ph↗

arXiv2022

Signal propagation and linear response in the delay Vicsek model

Retardation between sensation and action is an inherent biological trait. Here we study its effect in the Vicsek model, which is a paradigmatic swarm model. We find that: (i) a discrete time delay in the orientational interactions diminishes the ability of strongly aligned swarms to follow a leader and, in return, increases their stability against random orientation fluctuations; (ii) both longer delays and higher speeds favor ballistic over diffusive spreading of information (orientation) through the swarm; (iii) for short delays, the mean change in the total orientation (the order parameter) scales linearly in a small orientational bias of the leaders and inversely in the delay time, while its variance first increases and then saturates with increasing delays; (iv) the linear response breaks down when orientation conservation is broken.

cond-mat.soft↗

arXiv2022

Megaripple mechanics: bimodal transport ingrained in bimodal sands

Aeolian sand transport is a major process shaping landscapes on Earth and on diverse celestial bodies. Conditions favoring bimodal sand transport, with fine-grain saltation driving coarse-grain reptation, give rise to the evolution of megaripples with a characteristic bimodal sand composition. Here, we derive a unified phase diagram for this special aeolian process and the ensuing nonequilibrium megaripple morphodynamics by means of a conceptually simple quantitative model, grounded in the grain-scale physics. We establish a well-preserved quantitative signature of bimodal aeolian transport in the otherwise highly variable grain size distributions, namely, the log-scale width (Krumbein phi scale) of their coarse-grain peaks. A comprehensive collection of terrestrial and extraterrestrial data, covering a wide range of geographical sources and environmental conditions, supports the accuracy and robustness of this unexpected theoretical finding. It could help to resolve ambiguities in the classification of terrestrial and extraterrestrial sedimentary bedforms.

physics.geo-ph↗

arXiv2022

Information conduction and convection in noiseless Vicsek flocks

Physical interactions generally respect certain symmetries, such as reciprocity and energy conservation, which survive in coarse grained isothermal descriptions. Active many-body systems usually break such symmetries intrinsically, on the particle level, so that their collective behavior is often more naturally interpreted as a result of information exchange. Here, we study numerically how information spreads from a "leader" particle through an initially aligned flock, described by the Vicsek model without noise. In the low-speed limit of a static spin lattice, we find purely conductive spreading, reminiscent of heat transfer. Swarm motility and heterogeneity can break reciprocity and spin conservation. But what seems more consequential for the swarm response is that the dispersion relation acquires a significant convective contribution along the leader's direction of motion.

cond-mat.soft↗

arXiv2022

Single-dopant band bending fluctuations in MoSe$_2$ measured with electrostatic force microscopy

In this work, we experimentally demonstrate two-state fluctuations in a metal-insulator-semiconductor (MIS) device formed out of a metallic atomic force microscopy tip, vacuum gap, and multilayer MoSe$_2$ sample. We show that noise in this device is intrinsically bias-dependent due to the bias-dependent surface potential, and does not require that the frequency or magnitude of individual dopant fluctuations are themselves bias-dependent. Finally, we measure spatial nonhomogeneities in band bending (charge reorganization) timescales.

cond-mat.mes-hall↗

arXiv2022

Spontaneous vortex formation by microswimmers with retarded attractions

Collective states of inanimate particles self-assemble through physical interactions and thermal motion. Despite some phenomenological resemblance, including signatures of criticality, the autonomous dynamics that binds motile agents into flocks, herds, or swarms allows for much richer behavior. Low-dimensional models have hinted at the crucial role played in this respect by perceived information, decision-making, and feedback implying that the corresponding interactions are inevitably retarded. Here we present experiments on spherical Brownian microswimmers with delayed self-propulsion toward a spatially fixed target. We observe a spontaneous symmetry breaking to a transiently chiral dynamical state and concomitant critical behavior that does not rely on many-particle cooperativity. By comparison with the stochastic delay differential equation of motion of a single swimmer, we pinpoint the delay-induced effective synchronization of the swimmers with their own past as its key mechanism. Increasing numbers of swimmers self-organize into layers with pro- and retrograde orbital motion, synchronized and stabilized by steric and hydrodynamic interactions. Our results demonstrate how even most simple retarded non-reciprocal interactions can foster emergent complex adaptive behavior in relatively small ensembles.

cond-mat.soft↗