arXiv subjects

Hartmut Löwen

Publications and source records attributed to Hartmut Löwen.

At least 73 records · Page 4Linked to original sources

arXiv2024

Cybloids $-$ Creation and Control of Cybernetic Colloids

Colloids play an important role in fundamental science as well as in nature and technology. They have had a strong impact on the fundamental understanding of statistical physics. For example, colloids have helped to obtain a better understanding of collective phenomena, ranging from phase transitions and glass formation to the swarming of active Brownian particles. Yet the success of colloidal systems hinges crucially on the specific physical and chemical properties of the colloidal particles, i.e. particles with the appropriate characteristics must be available. Here we present an idea to create particles with freely selectable properties. The properties might depend, for example, on the presence of other particles (hence mimicking specific pair or many-body interactions), previous configurations (hence introducing some memory or feedback), or a directional bias (hence changing the dynamics). Without directly interfering with the sample, each particle is fully controlled and can receive external commands through a predefined algorithm that can take into account any input parameters. This is realized with computer-controlled colloids, which we term cybloids - short for cybernetic colloids. The potential of cybloids is illustrated by programming a time-delayed external potential acting on a single colloid and interaction potentials for many colloids. Both an attractive harmonic potential and an annular potential are implemented. For a single particle, this programming can cause subdiffusive behavior or lend activity. For many colloids, the programmed interaction potential allows to select a crystal structure at wish. Beyond these examples, we discuss further opportunities which cybloids offer.

cond-mat.soft↗

arXiv2024

Anisotropic Brownian particles under resetting

We study analytically the dynamics of an anisotropic particle subjected to different stochastic resetting schemes in two dimensions. The Brownian motion of shape-asymmetric particles in two dimensions results in anisotropic diffusion at short times, while the late-time transport is isotropic due to rotational diffusion. We show that the presence of orientational resetting promotes the anisotropy to late times. When the spatial and orientational degrees of freedom are reset, we find that a non-trivial spatial probability distribution emerges in the steady state that is determined by the initial orientation, particle asymmetry and the resetting rate. When only spatial degrees of freedom are reset while the orientational degree of freedom is allowed to evolve freely, the steady state is independent of the particle asymmetry. When only particle orientation is reset, the late-time probability density is given by a Gaussian with an effective diffusion tensor, including off-diagonal terms, determined by the resetting rate. Generally, the coupling between the translational and rotational degrees of freedom, when combined with stochastic resetting, gives rise to unique behaviour at late times not present in the case of symmetric particles. Considering recent developments in experimental implementations of resetting, our results can be useful for the control of asymmetric colloids, for example in self-assembly processes.

cond-mat.stat-mech↗

arXiv2024

Double Mpemba effect in the cooling of trapped colloids

The Mpemba effect describes the phenomenon that a system at a hot initial temperature cools faster than at an initial warm temperature in the same environment. Such an anomalous cooling has recently been predicted and realized for trapped colloids. Here, we investigate the freezing behavior of a passive colloidal particle by employing numerical Brownian dynamics simulations and theoretical calculations with a model that can be directly tested in experiments. During the cooling process, the colloidal particle exhibits multiple non-monotonic regimes in cooling rates, with the cooling time decreasing twice as a function of the initial temperature-an unexpected phenomenon we refer to as the Double Mpemba effect. Additionally, we demonstrate that both the Mpemba and Double Mpemba effects can be predicted by various machine learning methods, which expedite the analysis of complex, computationally intensive systems.

cond-mat.soft↗

arXiv2024

Partial stochastic resetting with refractory periods

The effect of refractory periods in partial resetting processes is studied. Under Poissonian partial resets, a state variable jumps to a value closer to the origin by a fixed fraction at constant rate, $x\to a x$. Following each reset, a stationary refractory period of arbitrary duration takes place. We derive an exact closed-form expression for the propagator in Fourier-Laplace space, which shows rich dynamical features such as connections not only to other resetting schemes but also to intermittent motion. For diffusive processes, we use the propagator to derive exact expressions for time dependent moments of $x$ at all orders. At late times the system reaches a non-equilibrium steady state which takes the form of a mixture distribution that splits the system into two subpopulations; trajectories that at any given time in the stationary regime find themselves in the freely evolving phase, and those that are in the refractory phase. In contrast to conventional resetting, partial resets give rise to non-trivial steady states even for the refractory subpopulation. Moments and cumulants associated with the steady state density are studied, and we show that a universal optimum for the kurtosis can be found as a function of mean refractory time, determined solely by the strength of the resetting and the mean inter-reset time. The presented results could be of relevance to growth-collapse processes with periods of inactivity following a collapse.

cond-mat.stat-mech↗

arXiv2024

Dynamical clustering and wetting phenomena in inertial active matter

Dynamical clustering represents a characteristic feature of active matter consisting of self-propelled agents that convert energy from the environment into mechanical motion. At the micron scale, typical of overdamped dynamics, particles with opposite motility block each other and show transient dynamical arrest that can induce cluster nucleation and motility-induced phase separation. However, for macroscopic agents, inertia plays a leading role, and clustering is strongly affected by bounce-back effects during collisions that could inhibit cluster growth. Here we present an experiment on the clustering of active granular particles, in which inertia can be systematically tuned. A plethora of new phenomena impacted severely by inertia is presented. Clusters display an inertia-induced transition from liquid-like to crystal-like inner structures that is accompanied by the suppression of cluster nucleation at system boundaries due to particle inertia. In contrast to microswimmers, where active particles wet the boundary by primarily forming clusters attached to the container walls, in an underdamped inertial active system, walls do not favor cluster formation and effectively annihilate motility-induced wetting phenomena.

cond-mat.soft↗

arXiv2024

Controlling Colloidal Flow through a Microfluidic Y-junction

Microscopic particles flowing through narrow channels may accumulate near bifurcation points provoking flow reduction, clogging and ultimately chip breakage. Here we show that the full flow behavior of colloidal particles through a microfluidic Y-junction (i.e. a three way intersection) can be controlled by tuning the pair interactions and the degree of confinement. By combining experiments with numerical simulations, we investigate the dynamic states emerging when magnetizable colloids flow through a symmetric Y-junction such that a single particle can pass through both gates with the same probability. We show that clogging can be avoided by repulsive interactions and branching into the two channels can be steered as well by interactions: attractive particles are flowing through the same gate, while repulsive colloids alternate between the two gates. Even details of the particle assembly such as buckling at the exit gate are tunable by interactions and the channel geometry.

cond-mat.soft↗

arXiv2024

Self-reverting vortices in chiral active matter

There is currently a strong interest in the collective behavior of chiral active particles that can propel and rotate themselves. In the presence of alignment interactions for many chiral particles, chiral self-propulsion can induce vortex patterns in the velocity fields. However, these emerging patterns are non-permanent, and do not induce global vorticity. Here we combine theoretical arguments and computer simulations to predict a so-far unknown class of collective behavior. We show that, for chiral active particles, vortices with significant dynamical coherence emerge spontaneously. They originate from the interplay between attraction interactions and chirality in the absence of alignment interactions. Depending on parameters, the vortices can either feature a constant vorticity or a vorticity that oscillates periodically in time, resulting in self-reverting vortices. Our results may guide future experiments to realize customized collective phenomena such as spontaneously rotating gears and patterns with a self-reverting order.

cond-mat.soft↗

arXiv2024

Dynamics of inertial particles under velocity resetting

We investigate stochastic resetting in coupled systems involving two degrees of freedom, where only one variable is reset. The resetting variable, which we think of as hidden, indirectly affects the remaining observable variable through correlations. We derive the Fourier-Laplace transform of the observable variable's propagator and provide a recursive relation for all the moments, facilitating a comprehensive examination of the process. We apply this framework to inertial transport processes where we observe particle position while the velocity is hidden and is being reset at a constant rate. We show that velocity resetting results in a linearly growing spatial mean squared displacement at late times, independently of reset-free dynamics, due to resetting-induced tempering of velocity correlations. General expressions for the effective diffusion and drift coefficients are derived as function of resetting rate. Non-trivial dependence on the rate may appear due to multiple timescales and crossovers in the reset-free dynamics. An extension that incorporates refractory periods after each reset is considered, where the post-resetting pauses can lead to anomalous diffusive behavior. Our results are of relevance to a wide range of systems, including inertial transport where mechanical momentum is lost in collisions with the environment, or the behavior of living organisms where stop-and-go locomotion with inertia is ubiquitous. Numerical simulations for underdamped Brownian motion and the random acceleration process confirm our findings.

cond-mat.stat-mech↗

arXiv2024

Universal Hyperuniformity in Active Field Theories

We show that dry scalar-order active field theories (AFTs) are universally hyperuniform, i.e., density fluctuations are anomalously suppressed in the long-time limit regardless of the integrability or functional form of the active contributions up to third order in gradient terms. These AFTs include Active model B, Active model B+, and effective Cahn-Hilliard models. Moreover, density variances and spectral densities are virtually indistinguishable from that of passive phase-separated hyperuniform fields. Higher moments of the density fluctuations, however, reveal activity-dependent higher-order correlations that are not captured by conventional two-point measures that characterize hyperuniformity.

cond-mat.stat-mech↗

arXiv2024

Dynamics of magnetic self-propelled particles in a harmonic trap

Artificial active particles, exemplified by Hexbugs (HB), serve as valuable tools for investigating the intricate dynamics of active matter systems. Leveraging their stochastic motion, Hexbugs provides an excellent experimental model. In this study, we utilize Hexbugs equipped with disk-like armor and embedded magnetic dipoles, transforming them into Magnetic Self-Propelled Particles (MSPP). We explore the emergence of collective and stationary states numerically and experimentally by confining these MSPPs within a parabolic domain acting as a harmonic potential. Our findings unveil a diverse range of metastable configurations intricately linked to the complex dynamics inherent in the system. We discern that particle number, activity, and the balance between magnetic and harmonic potential strengths predominantly influence the metastability of these structures. By employing these parameters as control factors, we compare and contrast the behavior of MSPPs with disk-like magnetic Active Brownian Particles (ABPs) in the overdamped limit of vanishing inertia. Our numerical predictions reproduce most of the experimental observations, highlighting the crucial role of magnetic dipole interactions in developing novel configurations for active particles within parabolic domains. These configurations include chains, clusters, and vortex formations characterized by a specific pattern in the particle spatial distribution. Notably, we observe that the influence of inertia is not fundamental in generating metastable configurations in these confined systems. Instead, the particle's shape, activity, and orientation are dominant factors. This comparative analysis provides insights into the distinctive features and dynamics of MSPP within confined environments, shedding light on the role of short-range polar interactions.

cond-mat.soft↗

arXiv2024

Active fluctuations in the harmonic chain: phonons, entropons and velocity correlations

Non-equilibrium random fluctuations of non-thermal nature are a salient feature of active matter. In this work, we consider the collective excitations of active systems at high density, focusing on a one-dimensional chain of elastically coupled inertial particles, whose activity is modeled with an Ornstein-Uhlenbeck process. Their excitation spectrum shows the presence of two kinds of fluctuations: the first ones are thermally excited phonons analogous to those of a passive crystal while the second ones have been termed entropons because associated with the entropy production due to the active forces. These two types of fluctuations show different properties: in fact, only entropons generate spatial velocity correlations and fail to satisfy a standard fluctuation-response relation. We derive the exact expression for the equal-time velocity and displacement correlations as well as for the structure factor and in each case identify the phonon and entropon contributions. Finally, we investigate the dynamical properties of the excitations in terms of steady-state two-time correlations, such as the intermediate scattering function and the mean-square displacement, and show that both phonon and entropon fluctuations are characterized by a long wavelength overdamped regime and a short wavelength underdamped regime. In the large persistence case, entropons decay slower than phonons, and in general, activity tends to suppress the oscillations typical of the underdamped regime.

cond-mat.soft↗

arXiv2023

Collective mechano-response dynamically tunes cell-size distributions in growing bacterial colonies

Mechanical stresses stemming from environmental factors are a key determinant of cellular behavior and physiology. Yet, the role of self-induced biomechanical stresses in growing bacterial colonies has remained largely unexplored. Here, we demonstrate how collective mechanical forcing plays an important role in the dynamics of the cell size of growing bacteria. We observe that the measured elongation rate of well-nourished \textit{Escherichia coli} cells decreases over time, depending on the free area around each individual, and associate this behavior with the response of the growing cells to mechanical stresses. Via a cell-resolved model accounting for the feedback of collective forces on individual cell growth, we quantify the effect of this mechano-response on the structure and composition of growing bacterial colonies, including the local environment of each cell. Finally, we predict that a mechano-cross-response between competing bacterial strains with distinct growth rates affects their size distributions.

cond-mat.soft↗

arXiv2023

Ultrafast entropy production in pump-probe experiments

The ultrafast control of materials has opened the possibility to investigate non-equilibrium states of matter with striking properties, such as transient superconductivity and ferroelectricity, ultrafast magnetization and demagnetization, as well as Floquet engineering. The characterization of the ultrafast thermodynamic properties within the material is key for their control and design. Here, we develop the ultrafast stochastic thermodynamics for laser-excited phonons. We calculate the entropy production and heat absorbed from experimental data for single phonon modes of driven materials from time-resolved X-ray scattering experiments where the crystal is excited by a laser pulse. The spectral entropy production is calculated for SrTiO$_3$ and KTaO$_3$ for different temperatures and reveals a striking relation with the power spectrum of the displacement-displacement correlation function by inducing a broad peak beside the eigenmode-resonance.

cond-mat.mtrl-sci↗

arXiv2023

Active smectics on a sphere

The dynamics of active smectic liquid crystals confined on a spherical surface is explored through an active phase field crystal model. Starting from an initially randomly perturbed isotropic phase, several types of topological defects are spontaneously formed, and then annihilate during a coarsening process until a steady state is achieved. The coarsening process is highly complex involving several scaling laws of defect densities as a function of time where different dynamical exponents can be identified. In general the exponent for the final stage towards the steady state is significantly larger than that in the passive and in the planar case, i.e., the coarsening is getting accelerated both by activity and by the topological and geometrical properties of the sphere. A defect type characteristic for this active system is a rotating spiral of evolving smectic layering lines. On a sphere this defect type also determines the steady state. Our results can in principle be confirmed by dense systems of synthetic or biological active particles.

cond-mat.soft↗

arXiv2023

Emergent memory from tapping collisions in active granular matter

In an equilibrium thermal environment, random elastic collisions between background particles and a tracer establish the picture of Brownian motion fulfilling the celebrated Einstein relation between diffusivity and mobility. In nature, environments often comprise collections of autonomously moving objects that exhibit fascinating non-equilibrium phenomena and are nowadays termed active matter. We investigate experimentally the impact of an active background on a passive tracer using vibrationally excited active particles. They produce multiple correlated tapping collisions with the tracer, for which a persistent memory emerges in the dynamics. The system is described by a generalized active Einstein relation that constrains fluctuations, dissipation, and effective activity, by taking the emerging tracer memory into account. Since the resulting persistence can largely be tuned by the environmental density and motility, our findings can be useful to engineer properties of various active systems in biomedical applications, microfluidics, chemical engineering, or swarm robotics.

cond-mat.stat-mech↗

arXiv2023

Giant Activity-Induced Stress Plateau in Entangled Polymer Solutions

We study the viscoelastic properties of highly entangled, flexible, self-propelled polymers using Brownian dynamics simulations. Our results show that the active motion of the polymer increases the height of the stress plateau by orders of magnitude due to the emergence of grip forces at entanglement points. Identifying the activity-induced energy of a single polymer and the ratio of polymer length to self-propulsion velocity as relevant energy and time scales, we find the stress autocorrelation functions collapse across Péclet numbers. We predict that the long-time viscosity scales with polymer length squared $\sim L^2$, in contrast to equilibrium counterparts $\sim L^3$. These insights offer prospects for designing new materials with activity-responsive mechanical properties.

cond-mat.soft↗

arXiv2023

Patchy landscapes promote stability of small groups

Group formation and coordination are fundamental characteristics of living systems, essential for performing tasks and ensuring survival. Interactions between individuals play a key role in group formation, and the impact of resource distributions is a vibrant area of research. Using active particles in a tuneable optical environment as a model system, we demonstrate that heterogeneous energy source distributions result in smaller, more stable groups with reduced individual exchange between clusters compared to homogeneous conditions. Reduced group sizes can be beneficial to optimise resources in heterogeneous environments and to control information flow within populations. Devoid of biological complications, our system provides insights into the importance of patchy landscapes in ecological dynamics and holds implications for refining swarm intelligence algorithms and enhancing crowd control techniques.

cond-mat.soft↗

arXiv2023

Inertial self-propelled particles in anisotropic environments

Self-propelled particles in anisotropic environments can exhibit a motility that depends on their orientation. This dependence is relevant for a plethora of living organisms but difficult to study in controlled environments. Here, we present a macroscopic system of self-propelled vibrated granular particles on a striated substrate that displays orientation-dependent motility. An extension of the active Brownian motion model involving orientation-dependent motility and inertial effects reproduces and explains our experimental observations. The model can be applied to general $n$-fold symmetric anisotropy and can be helpful for predictive optimization of the dynamics of active matter in complex environments.

cond-mat.soft↗