arXiv subjects

Thomas Speck

Publications and source records attributed to Thomas Speck.

At least 19 recordsLinked to original sources

arXiv2026

Designing topological edge currents in chiral active matter

Achieving robust functionality in active matter driven away from thermal equilibrium is a current theoretical and experimental challenge. Several recent studies have reported edge currents--persistent transport along walls and density inhomogeneities--in chiral active matter. Yet, the microscopic rules that render these edge currents robust with respect to the confinement geometry and defects remain elusive. Here, we introduce a simple particle model of two-dimensional chiral active swimmers that undergo chirality switching and demonstrate that the model exhibits robust edge currents, i.e., when a single particle is confined, edge currents arise regardless of the confinement geometry or the presence of defects. We also investigate the collective behavior of interacting particles in bulk and find that chirality switching induces phase separation accompanied by edge currents along interfaces. This phase separation is distinct from motility-induced phase separation and is qualitatively explained by an effective hydrodynamic theory derived via bottom-up coarse-graining. Furthermore, by analyzing the topological properties of the linearized hydrodynamic equations, we show that the edge currents in our system are genuine topological edge modes. Notably, phase separation induced by chirality switching can be regarded as the coexistence of two topologically distinct domains. Our results provide guidelines for designing robust edge currents in active matter systems.

cond-mat.soft↗

arXiv2026

Mutual Linearity is a Generic Property of Steady-State Markov Networks

Understanding and predicting how complex systems respond to external perturbations is a central challenge in nonequilibrium statistical physics. Here we consider continuous-time Markov networks, which we subject to perturbations along a single edge. We find that in steady state the probabilities of any two states are linearly related to one another. We show that this mutual linearity of probabilities extends to a broad class of observables, including currents but also generic counting and state-dependent observables. Moreover, we derive an exact relation between the relative response of any state's probability and the ratio of two steady-state probabilities. Leveraging the Markov chain tree theorem, we further show that probabilities and the considered observables are constrained by the topological and kinetic properties of the network and provide analytical expressions in terms of spanning tree polynomials. Our results are general, holding for arbitrary rate parameterizations and extending far from equilibrium.

cond-mat.stat-mech↗

arXiv2026

Topological chiral random walker

Understanding how biological and synthetic systems achieve robust function in noisy environments remains a fundamental challenge across the physical and life sciences. To connect robust behavior with non-trivial topological features present already in the dynamics of individual units, here we introduce the topological chiral random walker (TCRW) model. While exploring the system, a TCRW locates edges and boundaries in the system and develops topologically protected edge currents even in the presence of defects and disorder. Drawing on the bulk-boundary correspondence found in hard condensed matter systems allows us to rationalize the emergence of robust edge currents through topological features of the dynamic spectrum. We show that chiral motion and rotational noise with opposite chirality are two crucial components in our inherently non-Hermitian model. As proofs of principle, we first show that a topological walker outperforms diffusive motion to efficiently solve complex mazes due to its property of remaining on the edge with some rare detachments. Second, we use this model to design building blocks that can perform efficient self-assembly overcoming the timescale bottlenecks of diffusion-limited growth and reducing self-assembly times by approximately 80%.

cond-mat.stat-mech↗

arXiv2025

Thermo-responsive closing and reopening artificial Venus Flytrap utilizing shape memory elastomers

Despite their often perceived static and slow nature, some plants can move faster than the blink of an eye. The rapid snap closure motion of the Venus flytrap (Dionaea muscipula) has long captivated the interest of researchers and engineers alike, serving as a model for plant-inspired soft machines and robots. The translation of the fast snapping closure has inspired the development of various artificial Venus flytrap (AVF) systems. However, translating both the closing and reopening motion of D. muscipula into an autonomous plant inspired soft machine has yet to be achieved. In this study, we present an AVF that autonomously closes and reopens, utilizing novel thermo-responsive UV-curable shape memory materials for soft robotic systems. The life-sized thermo-responsive AVF exhibits closing and reopening motions triggered in a naturally occurring temperature range. The doubly curved trap lobes, built from shape memory polymers, close at 38°C, while reopening initiates around 45°C, employing shape memory elastomer strips as antagonistic actuators to facilitate lobe reopening. This work represents the first demonstration of thermo-responsive closing and reopening in an AVF with programmed sequential motion in response to increasing temperature. This approach marks the next step toward autonomously bidirectional moving soft machines/robots.

cs.RO↗

arXiv2025

Design and development of an electronics-free earthworm robot

Soft robotic systems have gained widespread attention due to their inherent flexibility, adaptability, and safety, making them well-suited for varied applications. Among bioinspired designs, earthworm locomotion has been extensively studied for its efficient peristaltic motion, enabling movement in confined and unstructured environments. Existing earthworm-inspired robots primarily utilize pneumatic actuation due to its high force-to-weight ratio and ease of implementation. However, these systems often rely on bulky, power-intensive electronic control units, limiting their practicality. In this work, we present an electronics-free, earthworm-inspired pneumatic robot utilizing a modified Pneumatic Logic Gate (PLG) design. By integrating preconfigured PLG units with bellow actuators, we achieved a plug-and-play style modular system capable of peristaltic locomotion without external electronic components. The proposed design reduces system complexity while maintaining efficient actuation. We characterize the bellow actuators under different operating conditions and evaluate the robots locomotion performance. Our findings demonstrate that the modified PLG-based control system effectively generates peristaltic wave propagation, achieving autonomous motion with minimal deviation. This study serves as a proof of concept for the development of electronics-free, peristaltic soft robots. The proposed system has potential for applications in hazardous environments, where untethered, adaptable locomotion is critical. Future work will focus on further optimizing the robot design and exploring untethered operation using onboard compressed air sources.

cs.RO↗

arXiv2025

Model to Model: Understanding the Venus Flytrap Snapping Mechanism and Transferring it to a 3D-printed Bistable Soft Robotic Demonstrator

The Venus flytrap (Dionaea muscipula) does not only serve as the textbook model for a carnivorous plant, but also has long intrigued both botanists and engineers with its rapidly closing leaf trap. The trap closure is triggered by two consecutive touches of a potential prey, after which the lobes rapidly switch from their concave open-state to their convex close-state and catch the prey within 100-500 ms after being triggered. This transformation from concave to convex is initiated by changes in turgor pressure and the release of stored elastic energy from prestresses in the concave state, which accelerate this movement, leading to inversion of the lobes bi-axial curvature. Possessing two low-energy states, the leaves can be characterized as bistable systems. With our research, we seek to deepen the understanding of Venus flytrap motion mechanics and apply its principles to the design of an artificial bistable lobe actuator. We identified geometrical characteristics, such as dimensional ratios and the thickness gradient in the lobe, and transferred these to two 3D-printed bistable actuator models. One actuator parallels the simulated geometry of a Venus flytrap leaf, the other is a lobe model designed with CAD. Both models display concave-convex bi-stability and snap close. These demonstrators are the first step in the development of an artificial Venus flytrap that mimics the mechanical behavior of the biological model and can be used as a soft fast gripper.

cs.RO↗

arXiv2025

A Bioinspired Aquatic Machine Mimicking Water Caltrop

Plants are increasingly becoming a source of inspiration for robotics and engineers to develop bioinspired, adaptive, and multifunctional machines. In this study, we propose a bioinspired aquatic machine that mimics the fruit of the water caltrop (Trapa natans L.). Among various plant species, T. natans produces unique woody fruits that can disperse passively via water currents or by clinging to boats or waterfowls. Inspired by the structures and dispersal mechanisms of T. natans, we designed miniaturized biomimetic machines capable of passive dispersion in aquatic ecosystems. In order to study our selected biological model, we collected natural fresh and dried mature samples of T. natans fruits. We designed biomimetic aquatic machines by extracting the main geometrical details from the natural samples, and by exploiting advanced three-dimensional reconstruction techniques, including x-ray micro-computed topography (Micro-CT). Then, we successfully fabricate the biomimetic machines at high-resolution in two configurations (hollow body and solid body) using light-based bioprinting of photo-responsive hydrogels. We also characterized the mechanical properties of the bioprinted materials through compression tests. Finally, we evaluated the floating behavior of the biomimetic machines in a flow chamber as a proof of concept. This biomimetic approach enhances the adaptability of the machine in aquatic environments, offering new design insights for underwater, soft, and microrobotics.

physics.app-ph↗

arXiv2025

Response to dynamic shape changes in suspensions of hard rectangles

While the autonomous assembly of hard nanoparticles with different shapes has been studied extensively both in experiment and simulations, little is known about systems where particle shape can be dynamically altered. DNA origami nanostructures offer an alternative route to synthesize nanoparticles that can change their shape on demand. Motivated by recent experiments, here we study the structure and dynamics of suspensions of hard squares in response to an elongation into a rectangle. Performing extensive hard-particle Monte Carlo simulations at constant volume and employing two protocols, we numerically analyze the collective diffusion and ordering during quenching and the subsequent relaxation to the new equilibrium state. We find that the cascading protocol, which mimics experimentally realized DNA origami, can become dynamically arrested due to the increase in effective packing fraction.

cond-mat.soft↗

arXiv2025

Statistics and morphologies of stable droplets in scalar active fluids

Conventional phase segregation is controlled by a positive interfacial tension, which implies that the system relaxes towards a state in which the interfacial area (or length) is minimized, typically manifesting as a single droplet that grows with the system size. Intriguingly, the extension of the underlying Model B paradigm by two non-potential terms (Active Model B+) is able to describe the stable coexistence of many finite droplets. Here we numerical study Active Model B+ in the vicinity of the transition between a single droplet (macrophase segregation) and multiple droplets (microphase segregation). Our results show that, although noise shifts transitions, the overall agreement with the mean-field theoretical predictions is very good. We find a strong correlation of droplet properties with a single parameter that determines the number, density, and the fractal dimension of droplets. Deeper inside the droplet phase we observe another transition to a hexagonal lattice of regular droplets.

cond-mat.soft↗

arXiv2025

Negative drag force on beating flagellar-shaped bodies in active fluids

We experimentally investigate the drag force exerted by a suspension of light-induced active particles (APs) on a translating and beating idealized flagellum-shaped object realized through negative phototactic interactions with the APs. We observe both positive and negative drag forces, depending on the beating frequency and translational velocity, driven by the dynamic redistribution of APs in response to the object's motion. These findings are supported by numerical simulations and an analytical model, extendable to a range of slender geometries. Our results illustrate the complex interplay between geometric body changes and the density distribution in active baths, which may also be relevant for microrobotic applications.

cond-mat.soft↗

arXiv2025

Universal limiting behaviour of reaction-diffusion systems with conservation laws

Making sense of complex inhomogeneous systems composed of many interacting species is a grand challenge that pervades basically all natural sciences. Phase separation and pattern formation in reaction-diffusion systems have been largely studied as two separate paradigms. Here we show that in reaction-diffusion systems composed of many species, the presence of a conservation law constrains the evolution of the conserved quantity to be governed by a Cahn-Hilliard-like equation. This establishes a direct link with the paradigm of coexistence and recent "active" field theories. Hence, even for complex many-species systems a dramatically simplified but accurate description emerges over coarse spatio-temporal scales. Using the nullcline (the line of homogeneous steady states) as the central motif, we develop a geometrical framework which endows chemical space with a basis and suitable coordinates. This framework allows us to capture and understand the effect of eliminating fast non-conserved degrees of freedom, and to explicitly construct coefficients of the coarse field theory. We expect that the theory we develop here will be particularly relevant to advance our understanding of biomolecular condensates.

cond-mat.soft↗

arXiv2024

Steady inhomogeneous shear flows as mechanical phase transitions

Inhomogeneous flows and shear banding are of interest for a range of applications but have been eluding a comprehensive theoretical understanding, mostly due to the lack of a framework comparable to equilibrium statistical mechanics. Here we revisit models of fluids that reach a stationary state obeying mechanical equilibrium. Starting from a non-local constitutive relation, we apply the idea of a "mechanical phase transition" and map the constitutive relation onto a dynamical system through an integrating factor. We illustrate this framework for two applications: shear banding in strongly thinning complex fluids and the coexistence of a solid with its sheared melt. Our results contribute to the growing body of work following a mechanical route to describe inhomogeneous systems away from thermal equilibrium.

cond-mat.stat-mech↗

arXiv2024

In pursuit of the tetratic phase in hard rectangles

We numerically investigate two-dimensional systems of hard rectangles at constant pressure through extensive hard-particle Monte Carlo simulations. We determine the complete phase diagram as a function of packing fraction and aspect ratio, which consists of four distinct phases. At very high packing fractions, particles form a smectic solid for all aspect ratios. Rod-like particles with large aspect ratio assemble in an intervening nematic phase, which is displaced by a "tetratic" phase (also called biaxial nematic) for moderately elongated rectangles. Surprisingly, we find evidence that the transition from tetratic to smectic is weakly discontinuous at variance with previously proposed two-step scenarios for the melting of hard particles.

cond-mat.soft↗

arXiv2024

Coarse-grained models for phase separation in DNA-based fluids

DNA is now firmly established as a versatile and robust platform for achieving synthetic nanostructures. While the folding of single molecules into complex structures is routinely achieved through engineering basepair sequences, much less is known about the emergence of structure on larger scales in DNA fluids. The fact that polymeric DNA fluids can undergo phase separation into dense fluid and dilute gas opens avenues to design hierachical and multifarious assemblies. Here we investigate to which extent the phase behavior of single-stranded DNA fluids is captured by a minimal model of semiflexible charged homopolymers while neglecting specific hybridization interactions. We first characterize the single-polymer behavior and then perform direct coexistence simulations to test the model against experimental data. We conclude that counterions not only determine the effective range of direct electrostatic interactions but also the effective attractions.

cond-mat.soft↗

arXiv2024

Dynamic renormalization of scalar active field theories

We study Active Model B+, a scalar field theory extending the paradigmatic Model B for equilibrium coexistence through including terms that do not arise from an underlying free energy functional and thus break detailed balance. In the first part of the manuscript, we provide a pedagogical and self-contained introduction to one-loop dynamic renormalization. We then address the technical challenge of complex vertex functions through developing a symbolic computer algebra code that allows us to obtain the graphical corrections of model parameters. We argue that the additional terms of Active Model B+ imply the generation of, potentially relevant, higher-order terms; strongly restricting the parameter regime in which we can apply a perturbative renormalization scheme. Moreover, we elucidate the role of the cubic coefficient, which, in contrast to passive Model B, is incessantly generated by the new terms. Analyzing its behavior with and without field shift near the Wilson-Fisher fixed point, we find that additional fixed points in the one-loop flow equations are likely artifacts. Additionally, we characterize the renormalization flow of perturbatively accessible field theories derived from Active Model B+.

cond-mat.stat-mech↗

arXiv2024

Thermodynamics of active matter: Tracking dissipation across scales

The concept of entropy has been pivotal in the formulation of thermodynamics. For systems driven away from thermal equilibrium, a comparable role is played by entropy production and dissipation. Here we provide a comprehensive picture how local dissipation due to effective chemical events manifests on large scales in active matter. We start from a microscopic model for a single catalytic particle involving explicit solute molecules and show that it undergoes directed motion. Leveraging stochastic thermodynamics, we calculate the average entropy production rate for interacting particles. We then show how the model of active Brownian particles emerges in a certain limit and we determine the entropy production rate on the level of the hydrodynamic equations. Our results augment the model of active Brownian particles with rigorous expressions for the dissipation that cannot be inferred from their equations of motion, and we illustrate consequences for wall aggregation and motility-induced phase separation. Notably, our bottom-up approach reveals that a naive application of the Onsager currents yields an incorrect expression for the local dissipation.

cond-mat.soft↗

arXiv2023

Motility-induced clustering of active particles under soft confinement

We investigate the structural and dynamic properties of active Brownian particles (APs) confined within a soft annulus-shaped channel. Depending on the strength of the confinement and the Péclet number, we observe a novel re-entrant behavior that is not present in unconfined systems. Our findings are substantiated by numerical simulations and analytical considerations, revealing that this behavior arises from the strong coupling between the Péclet number and the effective confining dimensionality of the APs. Beyond highlighting the important influence of soft boundaries on APs, our research holds significance for future applications of micro-robotic systems.

cond-mat.soft↗

arXiv2023

Collective Hall current in chiral active fluids: Coupling of phase and mass transport through traveling bands

Active fluids composed of constituents that are constantly driven away from thermal equilibrium can support spontaneous currents and can be engineered to have unconventional transport properties. Here we report the emergence of (meta-)stable traveling bands in computer simulations of aligning circle swimmers. These bands are different from polar flocks and we show that they can be understood as non-dispersive soliton solutions of the underlying non-linear hydrodynamic equations with constant celerity (phase propagation speed) that is much larger than the propulsion speed. In contrast to solitons in passive media, these bands can induce a bulk particle current with a component perpendicular to the propagation direction, thus constituting a collective Hall (or Magnus) effect. Traveling bands require sufficiently small orbits and undergo a discontinuous transition into a synchronized state with transient polar clusters for large orbital radii.

cond-mat.soft↗