arXiv subjects

Viktor Holubec

Publications and source records attributed to Viktor Holubec.

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

Spatially Structured Cohesion from Extremal Alignment in Topological Active Matter

Alignment interactions in active matter are typically modeled as relaxational dynamics toward local consensus. In unbounded systems, this makes alignment effectively decoupled from local density and therefore unable to sustain self-confined collective motion without additional attractive forces. Here we show that this limitation can be overcome by extremal alignment rules in which the interaction neighborhood depends on the candidate orientation. For a broad class of candidate- dependent rules with pairwise additive utilities, the decision utility factorizes into the product of an average interaction score and the number of selected neighbors. This multiplicative structure couples orientational decisions to local density and thereby generates an effective cohesive bias without explicit cohesive forces. In metric models, however, the same mechanism drives collapse toward globally connected, effectively mean-field states that suppress spatial structure. We show that topological interactions regularize this tendency, stabilizing self-confined flocks of finite extent in open space. The resulting dynamics exhibits a rich dynamical phase diagram as a function of noise intensity and turning rate, including polarized flocks, swarms, and persistent swirling states. Our results identify candidate-dependent extremal alignment as a simple mechanism for generating cohesive, spatially structured active matter beyond the standard relaxational paradigm.

cond-mat.soft↗

arXiv2026

Strong Violation of the Thermodynamic Uncertainty Relation in a Minimal Autonomous Heat Engine

Thermodynamic uncertainty relations (TURs) impose a universal trade-off between current precision and entropy production in autonomous steady states, constraining in particular the power, efficiency, and constancy of heat engines. We demonstrate strong violations of the long-time TUR in a minimal autonomous heat engine composed of a discrete ratchet generating work against a constant bias and an underdamped harmonic oscillator acting as an internal stochastic control. In the regime of time-scale separation, the model becomes exactly solvable and yields a closed analytical expression for the TUR ratio, where the influence of the continuous degree of freedom is fully captured by the Fano factor of oscillator zero crossings. We show that increasingly deterministic internal control drives the TUR ratio arbitrarily close to zero while the engine operates near maximal current and efficiency. In an appropriate limit, the model reduces to the classical pendulum-clock system of Pietzonka, Phys. Rev. Lett. 128, 130606 (2022).

cond-mat.stat-mech↗

arXiv2026

Feedback control and delayed interactions in active matter

Feedback control plays a central role in active matter, yet it is inevitably accompanied by noise and finite perception--action delays. This Perspective reviews recent advances on active systems with delayed interactions, showing how time delay can induce activity, chirality, transport, and collective pattern formation, and can act as an effective control parameter for switching between dynamical states. We discuss representative single-particle and many-body systems, highlight key experimental realizations, and argue that time delay constitutes an underexplored dimension of morphological intelligence--where intrinsic response dynamics, rather than explicit sensors or computation, enable functional behavior in active matter.

cond-mat.soft↗

arXiv2025

Programmable Hydrodynamics of Active Particles

Self-propelled microparticles create flow fields that determine how they interact with surfaces, external flows, and each other. These flow fields fall into distinct classes--pushers, pullers, and neutral swimmers--each exhibiting fundamentally different collective behaviors. In all existing synthetic systems, this hydrodynamic character is permanently set during fabrication, making it impossible to explore how adaptive switching between these classes might enable new functions or emergent phenomena. Here we demonstrate that the hydrodynamic character of a microswimmer can be programmed and switched on demand. Using patterned laser heating of surface-bound nanoparticles, we create tailored temperature gradients that drive controllable boundary flows at the particle surface. By changing the illumination pattern in real time, we dynamically transform the swimmers flow field continuously tuning from pusher to puller, while the particle continues to swim. Flow measurements confirm quantitative agreement with theory and allow us to simultaneously track how symmetry, power consumption, and efficiency change across modes. This control over hydrodynamic modes opens experimental access to questions that have remained largely theoretical: How do adaptive swimmers respond to crowding or confinement? Can mixtures with tunable pusher-puller ratios reveal new collective states? Our approach provides a platform to address these questions and explore the morphological developments of active matter systems under external physical constraints.

cond-mat.soft↗

arXiv2025

Inertia Tames Fluctuations in Autonomous Stationary Heat Engines

Thermodynamic uncertainty relations (TURs) provide fundamental constraints on the interplay between power fluctuations, entropy production, and efficiency in overdamped stationary autonomous heat engines. However, their validity in underdamped regimes remains limited and less explored. Here, we analytically and numerically study a physically realizable autonomous heat engine composed of two underdamped continuous degrees of freedom coupled to a two-level system. We show that this nonlinear setup can robustly violate TUR-based trade-offs by exploiting resonant coupling, effectively using one underdamped mode as an internal periodic drive. When this coupling is suppressed, the system recovers TUR-like bounds consistent with overdamped theory. Importantly, we demonstrate that the strongest suppression of current fluctuations occurs in a resonance regime that can be directly inferred from mean current measurements - a quantity typically much easier to access experimentally than fluctuations. Our results reveal new pathways to circumvent classical TUR constraints in underdamped systems and provide practical guidelines for designing efficient, precise microscopic engines and autonomous clocks.

cond-mat.stat-mech↗

arXiv2025

Order-Disorder Transition in Delay Vicsek Model

Interactions in active matter systems inherently involve delays due to information processing and actuation lags. We numerically investigate the impact of such delays on the phase behavior of the Vicsek model for motile active matter at a large but fixed system size. While the delayed Vicsek model retains the same three phases as the standard version -- an ordered state, a liquid-gas coexistence state, and a disordered state -- the presence of delay qualitatively alters the system's dynamics. At the relatively high velocity considered in this study, the critical noise for the transition between the ordered and coexistence states exhibits a non-monotonic dependence on delay, whereas the critical noise required for the transition to the disordered state increases with delay. Consequently, the width of the noise interval in which phase separation occurs broadens with increasing delay. Short delays stabilize the ordered phase, while long delays destabilize it in favor of the coexistence phase, which is consistently stabilized compared to the disordered state. Furthermore, the number of bands observed in the coexistence state at a given noise increases, and the time required for their formation decreases with delay. This acceleration is attributed to the emergence of swirling structures whose typical radius grows with increasing delay. Our results demonstrate that time delay in the Vicsek model acts as an effective control parameter for tuning the system's dynamic phase behavior.

cond-mat.soft↗

arXiv2025

Delayed Active Swimmer in a Velocity Landscape

Self-propelled active particles exhibit delayed responses to environmental changes, modulating their propulsion speed through intrinsic sensing and feedback mechanisms. This adaptive behavior fundamentally determines their dynamics and self-organization in active matter systems, with implications for biological microswimmers and engineered microrobots. Here, we investigate active Brownian particles whose propulsion speed is governed by spatially varying activity landscapes, incorporating a temporal delay between environmental sensing and speed adaptation. Through analytical solutions derived for both short-time and long-time delay regimes, we demonstrate that steady-state density and polarization profiles exhibit maxima at characteristic delays. Significantly, we observe that the polarization profile undergoes sign reversal when the swimming distance during the delay time exceeds the characteristic diffusion length, providing a novel mechanism for controlling particle transport without external fields. Our theoretical predictions, validated through experimental observations and numerical simulations, establish time delay as a crucial control parameter for particle transport and organization in active matter systems. These findings provide insights into how biological microorganisms might use response delays to gain navigation advantages and suggest design principles for synthetic microswimmers with programmable responses to heterogeneous environments.

cond-mat.soft↗

arXiv2025

Optimizing low-dissipation Carnot-like thermal devices with heat leak

Delimiting the bounds of optimal performance for heat engines (HEs), refrigerators (REs), and heat pumps (HPs) is a central goal in thermodynamics. While low-dissipation (LD) models have proven valuable for this purpose, the role of heat leak in such models has received limited attention. Here, we present a unified framework for LD Carnot-like (CL) HEs, REs, and HPs that incorporates heat leaks, and derive new results for the efficiency at maximum power and the power at maximum efficiency. We further investigate the relationship between the bounds of power at fixed efficiency and efficiency at fixed power, demonstrating that these bounds coincide and are described by identical curves across all thermal devices. Finally, we show that the optimal performance of all three devices can be achieved by optimizing the average entropy production rate over the cycle, a result that holds for any CL device and extends beyond the LD assumption.

cond-mat.stat-mech↗

arXiv2025

Active Matter Flocking via Predictive Alignment

Understanding collective self-organization in active matter, such as bird flocks and fish schools, remains a grand challenge in physics. Interactions that induce alignment are essential for flocking; however, alignment alone is generally insufficient to maintain group cohesion in the presence of noise, leading traditional models to introduce artificial boundaries or explicit attractive forces. Here, we propose a model that achieves cohesive flocking through purely alignment-based interactions by introducing predictive alignment, in which agents reorient to maximize alignment with the prevailing orientations of their anticipated future neighbors. Implemented in a discrete-time Vicsek-type framework, this approach delivers robust, noise-resistant cohesion without additional parameters. In the stable regime, flock size scales linearly with interaction radius, remaining nearly immune to noise or propulsion speed, and the group coherently follows a leader under noise. These findings reveal how predictive strategies enhance self-organization, paving the way for a new class of active matter models blending physics and cognitive-like dynamics.

cond-mat.soft↗

arXiv2025

Optimal constrained control for generally damped Brownian heat engines

Optimization of cyclic stochastic heat engines, a topic spanning decades of research, commonly assumes fixed control or response parameters at discrete points in the cycle-a limitation that often leads to experimentally impractical protocols. We overcome this with a general algorithm, adapted from optimal control theory, that optimizes full-cycle dynamics under realistic constraints, such as stiffness and temperature bounds, across diverse systems. Unlike geometric or mass transport methods, which rely on fixed endpoints and are unsuitable for unconstrained cycles, our approach simultaneously tunes both cycle time and control variations. Applied to a generally damped Brownian particle in a harmonic potential-an experimentally relevant case-our method is validated in the overdamped regime and extended to arbitrary damping rates. As damping decreases, maximum power vanishes and cycle time diverges; at fixed cycle times, efficiency follows a similar trend, with optimal protocols exhibiting non-monotonic complexity. Notably, optimizing temperature profiles-often overlooked-significantly enhances efficiency in intermediate damping regimes. Our work establishes the first systematic framework for optimizing cyclic stochastic processes under experimental constraints, broadening the scope of power and efficiency optimization in nonequilibrium thermodynamics.

cond-mat.stat-mech↗

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↗

arXiv2024

Run-and-tumble motion of ellipsoidal microswimmers

A hallmark of bacteria is their so-called "run-and-tumble" motion, consisting of a sequence of linear directed "runs" and random rotations that constantly alternate due to biochemical feedback. It plays a crucial role in the ability of bacteria to move through chemical gradients and inspired a fundamental active particle model. Nevertheless, synthetic active particles generally do not exhibit run-and-tumble motion but rather active Brownian motion. We show in experiments that ellipsoidal thermophoretic Janus particles, propelling along their short axis, can yield run-and-tumble-like motion even without feedback. Their hydrodynamic wall interactions under strong confinement give rise to an effective double-well potential for the declination of the short axis. The geometry-induced timescale separation of the in-plane rotational dynamics and noise-induced transitions in the potential then yields run-and-tumble-like motion.

cond-mat.soft↗

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↗

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

Optimal finite-time heat engines under constrained control

We optimize finite-time stochastic heat engines with a periodically scaled Hamiltonian under experimentally motivated constraints on the bath temperature $T$ and the scaling parameter $λ$. We present a general geometric proof that maximum-efficiency protocols for $T$ and $λ$ are piecewise constant, alternating between the maximum and minimum allowed values. When $λ$ is restricted to a small range and the system is close to equilibrium at the ends of the isotherms, a similar argument shows that this protocol also maximizes output power. These results are valid for arbitrary dynamics. We illustrate them for an overdamped Brownian heat engine, which can experimentally be realized using optical tweezers with stiffness $λ$.

cond-mat.stat-mech↗