arXiv subjects

Lorenzo Caprini

Publications and source records attributed to Lorenzo Caprini.

At least 19 recordsLinked to original sources

arXiv2026

Active embracement enables autonomous tweezing in active star polymers

The capacity for autonomous structural reconfiguration is a defining trait of living systems, yet it remains elusive in artificial active matter. Here, we report the discovery of active embracement, a non-equilibrium phenomenon where active star polymers, comprising a central core and self-propelled monomeric arms, transition from open configurations to tightly collapsed, ``hugging'' states. By combining polymer experiments using connected vibrobots with simulations, we demonstrate that internal self-propulsion fundamentally overrides the steric repulsion that keeps passive polymers dispersed. This active drive enables a suite of behaviors unattainable in equilibrium systems: individual star polymers undergo a globular-like self-collapse, multiple star polymers mutually intertwine and mutually embrace, and can spontaneously embrace and capture surrounding passive particles. Our findings reveal that active embracement is a distinct kinetic phase that allows star polymers to function as autonomous tweezers. By bridging the gap between macroscopic robotic collectives and microscopic polymer physics, this work provides a versatile blueprint for the design of smart materials capable of targeted cargo capture and self-directed assembly in complex environments.

cond-mat.soft↗

arXiv2026

Jerky Motion of Active Granular Particles

Abrupt transitions between rest and motion can render the standard Newtonian description -- based on position, velocity, and acceleration -- incomplete, requiring higher-order derivatives such as the jerk, the third time derivative of position. Here, we show that the interplay between activity and dry friction gives rise to robust jerk-dominated dynamics in self-propelled particles: the particle speed increases quadratically with time under an active force, in contrast to the linear growth expected for conventional Newtonian dynamics. We demonstrate this behavior analytically, numerically, and experimentally using active vibrobots self-propelling on a vertically vibrating plate at low vibration amplitudes, where surface asperities generate dry friction and, thus, give rise to jerky motion when combined with activity. Our results establish dry friction as a simple mechanism for realizing higher-order dynamics in active matter and suggest that jerky dynamics may arise broadly in nonequilibrium systems with frictional contacts.

cond-mat.soft↗

arXiv2026

Magnetic active matter across scales

Magnetic interactions provide a versatile and powerful tool for controlling and organizing active matter, where individual units continuously consume energy to drive autonomous motion. These interactions arise naturally in biological systems, such as magnetotactic bacteria, and can be engineered into synthetic platforms, including colloidal microswimmers, magnetic nanoparticles, and macroscopic granular robots. This review focuses on active, self-propelled particles that carry an intrinsic magnetic dipole moment, powered by their own energy consumption rather than driven by external fields; here, the dipole moment mediates interactions and self-organization, not propulsion. We survey experimental and theoretical studies across all length scales, showing how dipolar interactions shape single-particle dynamics, collective behavior, and self-organization. We discuss models incorporating pairwise dipolar forces and confinement, and examine emergent phenomena such as chaining, swarming, and tunable pattern formation. We close by outlining challenges and opportunities in the design, control, and application of magnetic active systems, from programmable materials and biomedical actuation to nonequilibrium physics.

cond-mat.soft↗

arXiv2026

Curvature-driven wall accumulation in chiral active particles

We study a dilute system of non-motile chiral active particles confined in geometries ranging from straight channels to circular enclosures. Activity is introduced through chiral particle-wall interactions, modeled as tangential wall forces that generate the edge currents characteristic of chiral active matter. Remarkably, although the particles lack self-propulsion, these boundary currents induce density inhomogeneities. We show that boundary curvature drives a wall accumulation phenomenon: particles remain uniformly distributed in straight channels but accumulate near the boundaries of circular confinements. Numerical simulations and a hydrodynamic theory for the density and momentum fields consistently capture this curvature-induced wall-accumulation. These results identify boundary curvature as a fundamental control parameter for chiral edge transport and confinement-induced organization, with potential experimental relevance to spinning colloids and granular spinners.

cond-mat.soft↗

arXiv2026

A particle-resolved rheological study of chirality transfer and odd transport

Chirality, or the breaking of mirror symmetry, appears across all scales in nature, from molecular conformations to the dynamics of bacterial collectives. Environments composed of such symmetry-breaking constituents can give rise to emergent physical phenomena, particularly in the transport and response of embedded tracers. Yet it remains unclear how chiral environments influence such tracers and through which microscopic mechanisms anomalous responses emerge. Here, we present a particle-resolved study of these systems, demonstrating chirality transfer and odd transport of an object embedded in a chiral active bath. In a rheological experiment, a symmetric passive tracer is driven through collisions with the particles of a non-equilibrium chiral bath. Combining table-top experiments, many-body simulations, and a reduced coarse-grained theory, we demonstrate that local collisions transfer chiral active dynamics to the tracer, which displays circular trajectories. We show that the same mechanism gives rise to a systematic transverse drift under a constant pulling force. Crucially, we identify nonlinear friction as an essential factor that rectifies these transferred chiral active fluctuations into a macroscopic odd response. Our results reveal a microscopic mechanism for odd transport in chiral active matter and provide general insights into transverse transport in driven non-equilibrium systems.

cond-mat.stat-mech↗

arXiv2026

Sparkling bubbles in chiral active fluids

We study an inertial chiral active fluid, formed by repulsive particles that transfer angular momentum through odd interactions, i.e. transverse forces. Chirality induces an inhomogeneous phase, consisting of rotating bubbles, whose formation is favored at an optimal packing fraction. In this regime, we discover that bubbles may be dynamically unstable, breaking up and reforming in the steady state, thereby showing a spontaneous sparkling-like behavior reminiscent of supersaturated liquids. Bubbles and sparkling bubbles are predicted by a coarse-grained hydrodynamic theory, revealing the intrinsic non-linearity of these collective phenomena, and call for experimental verifications in granular spinners or spinning colloids.

cond-mat.soft↗

arXiv2026

Kinetic Theory of Chiral Active Disks: Odd Transport and Torque Density

Parity-odd transport is a central signature of chiral fluids, yet analytical predictions are sparse. Here, we introduce a minimal two-dimensional hard-disk gas in which chirality arises solely from a collision-induced transverse impulse. Motivated by granular spinners, collisions are dissipative and inject orbital angular momentum through a fixed tangential ``kick'' at contact. Starting from a Boltzmann-Enskog description, we derive nonlinear hydrodynamic equations for density, momentum, and temperature, and show that chirality generates an antisymmetric homogeneous stress corresponding to a nonzero torque density. In the dilute limit, a Chapman-Enskog expansion yields analytical predictions for transport coefficients, including odd viscosity, odd thermal conductivity, and odd self-diffusivity, in good agreement with numerical simulations. This minimal kinetic model can serve as a foundation for systematic coarse-graining of chiral fluids and as a tractable benchmark for gaining insight into odd transport across a broader class of chiral systems.

cond-mat.stat-mech↗

arXiv2026

Emergent hydrodynamics of chiral active fluids: vortices, bubbles and odd diffusion

Starting from a microscopic multiparticle Langevin equation, we systematically derive a hydrodynamic description in terms of density and momentum fields for chiral active particles interacting via standard repulsive and nonlocal odd forces. These odd interactions are reciprocal but non-conservative: they are non-potential forces, as they act perpendicular to the vector joining any pair of particles. As a result, the torques that two particles exert on one another are non-reciprocal. The ensuing macroscopic continuum description consists of a continuity equation for the density and a generalized compressible Navier-Stokes equation for the fluid velocity. The latter includes a chirality-induced torque density term and an odd viscosity contribution. Our theory predicts the emergence of odd diffusivity, edge currents, and an inhomogeneous phase - characterized by bubble-like structures - recently observed in simulations. Specifically, the theory exhibits a linear instability arising from the interplay between odd viscosity and torque density, and admits steady-state inhomogeneous solutions featuring bubbles and vortices, in agreement with numerical simulations. Our findings can be tested experimentally in systems of granular spinners or rotating microorganisms suspended in a fluid.

cond-mat.soft↗

arXiv2025

Cooling mechanism controls motility-induced phase separation in inertial active liquids

Motility-induced phase separation (MIPS) is a central collective phenomenon in active matter, theoretically established in the overdamped regime. We discover that the dynamical origin of MIPS is fundamentally altered by inertia, which induces a cooling mechanism absent in overdamped active matter. This conclusion is supported by an active variant of the direct simulation Monte Carlo method and by a kinetic theory for inertial self-propelled hard spheres derived from the microscopic dynamics. In contrast to the overdamped case, both analyses demonstrate that inertial MIPS does not rely on volume exclusion but on a cooling mechanism involving density, polarization, and temperature fields. This mechanism emerges from the competition between activity and a density dependent collision rate, arising from spatial correlations between colliding particles. These findings open a pathway to fundamentally connect inertial active matter with granular physics.

cond-mat.soft↗

arXiv2025

Circling crystals in chiral active matter with self-alignment

We study a crystal composed of active units governed by self-alignment and chirality. The first mechanism acts as an effective torque that aligns the particle orientation with its velocity, while the second drives individual particles along circular orbits. We find that even a weak degree of chirality, when coupled with self-alignment, induces collective motion of the entire crystal along circular trajectories in space. We refer to this phase as a circling crystal. When chirality outweigh self-alignment, the circular global motion is suppressed in favor of vortex-like regions of coordinated motion. This state is characterized by oscillating spatial velocity correlations, a power law decay of the energy spectrum, and oscillatory temporal correlations. Our findings can be tested experimentally in systems ranging from epithelial tissues to swarming robots, governed by chirality and self-alignment.

cond-mat.soft↗

arXiv2025

Modeling chiral active particles: from circular motion to odd interactions

In this paper, we discuss microscopic models for chiral active particles, i.e., rotating active units that exhibit circular or spinning motion. While non-chiral active particles are typically governed by self-propulsion and conservative interactions, the rotating motion of chiral particles generates additional non-conservative forces that cannot be derived from a potential. These manifest as effective transverse forces, acting perpendicular to the line connecting the centres of two interacting particles, and are referred to as odd interactions, because they break the mirror symmetry of the system. Here, we demonstrate that odd interactions arise from a limiting case of a well-established model describing spinning granular objects. In addition, we show that these models for chiral active objects give rise to a novel collective phenomenon that emerges uniquely from transverse forces and, hence, chirality. Specifically, the system undergoes a transition from a homogeneous phase to an inhomogeneous one characterised by regions depleted of particles, referred to as bubbles. This collective behaviour, termed BIO (bubbles induced by odd interactions), is a general emergent phenomenon arising from chirality and odd interactions. In this work, we review theoretical approaches to this problem, including a scaling argument and predictions for spatial velocity correlations that account for the BIO phase. Finally, we outline perspectives and open challenges concerning this collective phenomenon.

cond-mat.soft↗

arXiv2025

Active particles in tunable compressible environments

Active particles affect their environment as much as the environment affects their active motion. Here, we present an experimental system where both can be simultaneously adjusted in situ using an external AC electric field. The environment consists in a two-dimensional bath of colloidal silica particles, whereas the active particles are gold-coated Janus spheres. As the electric field orthogonal to the planar layer increases, the former become stiffer and the latter become faster. The active trajectories exhibit enhanced rotational motion where the reorientation frequency increases with the particle speed, an effect that culminates in a chiral active motion. We demonstrate that self sustained reorientations arise from local compressions and interaction asymmetries, revealing a general particle-level mechanism where changes in the mechanical properties of the environment reshape active trajectories.

cond-mat.soft↗

arXiv2025

Optimal area exploration by resetting active particles

Identifying optimal strategies for efficient spatial exploration is crucial, both for animals seeking food and for robotic search processes, where maximizing the covered area is a fundamental requirement. Here, we propose position resetting as an optimal protocol to enhance spatial exploration in active matter systems. Specifically, we show that the area covered by an active Brownian particle exhibits a non-monotonic dependence on the resetting rate, demonstrating that resetting can optimize spatial exploration. Our results are based on experiments with active granular particles undergoing Poissonian resetting and are supported by active Brownian dynamics simulations. The covered area is analytically predicted at both large and small resetting rates, resulting in a scaling relation between the optimal resetting rate and the self-propulsion speed.

cond-mat.soft↗

arXiv2025

Active thermodynamics of inertial chiral active gases: equation of state and edge currents

One of the most fundamental quests in the physics of active matter concerns the existence of a comprehensive theory for its macroscopic properties, i.e. an ``active thermodynamics''. Here, we derive and experimentally verify key elements of the active thermodynamics of ideal chiral active gases, unveiling edge currents and odd diffusivity as their peculiar features. Our main results are the derivation of an equation of state relating density and pressure via a chirality-dependent effective temperature, the derivation of Fick's law including the full diffusion matrix predicting odd diffusion, and the exact prediction of edge currents at container walls that nonmonotonically depend on chirality.

cond-mat.soft↗

arXiv2025

Flocking as a second-order phase transition in self-aligning active crystals

We study a two-dimensional crystal composed of active units governed by self-alignment. This mechanism induces a torque that aligns a particle's orientation with its velocity and leads to a phase transition from a disordered to a flocking crystal. Here, we provide the first microscopic theory that analytically maps the crystal dynamics onto a Landau-Ginzburg model, in which the velocity-dependent effective free energy undergoes a transition from a single-well shape to a Mexican-hat profile. As confirmed by simulations, our theory quantitatively predicts the transition point and characteristic spatial velocity correlations. The continuous change of the order parameter and the diverging behavior of the analytically predicted correlation length imply that flocking in self-aligning active crystals is a second-order phase transition. These findings provide a theoretical foundation for the flocking phenomenon observed experimentally in active granular particles and migrating cells.

cond-mat.soft↗

arXiv2025

Active Gaussian Network Model: a non-equilibrium description of protein fluctuations and allosteric behavior

Understanding the link between structure and function in proteins is fundamental in molecular biology and proteomics. A central question in this context is whether allostery - where the binding of a molecule at one site affects the activity of a distant site - emerges as a further manifestation of the intricate interplay between structure, function, and intrinsic dynamics. This study explores how allosteric regulation is modified when intrinsic protein dynamics operate under out-of-equilibrium conditions. To this purpose, we introduce a simple nonequilibrium model of protein dynamics, inspired by active matter systems, by generalizing the widely employed Gaussian Network Model (GNM) to incorporate non-thermal effects. Our approach underscores the advantage of framing allostery as a causal process by using, as a benchmark system, the second PDZ domain of the human phosphatase hPT1E that mediates protein-protein interactions. We employ causal indicators, such as response functions and transfer entropy, to identify the network of PDZ2 residues through which the allosteric signal propagates across the protein structure. These indicators reveal specific regions that align well with experimental observations. Furthermore, our results suggest that deviations from purely thermal fluctuations can significantly influence allosteric communication by introducing distinct timescales and memory effects. This influence is particularly relevant when the allosteric response unfolds on timescales incompatible with relaxation to equilibrium. Accordingly, non-thermal fluctuations may become essential for accurately describing protein responses to ligand binding and developing a comprehensive understanding of allosteric regulation.

cond-mat.stat-mech↗

arXiv2025

Self-alignment and anti-self-alignment suppress motility-induced phase separation in active systems

In this article, we investigate the impact of self-alignment and anti-self-alignment on collective phenomena in dense active matter. These mechanisms correspond to effective torques that align or anti-align a particles orientation with its velocity, as observed in active granular systems. In the context of motility-induced phase separation (MIPS) - a non-equilibrium coexistence between a dense clustered phase and a dilute homogeneous phase - both self- and anti-self-alignment are found to suppress clustering. Specifically, increasing self-alignment strength first leads to flocking within the dense cluster, and eventually to the emergence of a homogeneous flocking phase. In contrast, anti-self-alignment induces a freezing phenomenon, progressively reducing particle speed until MIPS is suppressed and a homogeneous phase is recovered. These results are supported by scaling arguments and are amenable to experimental verification in high-density active granular systems exhibiting self- or anti-self-alignment.

cond-mat.soft↗

arXiv2025

Self-sustained frictional cooling in active matter

Cooling processes in nature are typically generated by external contact with a cold reservoir or bath. According to the laws of thermodynamics, the final temperature of a system is determined by the temperature of the environment. Here, we report a spontaneous internal cooling phenomenon for active particles, occurring without external contact. This effect, termed ``self-sustained frictional cooling'', arises from the interplay between activity and dry (Coulomb) friction, and in addition is self-sustained from particles densely caged by their neighbors. If an active particle moves in its cage, dry friction will stop any further motion after a collision with a neighbor particle thus cooling the particle down to an extremely low temperature. We demonstrate and verify this self-sustained cooling through experiments and simulations on active granular robots and identify dense frictional arrested clusters coexisting with hot, dilute regions. Our findings offer potential applications in two-dimensional swarm robotics, where activity and dry friction can serve as externally tunable mechanisms to regulate the swarm's dynamical and structural properties.

cond-mat.soft↗