Chiral Active Granular Matter

Project Information

CHIAGRAM

Grant agreement ID: 101103751

DOI

10.3030/101103751

Project closed

EC signature date 16 May 2023

Start date 1 October 2023

End date 30 September 2025

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 173 847,36

Coordinated by

HEINRICH-HEINE-UNIVERSITAET DUESSELDORF
Germany

Periodic Reporting for period 1 - CHIAGRAM (Chiral Active Granular Matter)

Reporting period: 2023-10-01 to 2025-09-30

Summary of the context and overall objectives of the project

Active matter is ubiquitous in nature and encompasses a broad range of systems, from microscopic to macroscopic scales, that extract energy from their surroundings and convert it into directed motion. Examples include living microswimmers such as bacteria, spermatozoa, and cells, as well as collective groups of animals, from insects to birds. Most of these systems break rotational symmetry and, consequently, possess an intrinsic chirality, allowing one to distinguish between left- and right-handed active particles that move along clockwise or counterclockwise circular trajectories. The nonequilibrium dynamics of such chiral active systems have inspired the design of artificial microswimmers, such as active colloids, and macroscopic robots capable of efficiently exploring their environment or cooperating to move collectively in self-organized structures.

The main goal of the CHIAGRAM project is to explore chirality in active granular matter, focusing on the dynamics of 3D-printed self-propelled robots, termed vibrobots. These vibrobots constitute a new class of self-propelled and self-rotating active materials that follow circular trajectories. Combining experiments and theory, the project investigates both single-particle and collective phenomena, their interactions with the environment, and the role of deformability - particularly in chain-like assemblies.

CHIAGRAM opens a novel research direction at the interface between chirality and granular matter. This connection paves the way for systematic studies combining theoretical modeling and experimental realization of chiral granular systems. The rich experimental and theoretical results achieved within the project—ranging from single-particle properties to emergent collective behaviors—will inspire future investigations into the interplay between chirality and inertia. Furthermore, CHIAGRAM establishes an innovative framework to experimentally probe chirality in active matter and lays the groundwork for the design of active, deformable robotic materials whose properties can be tuned and optimized for self-organization and adaptive functionalities.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

During this project, I designed chiral active granular particles, termed vibrobots, by 3D-printing plastic bodies with a cylindrical shape and multiple attached legs. When placed on a vibrating plate actuated by an electromagnetic shaker, these particles move autonomously, mimicking the behavior of microorganisms or small animals.

i) First, we examined the motion of a 3D-printed passive tracer immersed in a nonequilibrium environment of self-propelled vibrobots. We discovered a novel collisional mechanism, which we termed tapping collisions, in contrast to the random elastic collisions of passive systems. This mechanism leads to a deviation from Einstein’s relation between fluctuation and dissipation, while our theory allowed us to propose a generalized fluctuation–dissipation relation (Caprini et al., Commun. Phys. 7, 52, 2024).

ii) Next, we investigated the collective properties of dense vibrobot assemblies by increasing their surface density on the vibrating plate. We explored clustering and motility-induced phase separation, corresponding to a nonequilibrium coexistence between dense and dilute phases. We observed that increasing inertia suppresses clustering and induces an unexpected transition in the cluster’s internal structure—from a solid-like to a liquid-like state - driven purely by inertial effects (Caprini et al., Commun. Phys. 7, 343, 2024).

iii) We then explored the impact of chirality on collective phenomena in active matter through numerical simulations of chiral active Brownian particles. These self-propelled, rotating particles, interacting via a Lennard–Jones potential, form high-density clusters. Combining theory and simulations, we predicted a new collective behavior characterized by spontaneously forming vortices that periodically reverse their vorticity, termed self-reverting vortices (Caprini et al., Commun. Phys. 7, 153, 2024).

iv) Finally, we investigated chirality in active granular systems experimentally by twisting the legs of vibrobots. By linking them through 3D-printed connectors, we realized chiral active granular polymers. Experiments and simulations revealed a spontaneous folding–unfolding transition uniquely induced by chirality - a behavior absent in passive polymers. This transition arises from a self-wrapping mechanism that drives dynamic polymer collapse even in the absence of attractive interactions (Caprini et al., Newton 2025, online).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

In this project, we went beyond the state of the art in the broad field of active matter, achieving experimental and theoretical advances in active granular systems and uncovering the fundamental role of chirality. Specifically, our experimental findings led to several achievements that extend existing knowledge.

i) We provided one of the first experimental realizations of active granular particles governed by inertial dynamics. The project introduced an innovative method to control inertia by tuning the effective friction, enabling future systematic investigations of inertial effects in active granular matter and the validation of inertia-based phenomena previously explored only theoretically. This approach led to the first experimental observation of clustering suppression (Caprini et al., Commun. Phys. 7, 153, 2024) and revealed novel inertia-induced effects, including tapping collisions and the generalized Einstein relation (Caprini et al., Commun. Phys. 7, 343, 2024).

ii) A central goal of the project concerned the exploration of chirality in active granular matter, which we addressed experimentally and theoretically. We designed one of the first controllable realizations of chiral active particles performing circular trajectories at the macroscopic scale. This technique opens broad possibilities for future studies of both single-particle and collective behaviors, ranging from clustering (Caprini et al., Commun. Phys. 7, 153, 2024) and hyperuniform phases to measurements of chirality-induced effective temperatures and anomalous pressures.

iii) We achieved the first experimental realization of a chiral polymer composed of active and chiral units (Caprini et al., Newton 2025, online). Our strategy, based on connecting multiple granular particles, goes beyond the state of the art and paves the way for experimental studies of nonequilibrium polymer physics, i.e. the dynamics of polymers not in thermal equilibrium with their surroundings. These results may also shed light on the behavior of flexible biological structures that exploit self-encapsulation mechanisms to fold and unfold, thereby modulating their functionality.

Overall, this project provides innovative strategies for robotic applications that exploit the interplay between chirality, deformability, and inertia, enabling the design of adaptive robotic materials capable of self-organization, reconfiguration, and controlled morphogenesis in complex environments.

Experimental realization of a granular active chiral polymer

Inertia suppresses clusering in active matter