Active Particle Chirality Transitions Drive Circulation and Hyper-Uniformity in Suspensions

The ability of living systems to coordinate collective behaviour remains a fascinating puzzle, and researchers are now discovering how simple rules can generate complex order. Yuxin Zhou, Qingqing Yin, and colleagues from Tongji University, alongside Shubhadip Nayak, Poulami Bag, and Pulak K. Ghosh from Presidency University, investigate this phenomenon in artificial systems, exploring how particles can self-organise through a process akin to bacterial quorum sensing. Their work demonstrates that when active particles ‘perceive’ a sufficient density of neighbours, they change behaviour, triggering transitions from disordered motion to organised phases, and even large-scale circulating currents. Remarkably, these collective behaviours emerge even without physical contact between particles, and can be initiated with a surprisingly small number of ‘quorum-sensing’ particles, offering potential design principles for new active materials and coordinated micro-robotic swarms.

Induced phase separation represents an efficient aggregation mechanism for active matter, yet biological systems frequently exhibit more complex organisation through communication between their constituent parts. This research investigates suspensions of active particles that alter their chirality when the local density of neighbours within their visual cone surpasses a defined threshold, representing a communication-based, non-reciprocal interaction analogous to quorum sensing. By tuning the parameters of the visual cone, researchers demonstrate programmable transitions between states of disorder, phase separation, and hyperuniformity. Importantly, the onset of phase separation triggers large-scale circulation, with robust edge currents persistently flowing around dense clusters, while the particle distributions within these clusters become effectively hyperuniform.

Active Colloidal Particles Exhibit Quorum Sensing

This research explores the collective behaviour of active colloidal particles, microscopic particles capable of self-propulsion, and how they respond to their surroundings. These particles possess a unique visual perception mechanism, influencing their movement based on the presence and movement of other particles within their range of sight. This behaviour draws inspiration from flocking animals and quorum sensing in bacteria. The key finding is that particles exhibit a form of quorum sensing, changing their behaviour based on the local density of other particles, leading to spontaneous clustering and a state of hyperuniformity, a highly ordered arrangement minimizing density fluctuations.

The particles also exhibit coordinated collective motion and signs of phase separation. The research team employed computer simulations to model particle behaviour, using equations to describe particle motion and statistical techniques to analyse the simulation data, including methods to characterize particle arrangement, quantify order and hyperuniformity, measure particle diffusion, and analyse the local environment around each particle. This work is inspired by flocking behaviour in animals and quorum sensing in bacteria, and connects to concepts in materials science and physics.

Active Particles Self-Organize Via Density Sensing

Researchers have discovered a novel mechanism by which active particles can spontaneously organize into complex structures. This organization doesn’t rely on direct attraction or repulsion, but emerges from a form of communication based on sensing neighboring particles. The team demonstrated that when particles perceive a sufficient density of neighbours within their “visual cone”, they change their direction of motion, triggering coordinated movement and transitions between different phases of organization. Tuning the range of this perception controls the resulting structure. Initially, increasing the sensing range leads to the formation of clusters and cavities.

Surprisingly, these aren’t static; the density inside clusters decreases as they grow and cavities expand, contrasting with conventional phase separation. The team observed the emergence of extended, periodic structures spanning the simulation area, indicating long-range order. Further increasing the sensing range drives the system towards a state of “hyper-uniformity”, where particles are arranged in a highly ordered, yet not crystalline, pattern. Even a small percentage, just 5%, of particles capable of this perception-based communication is enough to induce collective circulation and herd passive particles, demonstrating a powerful level of control over the system’s behaviour. This discovery has significant implications for the design of active materials and micro-robotic swarms, offering a new paradigm for creating self-assembling systems.

Perception Drives Collective Order and Flow

The research demonstrates that simple communication rules among active particles can lead to complex, collective behaviours, moving beyond mere aggregation to organised circulation and spatial patterning. Specifically, the team investigated how particles change their behaviour based on the density of neighbours within their perceptual range, a process akin to quorum sensing. This perception-based interaction triggers transitions from disordered states to phase separation and, remarkably, to hyper-uniformity, where particles are arranged with exceptional order. Notably, the formation of dense clusters is accompanied by robust edge currents and an unexpectedly uniform distribution of particles within them.

These findings reveal that this collective behaviour does not rely on physical repulsion between particles, but emerges from the communication protocol itself. The researchers confirmed this by observing the same patterns even when removing repulsive forces between particles. Furthermore, they showed that only a small percentage of communicating particles, just five percent, is sufficient to induce these collective flows in mixtures of active and passive particles.