Dynamical clustering interrupts motility-induced phase separation in chiral active Brownian particles

Area of Science:

• Active matter physics
• Soft condensed matter
• Statistical mechanics

Background:

• Motility-induced phase separation (MIPS) is a key phenomenon in repulsive active particles, mimicking gas-liquid behavior.
• Real-world active particles are rarely perfect spheres, often possessing asymmetry leading to active torque and chirality.
• The effect of chirality on MIPS in active matter systems remains an open research question.

Purpose of the Study:

• To investigate the impact of active torque on phase separation in active Brownian particles.
• To explore the emergence of novel collective behaviors beyond conventional MIPS in chiral active systems.
• To understand the underlying mechanisms of dynamical clustering in two-dimensional chiral active matter.

Main Methods:

• Computer simulations of circle active Brownian particles in two dimensions.
• Dynamic mean-field theory to analyze non-equilibrium steady states.
• Investigation of particle dynamics, including rotation and collective motion.

Main Results:

• Sufficiently large active torque in chiral particles disrupts conventional MIPS, leading to a dynamical clustering state.
• Multiple clusters form due to a balance between MIPS cohesion and disintegration driven by circulating currents.
• A non-vanishing current in non-equilibrium steady states arises from motility 'relieved' by automatic rotation, breaking detailed balance.

Conclusions:

• Chiral active colloids, even with small active torque, cannot be described by equilibrium-like phase separation theories.
• The observed dynamical clustering mechanism provides insights into similar phenomena in diverse active matter systems.
• Active torque fundamentally alters the phase behavior of active matter, leading to non-equilibrium collective states.