Spontaneous vortex formation by microswimmers with retarded attractions

Area of Science:

• Active matter physics
• Non-equilibrium statistical mechanics
• Collective behavior

Background:

• Collective states of inanimate particles arise from physical interactions and thermal motion.
• Motile agents exhibit richer behaviors than inanimate particles, involving perceived information, decision-making, and feedback.
• Retarded interactions are crucial for complex dynamics in active systems.

Purpose of the Study:

• Investigate emergent collective behaviors in microswimmers with delayed self-propulsion.
• Explore the role of retarded interactions in active particle systems.
• Understand the mechanisms behind spontaneous symmetry breaking and critical phenomena.

Main Methods:

• Experimental study of spherical Brownian microswimmers with delayed self-propulsion towards a fixed target.
• Comparison with stochastic delay differential equations of motion for single swimmers.
• Analysis of emergent collective states, including chiral dynamics and layered structures.

Main Results:

• Observed spontaneous symmetry breaking to a transiently chiral dynamical state.
• Identified delay-induced effective synchronization with the swimmer's own past as a key mechanism.
• Demonstrated self-organization into layers with synchronized pro- and retrograde orbital motion, stabilized by physical interactions.

Conclusions:

• Simple retarded interactions can induce complex adaptive behavior in small active-particle ensembles.
• Emergent phenomena like chirality and layered motion do not require many-particle cooperativity.
• Delay-induced synchronization is a fundamental mechanism driving collective dynamics in microswimmer systems.