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Flocking by Turning Away (2401.17153v2)

Published 30 Jan 2024 in cond-mat.soft, cond-mat.stat-mech, and physics.bio-ph

Abstract: Flocking, as paradigmatically exemplified by birds, is the coherent collective motion of active agents. As originally conceived, flocking emerges through alignment interactions between the agents. Here, we report that flocking can also emerge through interactions that turn agents away from each other. Combining simulations, kinetic theory, and experiments, we demonstrate this mechanism of flocking in self-propelled Janus colloids with stronger repulsion on the front than on the rear. The polar state is stable because particles achieve a compromise between turning away from left and right neighbors. Unlike for alignment interactions, the emergence of polar order from turn-away interactions requires particle repulsion. At high concentration, repulsion produces flocking Wigner crystals. Whereas repulsion often leads to motility-induced phase separation of active particles, here it combines with turn-away torques to produce flocking. Therefore, our findings bridge the classes of aligning and non-aligning active matter. Our results could help to reconcile the observations that cells can flock despite turning away from each other via contact inhibition of locomotion. Overall, our work shows that flocking is a very robust phenomenon that arises even when the orientational interactions would seem to prevent it.

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Citations (6)
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Summary

  • The paper reveals that turn-away repulsion among self-propelled particles induces stable polar flocking, challenging traditional alignment models.
  • The study integrates experiments, kinetic theory, and simulations to show how particle density and repulsive forces drive emergent collective behavior.
  • The findings offer new insights for designing biological, robotic, and material systems that exploit adaptive, non-alignment interactions.

Insight into "Flocking by Turning Away"

The paper "Flocking by Turning Away" explores a non-traditional approach to understanding flocking behavior in active matter systems. Traditional models of flocking, such as the Vicsek model, suggest that alignment interactions between agents, akin to spins in the XY model, drive the emergence of collective motion. This paper, however, challenges the conventional understanding by demonstrating that flocking can emerge from turn-away interactions, where agents actively repel and orient themselves away from each other.

Summary of Findings

  1. Experimental and Theoretical Framework: The paper investigates self-propelled Janus colloids, whose interactions are characterized by stronger repulsion at the front compared to the rear. By combining empirical experiments with kinetic theory and simulations, the authors uncover that these particles can exhibit stable polar states, resulting in flocking behavior without traditional alignment interactions.
  2. Parametric Influence on Flocking: The paper reveals that the emergence of polar order from turn-away interactions necessitates particle repulsion. When the system is densely populated, interactions lead to the formation of structures resembling flocking Wigner crystals, where coherent motion is maintained despite independent orientation hindrance among neighboring particles.
  3. Contradictory Phenomenon: A significant contradiction is observed whereby repulsion typically leads to phase separation; here, repulsion in conjunction with turn-away torques instead culminates in collective flocking behavior.
  4. Broader Implications and Applications: The findings offer a potential explanation for the observation that certain cellular systems exhibit collective motion despite repelling interactions prompted by contact inhibition of locomotion. This novel understanding could bridge disparate classes of active matter, linking repulsion-based phenomena to alignment-driven collective behaviors.
  5. Particular Features of the Model: The authors examine active Brownian particles with turn-away interactions both through small-scale simulations and large-system analyses, finding that the mechanism stabilizes polar order even at larger scales. Notably, they develop a defining theoretical framework accounting for the torques and repulsions indicative of non-reciprocal interactions, essential for both analytical predictions and practical implementation.

Implications for Future Research

The implications of these findings extend to several domains:

  • Engineering: The insights could inform the design of robotic systems capable of adaptive collective locomotion in complex environments, favoring collision avoidance.
  • Biology: Understanding how cells overcome repulsive interactions to exhibit collective motility has profound implications for developmental biology and cancer metastasis research.
  • Material Science: The paper opens pathways for the development of materials demonstrating dynamic structural responses to environmental stimuli without losing collective coherence.

Conclusion

The work primarily suggests that flocking, a collective behavior observed across scales in nature, doesn't necessitate traditional paradigms of directional alignment. Instead, even seemingly conflicting interaction rules such as turn-away repulsions can suffice to induce robust flocking dynamics. This insight invites a re-examination of collective phenomena in natural and synthetic systems, proposing that the governing dynamics of flocking are more diverse and adaptive than traditionally conceived. As such, the paper makes a valuable contribution to the active matter literature, broadening the horizon for future explorations in non-equilibrium statistical mechanics and complex systems theory.

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