Trapping and sorting active particles: motility-induced condensation & smectic defects

TL;DR

This paper experimentally and theoretically investigates a phase transition in active particles where motility-induced condensation leads to trapping, with implications for particle sorting based on activity persistence.

Contribution

It provides the first experimental realization and theoretical understanding of the trapping phase transition in active particles with a V-shaped obstacle.

Findings

Trap fills when trap angle is below critical threshold

Particles escape when trap angle exceeds critical threshold

Critical angle decreases with increasing rotational noise

Abstract

We present an experimental realization of the collective trapping phase transition [Kaiser et al., PRL 108, 268307 (2012)], using motile polar granular rods in the presence of a V-shaped obstacle. We offer a theory of this transition based on the interplay of motility-induced condensation and liquid-crystalline ordering and show that trapping occurs when persistent influx overcomes the collective expulsion of smectic defect structures. In agreement with the theory, our experiments find that a trap fills to the brim when the trap angle $\theta$ is below a threshold $\theta_c$, while all particles escape for $\theta > \theta_c$. Our simulations support a further prediction, that $\theta_c$ goes down with increasing rotational noise. We exploit the sensitivity of trapping to the persistence of directed motion to sort particles based on the statistical properties of their activity