The interplay between activity and confinement leads to unexpected features in suspensions of active polymer chains and rings

Polymers are crucial in biological and artificial systems at several length scales, ranging from microtubules, bacteria, worms to robots. Active polymers are self-propelled units tailored to couple activity with conformational degrees of freedom. They present phenomena such as anomalous diffusion and a non-equilibrium coil-to-globule transition. The latter can be described by an active Rouse model (accounting for finite bending rigidity) [1].

In two dimensions [2], active filaments can either be in an open chain state (at low activity) or form spirals (at high activity), independently on their bending rigidity and the model used to implement activity along the chain. When considering length polydispersity, filament length is the control parameter for hierarchical self-assembly [3,4], displaying a collective wrapping mechanism absent in length monodisperse filaments.

Active filaments confined in a cylindrical channel show anomalous behaviours under different confinement and activity: the ratio between the end-to-end distance of passive polymer chains in channels and in bulk is broken by activity [5].

While passive polymers confined in corrugated channels display multiple dynamical features upon increasing their degree of polymerization. Such behaviour can be described via a Fick-Jacobs approach which can be used to design porous filters capable of separating polymers by their size [6].

The interplay between activity, topology, and confinement can lead to a spontaneous segregation from an initially one-component solution. In a suspension of active polymer rings confined between two parallel plates, a self-organized dynamical state emerges, characterized by the coexistence of slowly diffusing clusters of rotating disks and faster rings moving in between them [7]. This self-organization displays a non-monotonic dependence on the activity-induced entanglement, since increasing entanglement interferes with the planar conformations required to form aligned stacks [8].