Active boundary layers in confined active nematics

Boundary layers play a central role in fluid dynamics and in soft material science. They are regions whose extent is much smaller than any typical system size, yet, the interfacial transport processes associated with localized profiles of scalar fields, such as pressure or ionic charge density control the distant dynamics. In conventional liquid crystals, molecular interactions in this region determine the equilibrium orientational field of the passive material. However, the role of boundaries in active liquid crystals has just begun to be studied.

Here, we work with the well-known kinesin/tubulin active nematic under confinement and show that the self-sustained flows can develop active boundary layers that topologically polarize the confining walls and determine the bulk flows. Differently from the polarization by electric charges that drive electrokinetic phenomena and stabilize colloidal suspensions, here the boundary is populated by distortions of the orientational field, or topological defects, of identical negative charge. In contrast to their bulk counterparts both in passive and active liquid crystals, defects in the active boundary layer feature the ability to merge between like-sign distortions and exhibit collective dynamics that are reminiscent of a one-dimensional Kuramoto-Sivashinsky-like description of spatiotemporal chaos.