  The role of boundary layers in conventional liquid crystals is commonly
subsumed in their anchoring on confining walls. In the classical view,
anchoring enslaves the orientational field of the passive material under
equilibrium conditions. In this work, we experimentally explore the role of
confining walls in the behavior of an active nematic. We find that, under slip
boundary conditions, the wall induces the accumulation of negatively charged
topological defects in its vicinity, resulting in the formation of a
topological boundary layer that polarizes the wall. While the dynamics of wall
and bulk defects are decoupled, we find that the active boundary layer
influences the overall dynamics of the system, to the point of fully
controlling the behavior of the active nematic in situations of strong
confinement. Finally, we show that wall defects exhibit behaviors that are
essentially different from those of their bulk counterparts, such as high
motility or the ability to recombine with another defect of like-sign
topological charge. These exotic behaviors result from a change of symmetry
induced by the wall in the director field around the defect. Finally, we show
that the collective dynamics of wall defects can be described in terms of a
one-dimensional Kuramoto-Sivashinsky -like description of spatio-temporal
chaos.
