Confinement of active nematics using virtual boundaries imposed by submerged rectangular structures

Active nematics consist of energy-driven particles or subunits that carry out collective motion inherently in an out-of-equilibrium steady state. These active systems are capable of self-organization where energy is pulled from the environment and +1/2 and -1/2 disclinations are continuously created and annihilated in a quasi-2D configuration. The active material is confined between two immiscible fluids, an oil layer and an aqueous layer. In this work, we report phenomena where we introduce submerged retactangular structures, trenches and via microfabrication. The submerged trenches generate a virtual boundary, close to which -1/2 defects tend to stagnate. +1/2 defects tend to locate away from the boundary reminiscent to hard boundaries. When examining an array of submerged steps, we find similar defect distributions to the submerged trench. We also report that the thickness of the oil can change in defect concentration and morphology. We quantify these results by analyzing defect distributions with time and the impact of oil layer thickness on defect morphology.