Active nematic flows confined in a two dimensional channel with hybrid alignment at the walls: a unified picture

Abstract: Active nematic fluids confined in narrow channels generate spontaneous flows when the activity is sufficiently intense. Recently, it was shown that if the molecular anchoring at the channel walls is conflicting flows are initiated even in the zero activity limit. An analytical laminar velocity profile for this specific configuration was derived within a simplified nematohydrodynamic model in which the nematic order parameter is a fixed-magnitude unit vector n. In this study we explore systematically active flows in this confined geometry with a more general theoretical model that uses a second-rank tensor order parameter Q to express both the magnitude and orientation of the nematic phase. The Q-model allows for the presence of defects and biaxial, in addition to uniaxial, molecular arrangements. Our aim is to provide a unified picture, beyond the limiting regime explored previously, to serve as a guide for potential microfluidic applications. We reveal how the nematic-flow coupling is not only dependent on geometrical constraints but also highly sensitive to material and flow parameters. We specifically stress the key role played by the activity and the flow aligning parameter and we show that solutions depend on two dimensionless parameters. We find that for large values of the activity parameter the flow is suppressed for contractile particles while is either sustained or suppressed for extensile particles depending on whether they tend to align or tumble when subject to shear. We explain these distinct behaviors by an argument based on the results of the stability analysis applied to simpler configurations. We finally provide a numerical example of a biaxial three-dimensional thresholdless active flow for which we show that biaxiality is specially relevant for a weakly first-order isotropic-nematic phase transition.