Cooperativity of Confined Nematic Microswimmers: From One to Many

Controlling the behavior of microswimmers is a major challenge to extract work for novel active matter applications. Geometric confinement is often used for controlling soft matter systems. However, in comparison to the case of Newtonian fluids, the effects of solid interfaces on microswimmers moving through an anisotropic fluid are far less understood. By means of nematic multiparticle collision dynamics simulations and analytical modeling, we investigate the dynamical behavior of swimmers immersed in a nematic liquid crystal and confined by solid walls. For isolated squirmers, we find a rich phase diagram including oscillatory dynamics for weak pushers, depending on the strength of their propulsion and degree of confinement. Our theoretical model shows that, unlike in the isotropic case, in a nematic fluid, force dipole, source dipole, and source quadrupole singularities all are required for the onset of oscillations. Increasing the number of squirmers shows the emergence of cooperativity in pusher-type squirmers, while pullers' flow fields hinder each other's motion. The interplay of nematodynamic torque, wall-induced elastic repulsion, and active flows thus offers the opportunity for both control and transport in active nematic systems.