Interrogation and manipulation of active nematic films using colloidal probes

We report studies of the interactions between active nematic films and disk-shaped colloids. The active nematics are driven by molecular motors that cause the constituent microtubular bundles to slide past each other, generating extensional flows that continuously create pairs of +1/2 and -1/2 defects. Rotating magnetic microdisks in proximity to the films produce hydrodynamic flows that compete with the films’ intrinsic flow, leading to significant effects on the director field and defect landscape. Near the disks, the self-propelled +1/2 defects take on a preferred direction that is tangential to the direction of imposed stress, creating orientational ordering of the defect motion. At sufficient rotation rates, a more significant alteration of the director field is observed wherein a vortex-like structure within the director field with topological charge of +1 forms. Additionally, we are able to use the disks as effective microrheological probes to gain insight into the physical properties of the film. For example, by analyzing the velocity profile in the film produced by the spinning disk, we obtain an estimate of the film’s viscosity.