Spontaneous flow transition in confined 3D Active nematic droplets

Active matter collectively organizes mesoscale active stresses that drive a variety of emergent phenomena at the macroscopic scale including spontaneous large-scale flows. In 3D active nematics, the characteristic length scale of these flows is set by the ratio of nematic elasticity and active stress. Continuum hydrodynamic theory predicts that confinement below the active length scale stabilizes the otherwise chaotic active dynamics of the active nematic. This transition between a quiescent and a flowing state is reminiscent of the Fréedericksz transition. In this work, we probe how the interplay between confinement, active stress, and nematic elasticity controls the flow to no-flow transition in 3D microtubule-based active nematics confined in oil-water emulsions. This study provides a first novel method to measure active stresses in biomimetic active matter, which is a requirement for a quantitative comparison between experiments and theory.