The Three-Dimensional Velocity Field of Kinesin-Driven Microtubules in Torroidal Channels

We study two regimes of flow in multiple three-dimensional toroidal channels by tracking the fluorescent spherical particles in the kinesin-driven microtubule systems: chaotic'' flow and coherent’’ flow. In the smallest aspect ratio torus, where the channel height $h$ is a quarter of the width $w$, the active system shows zero mean velocity, small-scale isotropy in fluctuation and no persistent flow structure. In other tori with higher aspect ratios $h\mathrm{/}w$ close to 1, we find faster coherent flows along the azimuthal direction and increasing fluctuation strengths with growing confinement geometries. Regardless of flow regimes, the flow profiles at $r-z$ cross-section and $r-\theta$ plane are symmetric. The coherent'' profiles show two criteria: Poiseuille-like’’ profiles, which have the peak velocities near the centers of channels; a peak-separated'' profile, which has four peak velocities near a certain distance to four confining surfaces. These flow profiles, after scaled by the local isotropic fluctuation strength, reveal universal three-dimensional flow structures among the Poiseuille-like’’ criterion and the same level of scaled peak velocity at the ``peak-separated’’ one. These results illustrate scalable flow structures in this kinesin-driven microtubule active system.