Effects of confinement on the dynamics and correlation scales in active fluids

We study the influence of solid boundaries on dynamics and structure of active fluids as the height of the container, $z$, changes. Along the varying dimension, the geometry systematically increases, therefore, the confinement ($z$) transits from "strong confinement", to "intermediate confinement" and to "weak confinement" (close to "unconfined"). In horizontal dimensions ($x,y$), the system remains "unconfined". Through tracking the tracers dispersed in the active fluids in three dimensions we observed that activity level, characterized by velocity fluctuations of flow tracers, increases as system size increases. Concomitantly, the velocity-velocity temporal correlation changes from weak correlation to strong positive correlation, indicating "memory" in active flows. We estimate the characteristic size of the flow structure by integrating the velocity-velocity spatial correlation function. The integral increases as confinement becomes weaker and saturates at approximately 400 microns as the system becomes "unconfined". This saturation indicates an intrinsic length scale which, along with the small-scale isotropy, demonstrates the multi-scale nature of this kinesin-driven bundled microtubule system.