Speaker
Description
Transport in active media has been extensively studied in bacterial suspensions, where tracers exhibit short-time superdiffusion followed by long-time diffusion, typically without a clearly resolved ballistic regime or an intrinsic length scale for optimal transport. In microtubule-based active nematics, research has mainly focused on controlling flows through confinement and boundary geometry. Only recently have freely moving rigid inclusions been investigated, mainly in confined settings where rotational motion is tuned via ratchet-shaped boundaries. However, their translational dynamics in unconfined active nematics remain largely unexplored.
Here, we experimentally investigate the motion of rigid polymer microdisks (30–300 μm in diameter) freely advected at the oil–water interface of a two-dimensional microtubule-based active nematic. Mean-squared displacement measurements reveal a clear crossover from a well-defined ballistic regime at short times to diffusion at long times, enabling direct extraction of propulsion speed, persistence time, and effective diffusivity. This crossover is strongly size-dependent: disks in the 50–100 μm range exhibit the highest propulsion speeds and diffusivities, together with the shortest crossover times, while smaller (30 μm) and larger (≥150 μm) disks show reduced propulsion and longer ballistic persistence before transitioning to the diffusive regime. Consequently, both propulsion speed and long-time diffusivity exhibit a pronounced non-monotonic dependence on disk diameter, peaking at ~75 μm. This characteristic size closely matches the intrinsic active nematic length scale, estimated from velocity correlation lengths (≈50–130 μm), revealing a direct coupling between inclusion size and the mesoscale structure of the active flow.
We extend this analysis to anisotropic inclusions, namely elliptical and crescent-shaped geometries. Anisotropy enhances transport: ellipses exhibit faster motion and higher diffusivity than circular disks, consistent with more efficient coupling to the spatiotemporal structure of the active nematic. In contrast, crescent-shaped inclusions display pronounced directional anisotropy, with motion along their long axis showing higher diffusivity and longer persistence than along the short axis. Together, these results demonstrate that both size and geometry act as effective control parameters for tuning transport in active nematics, providing design principles for directing motion in structured active materials.
