Anisotropic dynamics of a self-assembled colloidal chain in an active bath

Mehdi Shafiei Aporvari, Mustafa Utkur, and Emine Ulku Saritas have published an article titled “Anisotropic Dynamics of a Self-assembled Colloidal Chain in an Active Bath” in Soft Matter. This paper was a collaborative work with Giovanni Volpe (previously with the Department of Physics in Bilkent, now with University of Gothenburg) and Joakim Stenhammar (with Lund University). This paper presents an experimental investigation of the dynamics of isolated self-assembled chains moving at an interface between a suspension of swimming bacteria and air, finding an anomalous, non-monotonic size dependence in the anisotropic center-of-mass diffusivity.



Anisotropic macromolecules exposed to non-equilibrium (active) noise are very common in biological systems, and an accurate understanding of their anisotropic dynamics is therefore crucial. Here, we experimentally investigate the dynamics of isolated chains assembled from magnetic microparticles at a liquid-air interface and moving in an active bath consisting of motile E. coli bacteria. We investigate both the internal chain dynamics and the anisotropic center-of-mass dynamics through particle tracking. We find that both the internal and center-of-mass dynamics are greatly enhanced compared to the passive case, i.e., a system without bacteria, and that the center-of-mass diffusion coefficient D features a non- monotonic dependence as a function of the chain length. Furthermore, our results show that the relationship between the components of D parallel and perpendicular with respect to the direction of the applied magnetic field is preserved in the active bath compared to the passive case, with a higher diffusion in the parallel direction, in contrast to previous findings in the literature. We argue that this qualitative difference is due to subtle differences in the experimental geometry and conditions and the relative roles played by long-range hydrodynamic interactions and short-range collisions.