Softening and Enhanced Transport of Colloidal Chains in a Bacterial Bath

Semi-flexible filaments immersed in an active environment are ubiquitous in biological systems. Therefore, understanding their dynamics is of fundamental importance to both biology and physics. Recent theoretical and simulation studies of semi-flexible filaments in an active bath predict a rich diversity of phenomena such as activity-induced collapse and enhanced diffusion. However, there is a notable lack of experimental investigations on this topic. To address this issue, we construct a model system consisting of colloidal chains and the motile bacteria Bacillus subtilis. We experimentally study the non-equilibrium dynamics of the chains by controlling the concentration and the activity of the bacteria and the rigidity of the chains. We observe 'softening' of the colloidal chains: the effective persistence length of the chains is reduced by over an order of magnitude. Consequently, we see a variety of conformations adopted by the colloidal chains which we quantitatively describe using shape parameters including radius of gyration and acylindricity. We also observe enhanced diffusion of the centers of mass of the colloidal chains. We attribute both effects to the large-scale, structured flow in the active bath.