(462e) Swimmers at Interfaces Enhance Interfacial Transport

We study Pseudomonas aeruginosa PA01as active colloids or micro-swimmers to understand how active colloids alter interfacial transport. Interfaces differ from bulk in important ways. First, swimmers can become trapped at interfaces in diverse configurations and swim persistently in these adhered states. Thus, identical swimmers move along diverse trajectories which differ because they are trapped in different orientations with respect to the interfacial plane. Our swimmers move primarily in circular paths with differing curvatures whose centers decorrelate over long times owing to active noisy processes. Second, the flow field generated by the swimmers is altered by the interface because of the importance of interfacial tension and Marangoni stresses that render the interface incompressible. We had previously visualized the interfacial flow around a pusher bacterium¹ using correlated displacement velocimetry (CDV) and described the flow field using two dipolar hydrodynamic modes; one stresslet mode whose symmetries differ from those in bulk, and another bulk mode unique to incompressible fluid interfaces. Here, we ask, how do swimmers at interfaces generate biomixing? We find the Lagrangian paths of tracer particles moving via hydrodynamic interaction with the swimmers. We also infer tracer paths in experiment, exploiting the flow measured by CDV which allows hydrodynamic interactions to be determined even in the presence of multiple swimmers and neighboring tracers undergoing Brownian motion in the interface. We show that active noisy processes in bacteria motion play a central role in enhancing interfacial biomixing.

1. Deng, Jiayi, Mehdi Molaei, Nicholas G. Chisholm, and Kathleen J. Stebe. "Interfacial flow around a pusher bacterium." Journal of Fluid Mechanics976 (2023): A18.
2. Swimmers at Interfaces Enhance Interfacial Transport, Jiayi Deng, Mehdi Molaei, Nicholas G. Chisholm, Scarlett Clarke, and Kathleen J. Stebe, Deng, Jiayi, Mehdi Molaei, Nicholas G. Chisholm, and Kathleen J. Stebe. arXiv preprint arXiv:2402.04277(2024).