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BP: Fachverband Biologische Physik

BP 5: Active Biological Matter II (joint session BP/CPP/DY)

BP 5.4: Vortrag

Montag, 22. März 2021, 12:00–12:20, BPb

Sedimentation and Convection of Bottom-Heavy Squirmers — •Felix Rühle, Jan-Timm Kuhr, and Holger Stark — TU Berlin, Institut für Theoretische Physik, Berlin, Germany

Active particles form appealing patterns, in particular, when hydrodynamic interactions are present [1-3]. A fascinating example known from biology is bioconvection of microswimmers under gravity [4]. In order to study such systems, we simulate bottom-heavy squirmers (neutral squirmers, pushers, and pullers) under different gravitational forces and torques [3]. The relevant parameters are the ratio of swimming to bulk sedimentation velocity α and the normalized torque.

In the state diagram of these parameters, for neutral squirmers at low α we observe sedimentation states, where bottom-heaviness leads to the formation of clusters of different sizes. For high α, finite torques lead to inverted sedimentation. In between, we identify plumes of collectively sinking squirmers that feed convective rolls of circling squirmers at the bottom of the simulation cell. At α 1 and large torques squirmers form a spawning cluster above the wall, from which squirmers occasionally escape. For strong pushers and pullers, we find that the dipolar flow fields weaken the formation of plumes and convective rolls.
[1] M. Hennes, et al., PRL 112, 238104 (2014)

[2] H. Jeckel, et al., PNAS 116, 1489 (2019).

[3] F. Rühle, and H. Stark, Eur. Phys. J. E 43, 26 (2020).

[4] T.J. Pedley, and J.O. Kessler, Annu. Rev. Fluid Mech. 24, 313 (1992).