Dresden 2020 – scientific programme

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BP: Fachverband Biologische Physik

BP 12: Active Matter II (joint session BP/CPP/DY)

BP 12.1: Talk

Tuesday, March 17, 2020, 09:30–09:45, HÜL 386

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. The relevant parameters are the ratio of swimming to bulk sedimentation velocity and the normalized torque.

In the state diagram of these parameters, for neutral squirmers we observe sedimentation at strong gravitational forces and inverted sedimentation at finite torques, when activity dominates. In between, we identify plumes of collectively sinking squirmers that feed convective rolls of circling squirmers at the bottom of the simulation cell. At velocity ratios slightly above one and large torques squirmers form a spawning cluster, which floats above the bottom wall and 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] J.-T. Kuhr, et al., Soft Matter 13, 7548 (2017).

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

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