Elongated microswimmers: influence of hydrodynamics

Our nature is full of microscopic organisms such as algae, which are at the basis of the marine food chain and thus establish entire ecosystems, or bacteria, which are important for our nutrition and health, and also find a broad application in industrial processes. Thus, a broad research field is concerned with the many aspects of the biology, chemistry and physics of microorganisms. Physicists are particularly interested in the swimming motion of organisms such as algae, bacteria, spermatozoa and others. In collective motion, these microswimmers exhibit a diverse behavior such as the formation of swarms, vortices, convection rolls and plumes, and active turbulence. Therby, most microswimmers are elongated, and are otherwise characterized by different swimming mechanisms. Consequently, their dynamics arises from the interplay of different interactions, i.e., direct collisions and long-range flow fields. It is subject of current research to determine the role of the different interactions for the fascinating collective dynamics. In this context, we consider the squirmer rod, a model microswimmer that consists of several spherical squirmers, in order to create an elongated body shape. To realize the swimming mechanisms of bacteria and algae, i.e., pushers and pullers, we concentrate the surface slip-velocity field on the rear end or front side of the rod. This generates the force dipole, which these swimming mechanisms exert on the fluid. Using the simulation method of multi-particle collision dynamics (MPCD), we analyze the flow fields of the squirmer rods both in the bulk fluid as well as in the Hele-Shaw geometry, i.e., in narrow confinement between two parallel plates. Using hydrodynamic multipole expansions, we categorize the different multipole contributions created by neutral, and pusher squirmer rods in the bulk fluid as well as in the Hele-Shaw geometry. Thereby, we show how the confinement changes the radial decay of the flow fields of the force or source multipoles, and hence the characteristic flow fields compared to the bulk fluid. Further, we present a detailed study of the collective dynamics of neutral squirmer rods moving in the midplane of the Hele-Shaw geometry. From small to a large aspect ratio and density, we observe a disordered state, dynamic swarms, a single swarm, and a jammed cluster state and characterize them accordingly. We also investigate a wide range of aspect ratios and densities for pushers and pullers and provide corresponding state diagrams. The flow field of pushers destabilizes ordered structures and favors the disordered state at small densities and aspect ratios. As soon as geometric interactions become relevant for longer squirmer rods, we observe a turbulent state, as well as a dynamic cluster, while a single swarm and jammed clusters reappear at large aspect ratios. The power spectrum of the turbulent state shows two distinct energy cascades at small and large wave numbers, which follow power laws with non-universal exponents. Pullers show a strong tendency to form swarms so that no disordered state occurs at the investigated densities. For larger swarms so that no disordered state occurs at the investigated densities. Another part of this work deals in more detail with the multi-particle collision dynamics (MPCD) method, which is widely used in soft matter physics to simulate fluid flows at micrometer scale. In general, in this model the fluid exhibits the equation of state of an ideal gas, making it highly compressible. This is in contrast to real fluids, which are incompressible for velocities well below the speed of sound. Therefore, we propose a modified collision rule that leads to a MPCD algorithm with a non-ideal equation of state and a significantly reduced compressibility. At the same time, our algorithm requires less computational resources compared to conventional MPCD algorithms. To further establish the algorithm, we provide analytic expressions for the equation of state and shear viscosity, which show a good agreement with simulations of the pressure in a fluid at rest, the shear viscosity in linear shear flow, and the velocity field of a Poiseuille flow. Using two exemplary squirmer rod systems, we further compare the results of the dynamics under the extended MPCD method to those with the established MPCD version with Andersen thermostat. Thereby, we investigate the dynamic swarm state and single swarm state, which create large pressure gradients due to the sum of the many individual squirmer-rod flow fields. For the single swarm state the extended MPCD fluid shows more homogeneous fluid density, and we make the interesting observation that dynamic swarms are more pronounced and exhibit a higher polar order for the extended MPCD method.

Author dc:creator

• Zantop, Arne Wolf

Advisor dc:contributor.advisor

• Stark, Holger

Licence dc:rights.uri

• https://creativecommons.org/licenses/by/4.0/

Language dc:language.iso

en

Identifier URI

https://doi.org/10.14279/depositonce-16766

OAI identifier oai:identifier

oai:depositonce.tu-berlin.de:11303/17974