Hydrodynamic interaction of two unsteady model microorganisms

A physics-based model of swarming jellyfish

We propose a model for the structure formation of jellyfish swimming based on active Brownian particles. We address the phenomena of counter-current swimming, avoidance of turbulent flow regions and foraging. We motivate corresponding mechanisms from observations of jellyfish swarming reported in the literature and incorporate them into the generic modelling framework. The model characteristics is tested in three paradigmatic flow environments.

Flow-driven competition between two capsules passing through a narrow pore

By incorporating a distance function into the finite element simulation, we investigate the flow-driven competition between two soft capsules passing through a narrow pore, employing the arbitrary Lagrangian-Eulerian formulation to satisfy the boundary conditions for fluid flow and capsule deformation. In our simulations, the motion and deformation of the capsules can be described in an intuitive manner, and the order in which capsules of different sizes pass through a pore can be clearly determined. Meanwhile, when the capsules are near the narrow pore, the change of the flow field is also very interesting and can be expressed intuitively. It is shown that, driven by the Poiseuille flow, the larger capsule has a stronger tendency to pass through the pore than the small one, which can be attributed to the greater resistance and the volume advantage of the larger capsule. In addition, we demonstrate that this tendency can be reversed by changing the inlet velocity and setting the initial position of the smaller capsule closer to the axis of the pore. And as long as the large one passes through first, the small one will offset the axis to the same orientation as the initial, while the large one always moves along the axis.

Do hydrodynamically assisted binary collisions lead to orientational ordering of microswimmers?

We have investigated the onset of collective motion in systems of model microswimmers, by performing a comprehensive analysis of the binary collision dynamics using three-dimensional direct numerical simulations (DNS) with hydrodynamic interactions. From this data, we have constructed a simplified binary collision model (BCM) which accurately reproduces the collective behavior obtained from the DNS for most cases. Thus, we show that global alignment can mostly arise solely from binary collisions. Although the agreement between both models (DNS and BCM) is not perfect, the parameter range in which notable differences appear is also that for which strong density fluctuations are present in the system (where pseudo-sound mound can be observed (N. Oyama et al., Phys. Rev. E 93, 043114 (2016))).

Hydrodynamic persistence within very dilute two-dimensional suspensions of squirmers

In this study, we consider suspensions of swimming microorganisms in situations where we might expect the promotion of two-dimensional flow, such as within thin fluid films. Given that two-dimensional, inertialess flows are notoriously long-ranged (although not afflicted by Stokes paradox in the case of self-motile bodies), this raises interesting questions around the care which must be taken with the semi-dilute assumption in such situations. Adopting the prototype squirmer as a model of a swimming microorganisms of the type previously considered, we find that although the flowfield decays algebraically with the characteristic separation distance between microorganisms, there remains a finite interaction between the squirmers even at asymptotically large distances. This finding is further borne out by asymptotic analysis, which confirms that the limiting form of the far-field interaction depends solely upon the relative orientation between the microorganisms. Those which swim in the same general direction are seen to experience very large lateral displacements (many times the size of the displacements experienced owing to interactions between less well-aligned swimmers). This clearly has potential implications for very dilute suspensions in which squirmers become broadly aligned in their swimming direction (e.g. during chemotaxis). We show that hydrodynamically enhanced cell spreading, previously reported for denser suspensions, can persist even at extreme dilutions. Moreover, we demonstrate that this induced spreading can continue in the presence of potentially decohering Brownian effects.