Coherent Structures in Monolayers of Swimming Particles

We report various types of coherent structures in suspensions of spherical particles swimming in a monolayer. We solve the fluid dynamics precisely from far-field hydrodynamic interactions to lubrication between two near-contact surfaces. The simulation results clearly illustrate that coherent structures, such as aggregation, mesoscale spatiotemporal motion, and band formation, can be generated by purely hydrodynamic interactions.

Figure 1

A sketch of the arrangement of a bottom-heavy squirmer. Gravity acts in the $\mathsf{g}$ direction, while the squirmer has orientation vector $\mathbf{e}$. $G$ is the center of gravity. Gray arrows schematically show the squirming velocity on the surface.

Figure 2

Velocity vectors relative to the average squirmer velocity ($\beta =5$). The computational cell is located at the center as indicated by the solid lines.

Figure 3

Correlation of the velocity of squirmers with $\beta =5$ under $c_a=0.1$, 0.3, and 0.5 conditions.

Figure 4

Effect of $\beta$ on the time change of the length of squirmer averaged orientation vectors for $c=0.3$.

Figure 5

Velocity vectors of bottom-heavy squirmers with $\beta =1$, relative to the average squirmer velocity ($c_a=0.3$).