Generating Large Scale Flocks of Sperm in Viscoelastic Fluid*

Sperm collective swimming in viscoelastic fluid provides a biologically relevant model system to study the behavior of active matter. To study the statistical mechanics of the flocking of sperm, it will be important to study the variation of sperm orientation within a large flock. While early experiments show that increasing cell density increases the average flock size, packing the field of view (570×426 µm) with sperm was not sufficient to generate large flocks. On the other hand, by transiently aligning sperm orientation with a flow, we were able to observe flock sizes close to the height of the field of view (across 435 µm or 240 cells) forming after the flow was turned off. This suggests that the sperm flock sizes depend on the history of the flock orientation. Furthermore, alignment due to cell-cell interactions through viscoelastic medium is not enough to overcome the vigorous swimming of sperm and align two flocks with different orientations, yet enough to prevent rotational diffusivity from efficiently breaking down the large flocks. We will also discuss the orientation variation within a large flock.