Understanding the determination factors of the formation of large flocks of swimming sperm in viscoelastic fluid*

When bovine sperm swim in viscoelastic fluid, they form dynamic clusters that coexist with individually swimming sperm in a steady state. By tuning the viscoelasticity of the fluid, we are able to generate clusters in the size of ~20 cells. By using a pulse of flow to align them upstream momentarily, we were able to generate large flocks, from a range of 60-200 cells. This large flock formation is also known as mass motility in a clinical setting, which is used for semen motility evaluation from bulls and other ruminants in the field for veterinarians, where environmental control is not available, and sperm die quickly on an optical system. We employ a numerical self-propelled particle (SPP) model to identify the factors that lead to bigger flock formation, or better mass motility. Our model includes nearby sperm orientation alignment and attraction, volume exclusion, and heterogeneity among cells. We will compare the data from the numerical model with the experiments.