Swimming against the flow - An orientational disorder to order transition

Micro-organisms often need to swim against fluid flows for their survival. In native state, mammalian sperm swim against a flow to reach the egg. Using bull sperm as a model system, we studied the impact of fluid flows on sperm swimming behavior. Interestingly, we find that a directional swimming pattern emerges as the fluid flow rate exceeds a critical value. Using the average directional vector, $<$$S_{x}$$>$ or $<$$S_{y}$$>$ of the sperm head, as an order parameter, and fluid flow rate (along $x$-axis) as a control parameter, we find that $<$$S_{x}$$>$ or $<$$S_{y}$$>$ increases continuously with the increase of flow rate above the onset point, following a power law with an exponent close to 0.5. We will discuss the sources of this transition, and implications in both physics and biology.