Hydrodynamic behaviors of self-propelled sperms in confined spaces

The hydrodynamic behaviors of sperms in confined spaces (e.g. Poiseuille flow and quiescent flow between two parallel walls) are studied with a two-dimensional self-propelled sperm model based on the immersed boundary-lattice Boltzmann method (IB-LBM). For single-sperm swimming in Poiseuille flow, four typical swimming patterns, including wall-adhesion, focusing, tumbling and oscillating, are observed. The occurrence conditions including flow strength, inter-wall distance, initialization of sperm are studied in detail for the four patterns. For multiple-sperm swimming in quiescent flow between two parallel walls, it is observed that the sperm population tends to swim nearby the walls, leading to wall accumulation. To explore the hydrodynamic behaviors of the sperm population in Poiseuille flows, the effects of flow velocities on the sperm rheotaxis and the wall accumulation are investigated in detail. It is found that an appropriate flow velocity is essential to guide the swimming direction, as well as the wall accumulation. In addition, collision and adsorption between adjacent sperms may facilitate wall accumulation. Finally, we study the propulsion rate of different combination modes of sperms. We found that the wedge-shaped mode leads to the optimal propulsive speed, and reducing the combining distance can increase the propulsive speed.