FIG. 1.

Sperm structure, trajectory chirality, and near-surface swimming. (a) Schematic of experimental setup. Each bull sperm has a paddle-shape head, with approximate dimensions of 10 μm long, 5 μm wide, and roughly 1 μm thick. The head is directly connected to a single flagellum that is about 50 – 60 μm long and tapers from 1 μm in diameter at the connection to 200 nm at the tail end. Mean swimming speed is close to 120 μm/s. Sperm travelling along the surfaces of a microfluidic channel with 2.47 mm width and 120 μm height. Fluid flow is applied in −x direction. (b) A micrograph (570 × 426 μm) of sperm swimming along the lower surface of the central portion of the microfluidic channel. Scale bar: 100 μm. (c) Trajectories (1–6.30s long) of sperm in the absence of the flow. Each colored line is a sperm track that ends at the black dot. CW chirality is seen in the trajectories. Scale bar: 100 μm. (d) Trajectories (1.78 s long) of 30 sperm swimming toward the side wall of the microfluidic channel, and the coordinate (0,0) marks the point where the sperm hits the side wall. The sperm continue to move along the sidewall (or x-axis). Red/black dots are starting/ending positions of the tracks. Scale bar: 25 μm.