Fig. 1.

Turning behavior of human sperm under flow reversal reveals two kinematically distinct swimming states. (A) Trajectories of individual sperm cells swimming close to the channel boundary in the $\left(x,y\right)$ plane, with initial positions superimposed at time $t=0$ (viewed from inside the channel). Equal-time trajectory averages for left- and right-turning cells are shown as thick white-shaded lines. Flow was reversed at $t=0$, pointing in positive x direction for $t>0$ (white arrow). The shear velocity increases linearly in z direction. Color encodes time. (Scale bar, 200 μm.) (B) Normalized speed distributions before flow reversal at time $t=0$. Faint lines indicate left-turning cells, and the other lines indicate right-turning cells. (C) Distribution of the orientation angles $\phi \left(0\right)$, measured relative to the x axis before flow reversal at time $t=0$, signals two kinematically distinct cell populations. Colors in B and C indicate different viscosities (black, 1 cSt; red, 3 cSt; blue, 12 cSt).