Emergence of collective motion in suspensions of swimming cells

We report a study on transition to collective dynamics in suspensions of mammalian sperm cells in microfluidic confinement. The concentration of motile spermatozoa in the region of interest is altered either in steps through manual dilution of the original sample or gradually by a microfluidic ratcheted device, designed to direct and concentrate motile cells. Velocity fields have been measured using PIV (Particle Image Velocimetry) from videos of both passive fluorescent beads and of the motile cells. The establishment of the collective motion is identified through calculation of a number of fluid parameters such as vorticity, enstrophy, velocity spatial and temporal correlations, and average velocities. The measured parameters exhibit a step change indicating transition from random and unorganised motion of individual cells to locally organised motion upon reaching a critical concentration of motile spermatozoa. The shortest length scale in the collective dynamics remains nearly constant throughout the range of concentrations, while the largest scale depends on the spermatozoa concentration and the geometrical confinement.