New experimental evidence of self-motion of a
confined active suspension is presented. Depositing
fresh semen sample in an annular shaped micro-
fluidic chip leads to a spontaneous vortex state of
the fluid at sufficiently large sperm concentration.
The rotation occurs unpredictably clockwise or
counterclockwise and is robust and stable. Furthermore,
for highly active and concentrated semen, richer
dynamics can occur such as self-sustained or damped
rotation oscillations. Experimental results obtained
with systematic dilution provide a clear evidence of a
phase transition toward collective motion associated
with local alignment of spermatozoa akin to the
Vicsek model. A macroscopic theory based on
previously derived Self-Organized Hydrodynamics
(SOH) models is adapted to this context and provides
predictions consistent with the observed stationary
motion.