Abstract
New experimental evidence of self-motion of a confined active suspension is presented. Depositing fresh semen sample in an annular shaped microfluidic 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 towards collective motion associated with local alignment of spermatozoa akin to the Vicsek model. A macroscopic theory based on previously derived self-organized hydrodynamics models is adapted to this context and provides predictions consistent with the observed stationary motion.
Highlights
Biological fluids in physiological contexts are usually confined
We show that a fresh semen sample confined inside a ring displays a very robust and stable rotational motion which is uniform along the azimuthal direction
We show that concentrated active suspension of pushers, when confined into an annular microfluidic device, display non-trivial spontaneous motion
Summary
Biological fluids in physiological contexts are usually confined. For most of them, confinement is a hindrance to flow owing to viscous dissipation. I.e. fluids composed of a suspension of swimming microorganisms, is a new area of research where unexpected behaviours have been discovered (see reviews [1,2] and [3]). In the case of dilute suspensions of pushers Pull) the fluid ahead, such as spermatozoa In the case of pushers [6] and spermatozoa [7,8,9,10], recent observations have suggested that interactions with the boundaries might be key to guide spermatozoa towards the oviduct tract, or generate vortices in confined bacterial suspensions [11]. Similar unexpected effects have recently been observed in systems of colloidal rollers [12,13] confined in microfluidic arenas
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