Geoinspired bioreactors : from Earth's precession to cell production*

Inspired by the precession of the Earth, a new bladeless mixer has been designed, which consists of a tilted and rotating cylinder. The transition to turbulence in such rotating flows is strongly influenced by the Coriolis force. At specific heights of fluid, the inertial waves forced by the free surface interfere constructively leading to the resonance of a global eigen mode. It creates a strong overturning flow even for small tilt angles. At large enough Reynolds numbers, this base flow exhibits a parametric instability involving a triadic resonance. The viscous threshold of this instability can be predicted using a linear stability analysis, in excellent agreement with the experiments.

This simple set-up has been used to build large-scale mixers. The mixing is as efficient as using a classical Rushton turbine, but with a shear 20 times smaller. This soft mixer is thus particularly interesting for bioreactors which require an efficient mixing of oxygen and carbon dioxide but where a strong shear can damage fragile cells. Applications to micro-algae indicate that the growth rate is increased compared to classical bioreactors. It opens the way to a new generation of bioreactors that may be a technological breakthrough in biotechnologies.