Geoinspired soft mixers

The flow with low shear inside a bladeless mixer is characterized experimentally. This soft mixer, inspired by the precession of the Earth, consists of a cylindrical container rotating around its axis and tilted from the vertical. For low Froude numbers, the free surface remains horizontal, thus generating a forcing on the tilted rotating fluid. As in the case of a precessing cylinder, this forcing excites global inertial modes (Kelvin modes) which become resonant when the height of fluid is equal to a multiple of a half-wavelength. Ekman pumping saturates the amplitude of the mode at a value proportional to the square root of the Reynolds number. For sufficiently large tilt angles and Reynolds numbers, the global mode destabilizes via a parametric triadic instability involving two additional Kelvin modes. The viscous threshold of the instability can be predicted analytically with no fitting parameter and is in excellent agreement with the experimental results. This instability generates a strong mixing which is as efficient as the one achieved using a classical Rushton turbine, but with a shear 20 times smaller. This simple bladeless mixer is thus an excellent candidate for large scale bioreactors where mixing is needed to enhance gas exchanges but where shear is harmful for fragile cells. Preliminary results obtained for the growth of microalgae (dinoflagellates) in such photobioreactors suggest that it could be a technological breakthrough in biotechnologies.