Sébastien Cazin

Symmetry-breaking phase-transitions in highly concentrated semen

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.

Simultaneous PIV/PTV velocimetry technique in a turbulent particle-laden flow

In a three-dimensional two-phase flow, accessing the velocity fields of the two phases simultaneously is challenging. Nevertheless, information about the local relative motion between the two phases is particularly valuable to quantify the inter-phase interactions. In this article, the dynamics of neutrally buoyant finite-sized particles embedded in a three-dimensional turbulence is investigated using a simultaneous particle image velocimetry (PIV) and particle tracking velocimetry (PTV) measurement technique based on optical discrimination of the two phases prior to image acquisition. The implementation of this dual whole-field velocimetry technique is presented and detailed. With this technique, we were able to measure the instantaneous and local velocity differences between the particles and the underlying fluid. Our results show that, whereas the single-point velocity statistics of the two phases are very similar, the particles often have different local velocity than the velocity of the neighboring fluid. The relative velocity increases with the relative size of the particle to the Kolmogorov scale. In addition, the relative velocity exhibits an intermittent distribution.Graphical abstract