Résumé |
Active fluids display striking collective phenomena not possible in passive
fluids. Examples include spontaneous flows powered by microscopic constituents,
and phase separation in the absence of attractive interactions. In this talk, I
will discuss three aspects of the physics of active fluids, connected with
experiments in both biological and artificial soft matter systems. First, I
will show that dense colonies of the rod-shaped motile bacterium Myxococcus
xanthus form active liquid crystals. I will show that topological defects of
the cell alignment field induce flows that lead to the formation of new cell
layers, which triggers the development of multicellular structures called
fruiting bodies. Second, I will discuss chaotic flows in active liquid
crystals. I will show that, as in classic turbulence, the statistical
properties of these active flows at low Reynolds number are described by
universal scaling laws. Unlike classic turbulence, however, this type of active
turbulence requires no energy transfer across scales. Finally, I will present a
new mechanism of liquid-gas phase separation in active fluids. I will show
that, in addition to repulsive forces that oppose phase separation, artificial
self-propelled Janus colloids experience torques that reorient particle motion
toward high-density regions, providing a novel route to phase separation. |