  Self-organized collective behaviour of active units is inspiring new designs
of artificial swarms of micron-sized objects. However, active control at the
nanoscale remains elusive. We have accurately solved the collective optofluidic
dynamics of gold plasmonic 50 nm-radius nanoparticles moving in aqueous
solution under a nonconservative optical vortex lattice. Above a critical field
intensity and concentration, the interplay between optical forces, thermal
fluctuations and hydrodynamic pairing leads to a spontaneous transition towards
synchronised motion exhibiting a rich assortment of collective dynamics. The
critical phenomenon creates strong unidirectional currents of nanoparticles,
reaching speeds of centimeters per second along the diagonals of the lattice.
