  We investigate the collective dynamics of self-propelled droplets, confined
in a one dimensional micro-fluidic channel. On one hand, neighboring droplets
align and form large trains of droplets moving in the same direction. On the
other hand, the droplets condensates, leaving large regions with very low
density. A careful examination of the interactions between two "colliding"
droplets demonstrates that local alignment takes place as a result of the
interplay between the dispersion of their speeds and the absence of Galilean
invariance. Inspired by these observations, we propose a minimalistic 1D model
of active particles reproducing such dynamical rules and, combining analytical
arguments and numerical evidences, we show that the model exhibits a transition
to collective motion in 1D for a large range of values of the control
parameters. Condensation takes place as a transient phenomena which
tremendously slows down the dynamics, before the system eventually settles into
a homogeneous aligned phase.
