Over the past ten years, there has been a growing interest among physicists
for ‘active matter’, a codename that encompasses systems in which energy is
taken from the environment to generate self-propulsion at the single particle
level. Active particles, such as run-and-tumble bacteria, self-diffusiophoretic
colloids or actin filaments in motility assays, are strongly out-of-equilibrium
and exhibit much richer behaviours than their passive counterpart.
In this talk I will review recent progresses regarding the physics of active
particles. I will show how simple concepts like pressure, the force density
exerted by assemblies of particles on their container, play a new role for
active systems because of the lack of equation of state. I will also show how
new collective phenomena emerge, from the transition to collective motion
to the existence of cohesive matter without cohesive forces, that have no
counterpart in thermal equilibrium.