Sommerfeld Theory Colloquium, 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.