Walking grains, Rolling colloids, Swimming droplets: How universal are transition to collective motion in active matter?
Mon, Nov. 03rd 2014, 14:00-15:00
Salle Claude Itzykson, Bât. 774, Orme des Merisiers
Polar active liquids, composed of aligning self-propelled particle
generically exhibit large scale collective motion.
Historically, simulations of Vicsek-like models of constant-speed point
particles, aligning with their neighbors in the presence of noise, have
revealed the existence of a transition towards a true long range order
polar phase with giant density fluctuations. Quite generically, the
homogenous polar state is unstable; segregated dense and highly ordered
non-linear propagative structures develop in the transitional regime; and
the transition is discontinuous.
An intense theoretical effort towards the understanding of the long range
behavior of these systems has lead to the picture of a basic universality
class, at least for the simplest situation in which the surrounding fluid
can be neglected (dry flocking) and the sole interaction is some local
However, Vicsek-like models already contain some level of coarse graining
of the dynamics and as such are not just ``simple liquids''. For any given
system of particles, it is thus crucial to identify if it truly belongs to
the above universality class.
I will first discuss this matter in the context of two experimental
systems, namely walking grains and rolling colloids.
In all case we shall observe strong similarities with the above scenario,
but also qualitative differences.
I will then move to more basic questions, which were hindered by the
complexity of the dynamics close to the transition. Is there a simple way
to predict the existence and the order of a transition to collective
motion for a given microscopic dynamics? What would be the physically
meaningful and relevant quantity to answer this question? How universal
would such a quantity be? Answering such questions would open the way
towards a completely new paradigm in the field of active matter: the
design of microscopic particles with a desired macroscopic behavior in