Abstract

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 effective alignment.

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 is indeed in the above universality class.

I will first discuss this matter in the context of three experimental systems, namely walking grains, rolling colloids and swimming droplets. 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 mind.