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SUMMARY:Walking grains\, rolling colloids and swimming droplets: How unive
 rsal active matter really is? - Olivier Dauchot: ESPCI
DTSTART:20141017T150000Z
DTEND:20141017T160000Z
UID:TALK54716@talks.cam.ac.uk
CONTACT:Nathalie Vriend
DESCRIPTION:Polar active liquids\, composed of aligning self-propelled par
 ticle generically exhibit large scale collective motion.\n\nHistorically\,
  simulations of Vicsek-like models of constant-speed point particles\, ali
 gning with their neighbors in the presence of noise\, have revealed the ex
 istence of a transition towards a true long range order polar phase with g
 iant density fluctuations. Quite generically\, the homogenous polar state 
 is unstable\; segregated dense and highly ordered non-linear propagative s
 tructures develop in the transitional regime\; and the transition is disco
 ntinuous.\n\nAn intense theoretical effort towards the understanding of th
 e 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 surr
 ounding fluid can be neglected (dry flocking) and the sole interaction is 
 some local effective alignment.\n\nHowever\, Vicsek-like models already co
 ntain some level of coarse graining of the dynamics and as such are not ju
 st ``simple liquids''. For any given system of particles\, it is thus cruc
 ial to identify if it is indeed in the above universality class.\n\nI will
  first discuss this matter in the context of three experimental systems\, 
 namely walking grains\, rolling colloids and swimming droplets. In all cas
 e we shall observe strong similarities with the above scenario\, but also 
 qualitative differences.\n\nI will then move to more basic questions\, whi
 ch 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 trans
 ition 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 ope
 n the way towards a completely new paradigm in the field of active matter:
  the design of microscopic particles with a desired macroscopic behavior i
 n mind.
LOCATION:MR2\, Centre for Mathematical Sciences\, Wilberforce Road\, Cambr
 idge
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