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Active matter : model experiments - Olivier Dauchot

Laboratoire Gulliver, ESPCI, Paris

The ubiquity of collective motions observed at all scales in more or less complex situations ranging from the cooperative action of molecular motors to the behavior of large animal or human groups, has recently driven a surge of theoretical and numerical activity.

Within physics, most progress was achieved by studying microscopic point-particles models and their continuous descriptions. Among the landmark results are the possibility of true long-range orientational order in two dimensions, the generic presence of strong, long-range density correlations and the spontaneous formation of
segregated dense and highly ordered nonlinear structures.

In the present talk I will explore several experimental systems including walking grains, rolling colloids, and swimming droplets. In such model systems, one can in principle control the interactions amongst the agents and therefore provide firm experimental grounds in view of theoretical developments. In particular we shall discuss
(i) how and why collective motion emerge in a system of polar disks, for
which no alignment is a priori imposed at the microscopic level ;
(ii) the case of the colloidal rollers where interactions are of
hydrodynamics and electrostatic origins ;
(iii) the intriguing swimming of pure water droplet in oil.


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