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Colloidal flocks in challenging environments

Abstract : Directed collected motion within herds, swarms and flocks, is a phenomenon that takes place at all scales in living systems. Physicists have rationalized the emergence of such collective behavior. They have described these systems as active materials. These materials are assembled from self-propelled units that spontaneously move in the same direction. By experimentally studying synthetic flocks, this work uncovers some properties of polar active materials in situations that disfavor their self-organization: their dynamics in disordered environments and their response to external perturbations. Colloidal rollers with alignment interactions are confined within microfluidic devices. At high density, they spontaneously form a flock which is characterized by the emergence of orientational long-ranged order. These colloidal flocks are prototypical realizations of polar active matter. We have studied the response of a polar active liquid assembled from colloidal rollers. We have shown that they display a hysteretic response to longitudinal perturbations. We have theoretically accounted for this non-linear behavior. We have used this behavior to realize autonomous microfluidic oscillators. We have also studied the dynamics of colloidal flocks that propagate through heterogeneous environments. Randomly positioned obstacles focalize flocks along favored channels that form a sparse and tortuous network. Increasing disorder leads to the destruction of flocks. We have demonstrated that the suppression of collective motion is a discontinuous transition generic to all polar active materials.
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Submitted on : Tuesday, November 27, 2018 - 12:32:10 PM
Last modification on : Wednesday, November 20, 2019 - 3:09:28 AM
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  • HAL Id : tel-01936270, version 1

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Alexandre Morin. Colloidal flocks in challenging environments. Soft Condensed Matter [cond-mat.soft]. Université de Lyon, 2018. English. ⟨NNT : 2018LYSEN047⟩. ⟨tel-01936270⟩

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