Sommaire:

Active matter constitutes a particularly interesting class of systems
in non-equilibrium statistical mechanics and soft-condensed matter physics.

In the first part of this talk talk I will focus on the dynamics of passive colloidal tracers in a bath of self-propelled particles. We demonstrate that a bath of swimming E. coli is capable of mediating effective interactions between suspended (passive) colloids. In particular we observe that a bath of motile bacteria gives rise to a short range attraction similar to depletion forces in equilibrium colloidal suspensions[1]. However, using numerical simulations, we show how this effective interactions uniquely arise from the non-equilibrium dynamics, induced by the bacterial bath. Surprisingly excluded volume effects reduce the observed "attraction", in striking contrast with an equilibrium-like depletion scenario.

In the second part of talk I will describe how the intrinsic non-equilibrium nature of swimming bacteria appears evidently even when a simple constant external field is applied to a sample of motile E. coli bacteria[2]. To this aim we use centrifugation that is a widespread laboratory technique. We demonstrate that simple centrifugation can also be used to separate swimming cells having different motilities and we discuss the potential applications of our technique. In this experiment we apply an external centrifugal field to self-propelled bacteria and we use dynamic image correlation spectroscopy to measure the motility of bacteria after centrifugation. A significant spatial gradient in swimming-speed is observed for increasing centrifugal speeds. We show how these results can be reproduced by a diffusion equation that treats bacteria as "hot" colloidal particles having a diffusion coefficient that depends on their heterogeneous swimming speed.

In the final part of the talk I will discuss a recent experimental/numerical work studying the dynamics of a colloidal bead confined by a harmonic potential in a bath of swimming E. coli bacteria [3]. The resulting dynamics is well approximated by a Langevin equation for an overdamped oscillator driven by the combination of a white thermal noise and an exponentially correlated active noise [4]. We discuss how this scenario leads to a breakdown of the equipartition theorem and to the coexistence of two different effective temperatures that govern dynamics along the flat and the curved directions in the potential landscape.


[1] L. Angelani, C. Maggi, M. L. Bernardini, et al.
Phys. Rev. Lett. 107, 138302 (2011)

[2] C. Maggi, A. Lepore, J. Solari et al.
Soft Matter 9, 10885-10890 (2013)

[3] C. Maggi, M. Paoluzzi, N. Pellicciotta, et al.
arXiv:1409.3929

[4] N. Koumakis, C. Maggi, R. Di Leonardo,
Soft Matter, 10, 5695-5701, (2014)

Pour plus d'informations, merci de contacter Gross M.