Active matter

When many individuals are put together some spectacular collective effects can emerge such as flock of birds, swarms of bacteria, human crowds. Many efforts have recently attempted to create artificial systems mimicking their living counterpart. Active matter is a new class of complex systems, composed of particles that are made « active » through a local conversion of energy to create self-propulsion, which drives these systems very far from thermal equilibrium. This generates complex behaviors far richer than in equilibrium materials.

We experimentally study active soft matter and more particularly the spontaneous structure and dynamics of semi-dilute and dense active colloidal structures.

We are also interested in driven active matter: particles that have the ability to bias their motion in response to an external stimuli, indeed the ubiquitous ability of living active matter to bias its motion is of key importance in observed collective motions.

  

 

Members:  


Cottin-Bizonne, Cécile
Detcheverry, François
Ginot, Félix
Kervil, Ronan
Klongvessa, Natsuda
Leocmach, Mathieu
Ybert, Christophe

Highlights

 

Driven active matter: magnetotactic bacteria

We explore the behavior of magnetotactic bacteria as a benchmark system of driven active matter that offers great capabilities for physical and quantitative investigations. Such bacteria synthetize a permanent magnet, that can be easily remotely controlled by a magnetic field that orients the motion of the bacteria. We characterize the bacteria motion when facing a Poiseuille flow and evidence some structuring capabilities of active matter.

Destabilization of a flow focused suspension of magnetotactic bacteria. Waisbord et al., Phys. Rev. Fluids (2016).

 

    

Scketch and  visualization of a magnetotactic bacteria (left and right).

 

Effective adhesion between active particles

We show that artificial self-propelled colloids can self-assemble and form clusters. This behavior can be quantified in thermodynamic terms: we have measured the equation of state of an assembly of colloids and found  an effective adhesion, whose strength, surprisingly,increases with the activity. This work paves the way for quantifying non-equilibrium interactions between active particles.

Nonequilibrium equation of state in suspensions of active colloids, Ginot et al., Phys. Rev. X (2015).

 

      

Self-propelled colloids (sketched on the left) at increasing activity (right)

 

Random walks of swimming bacteria

Swimming bacteria exhibit a variety of motion patterns, in which persistent runs are punctuated by sudden turning events. The statistical properties of these random walks have been established for the particular case where the turning events follow a Poisson process. Extending the framework of continuous-time random walks, we show how to treat the general, non-Poissonian case.

Non-Poissonian run-and-turn motions, Detcheverry F., Europhys. Lett. (2015).

 

Left : example of bacteria trajectories, showing the run-reverse-flick swimming pattern. Right : basic model of motion, including rotational diffusion and sudden turns. 

 

Active colloids

We study experimentally dense suspensions of active colloids. Such a system of self-propelled particles is intrinsically out of equilibrium. We show that for intermediate volume fraction of colloides, there is a new phase of dynamical clusters, that continously break and recombine. The average cluster size increases linearly with activity. A chimiotactic agregation mechanism that was introduced for bacteries allow to rationalize those observations.

Dynamic  clustering  in active colloidal suspensions with chemical signaling, Theurkauff et al.,  Phys. Rev. Lett. (2012)

 

Left) microscopy observation of colloid sedimentation under effective gravity. Right) 2D images of colloids after sedimentation, for passive and active colloids (left and right respectively). In the latter case, one notes the appearance of clusters as well as an extended gaz phase.

 

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