Research

Active matter refers to a distinct class of nonequilibrium materials composed of units that are individually powered or self-driven. A large collection of such interacting active units can display striking collective behaviour emergent on large-scales.

My work is broadly focused on understanding the marvelous phases of active matter, revealing and emphasizing any geometric or topological aspects that may be at stake. My current research interests include emergent collective motion, behavior of colloidal flocks in disordered media, coarsening dynamics of active fluids or frustration of topological active materials. 

I address fundamental questions by performing and analyzing highly quantitative experiments, based on ensemble of millions of self-propelled colloids (Quincke rollers) that spontaneously organize into a polar active fluid at high density. Tools most commonly used involve soft lithography, microfluidics and microscopy, along with extensive data analysis tools (image analysis, PTV, PIV), in order to reveal and explain diverse properties of active materials, beyond the specificities of a model experiment.

See below short abstracts for different projects I have worked on so far. 


Coarsening dynamics of active fluids

Fig. 1 Snapshot of an active fluid during its coarsening process.
The emergence of collective motion in a population of millions of active colloids
results in the formation of an active fluid.

Objective: understand the ordering dynamics of flocking matter, and the particular role of topological excitations.

When motile units self-assemble into flocks where all particles propel along the same direction, they realize one of the most stable phase observed in Nature. Unlike in active nematics or passive systems such as ferromagnets or liquid crystals, the long range orientational ordered active fluids formed by flocking units are robust to defect proliferation even in two dimensions.
The velocity field of a colloidal flock initially marred by a number of topological defects (see Fig.2), heals and reaches pristine orientational order over large scales. Combining experiments, simulations and theory we elucidate the elementary excitations of 2D polar active matter and explain their phase ordering dynamics. 
Self-similar dynamics emerges from the annihilation of +/- 1 vortices along a filamentous network of domain walls with no counterparts in passive systems. Remarkably, the structure of this fully connected network is mainly determined by extended singularity lines growing from −1 vortices. The two body interactions between the defects correctly account for the self-similar coarsening of the density and flow excitations of flocking liquids.

See our article for more info “Topology-driven ordering of flocking matter”, Amélie Chardac, Ludwig Hoffmann, Yoann Poupart, Luca Giomi, Denis Bartolo. Phys. Rev. X 11, 031069 (2021)


Colloidal flocks in disordered media

Fig.2 Colloidal flock cruising a forest of physical obstacles in a circular microfluidic chamber of diameter 3mm.

Objective: explain the random geometry of the flowing river networks emerging when an active fluid is confined in a disordered medium.

Abstract: In equilibrium, disorder conspires with topological defects to redefine the ordered states of matter in systems as diverse as crystals, superconductors and liquid crystals. Far from equilibrium, however, the consequences of quenched disorder on active condensed matter remain virtually uncharted. Here, we reveal a state of strongly disordered active matter with no counterparts in equilibrium: a dynamical vortex glass. Combining high-content microfluidic experiments and theory, we show how colloidal flocks collectively cruise through disorder without relaxing the topological singularities of their flows. The resulting state is highly dynamical but the flow patterns, shaped by a finite density of frozen vortices, are stationary and exponentially degenerated. Quenched isotropic disorder acts as a random gauge field turning active liquids into dynamical vortex glasses. We argue that this robust mechanism should shape the collective dynamics of a broad class of disordered active matter, from synthetic active nematics to collections of living cells exploring heterogeneous media.

See our paper for more info: « Emergence of dynamic vortex glasses in disordered polar active fluids », Amélie Chardac, Suraj Shankar, M. Cristina Marchetti, Denis Bartolo. PNAS, 118-10 (2021).
Our experiments on the cover of PNAS.


Frustration of active materials

Fig. 3 Geometrical frustration of an active fluid

Objective: characterize the frustration of active fluids in hydrodynamics networks.

Now that the mesmerizing flows of polar active liquids are understood in simple geometries and that we have improved our knowledge about the influence of topological defects in their formation, we would like to address the question of their behavior in fluidic networks.
     Unlike Newtonian viscous fluids, active flows are not uniquely defined for a given geometry: they are intrinsically multistable. Even in simple geometries, such as periodic lattices, this highly nonlinear feature results in frustrated and highly degenerated patterns (see Fig.3). The aim of our research is to elucidate the degeneracy of the spontaneous flows of active fluids exploring hydrodynamic networks, and to lay out the foundation of active microfluidics.

More here soon!