Yaouen Fily

Athermal phase separation of self-propelled particles with no alignment

We study numerically and analytically a model of self-propelled polar disks on a substrate in two dimensions. The particles interact via isotropic repulsive forces and are subject to rotational noise, but there is no aligning interaction. As a result, the system does not exhibit an ordered state. The isotropic fluid phase separates well below close packing and exhibits the large number fluctuations and clustering found ubiquitously in active systems. Our work shows that this behavior is a generic property of systems that are driven out of equilibrium locally, as for instance by self-propulsion.

Active jamming: Self-propelled soft particles at high density

We study numerically the phases and dynamics of a dense collection of self-propelled particles with soft repulsive interactions in two dimensions. The model is motivated by recent in vitro experiments on confluent monolayers of migratory epithelial and endothelial cells. The phase diagram exhibits a liquid phase with giant number fluctuations at low packing fraction φ and high self-propulsion speed v(0) and a jammed phase at high φ and low v(0). The dynamics of the jammed phase is controlled by the low-frequency modes of the jammed packing.

Critical behavior of plastic depinning of vortex lattices in two dimensions: Molecular dynamics simulations

Using molecular dynamics simulations, we report a study of the dynamics of two-dimensional vortex lattices driven over a disordered medium. In strong disorder, when topological order is lost, we show that the depinning transition is analogous to a second-order critical transition: the velocity-force response at the onset of motion is continuous and characterized by critical exponents. Combining studies at zero and nonzero tempera

Cooperative self-propulsion of active and passive rotors

Using minimal models for low Reynolds number passive and active rotors in a fluid, we characterize the hydrodynamic interactions among rotors and the resulting dynamics of a pair of interacting rotors. This allows us to treat in a common framework passive or externally driven rotors, such as magnetic colloids driven by a rotating magnetic field, and active or internally driven rotors, such as sperm cells confined at boundaries. The hydrodynamic interaction of passive rotors is known to contain an azimuthal component ∼1/r2 to dipolar order that can yield the recently studied “cooperative self-propulsion” of a pair of rotors of opposite vorticity. It is also known, although not widely appreciated, that this interaction is identically zero for active rotors as a consequence of torque balance. In this paper we show that a ∼1/r4 azimuthal component of the interaction arises in active systems to octupolar order. This is a new result that allows us to discuss the dynamic behavior of pairs of passive and active interacting rotors in a unified manner and to show that cooperative self-propulsion, although weaker, can also occur for pairs of active rotors.

The elastic depinning transition of vortex lattices in two dimensions

Large-scale numerical simulations are used to study the elastic dynamics of two-dimensional vortex lattices driven on a disordered medium in the case of weak disorder. We investigate the so-called elastic depinning transition by decreasing the driving force from the elastic dynamical regime to the state pinned by the quenched disorder. Similarly to the plastic depinning transition, we find results compatible with a second-order phase transition, although both depinning transitions are very different from many viewpoints. We evaluate three critical exponents of the elastic depinning transition. β = 0.29 ± 0.03 is found for the velocity exponent at zero temperature, and from the velocity–temperature curves we extract the critical exponent δ−1 = 0.28 ± 0.05. Furthermore, in contrast with charge-density waves, a finite-size scaling analysis suggests the existence of a unique diverging length at the depinning threshold with an exponent ν = 1.04 ± 0.04, which controls the critical force distribution, the finite-size crossover force distribution and the intrinsic correlation length. Finally, a scaling relation is found between velocity and temperature with the β and δ critical exponents both independent with regard to pinning strength and disorder realizations.

Plastic depinning of superconductor vortices

We present numerical simulation results for driven vortex lattices in presence of random disorder at zero temperature. The dynamics of vortices is studied for strong disorder. Plastic regimes at the depinning transition are analysed. The scenario of a second order phase transition ν ~ (F — Fc)β for the velocity v emerges. The exponent β ~ 1.2 is extracted from the critical region of several lattice sizes. Furthermore, the dynamical regimes at the depinning threshold are shown to be chaotic.

Elastic depinning transition for superconductor vortices

We present 2D numerical simulation results of vortex lattices driven over a random disorder. We study the vortex dynamics at the depinning threshold Fc for weak disorder at zero temperature. Elastic regimes are analysed in the framework of a second order phase transition where the velocity response v to the driving force F behaves like v ~ (F − Fc)β. The exponent β = 0.27 ± 0.04 is extracted from the critical region of several large lattice sizes.

Stability of the moving Bose glass phase at finite temperature

We present 3D numerical simulation results for driven vortex lattices in presence of weak random columnar disorder at finite temperature and high driving force. A moving Bose glass phase is expected theoretically, characterized by a flow along rough static channels and dynamical transverse Meissner effect, the moving flux lines being localized along the correlated disorder direction. We find the moving Bose glass phase to be stable in a large range of temperature and velocity. Moreover, we confirm the existence of an effective static pinning potential in the tranverse direction, previously observed in zero temperature simulations, and leading to a kink structure of the vortex lines beyond a critical transverse field.