laboratoire de physique statistique
laboratoire de physique statistique


Nature Physics 


P A R M I :

Local equilibrium in bird flocks - Mora, Thierry and Walczak, Aleksandra M. and Del Castello, Lorenzo and Ginelli, Francesco and Melillo, Stefania and Parisi, Leonardo and Viale, Massimiliano and Cavagna, Andrea and Giardina, Irene
Nature Physics 121153+ (2016)

Abstract : The correlated motion of flocks is an example of global order emerging from local interactions. An essential difference with respect to analogous ferromagnetic systems is that flocks are active: animals move relative to each other, dynamically rearranging their interaction network. This non-equilibrium characteristic has been studied theoretically, but its impact on actual animal groups remains to be fully explored experimentally. Here, we introduce a novel dynamical inference technique, based on the principle of maximum entropy, which accommodates network rearrangements and overcomes the problem of slow experimental sampling rates. We use this method to infer the strength and range of alignment forces from data of starling flocks. We find that local bird alignment occurs on a much faster timescale than neighbour rearrangement. Accordingly, equilibrium inference, which assumes a fixed interaction network, gives results consistent with dynamical inference. We conclude that bird orientations are in a state of local quasi-equilibrium over the interaction length scale, providing firm ground for the applicability of statistical physics in certain active systems.
Breakdown of elasticity in amorphous solids - Biroli, Giulio and Urbani, Pierfrancesco
Nature Physics 121130-1133 (2016)

Abstract : What characterizes a solid is the way that it responds to external stresses. Ordered solids, such as crystals, exhibit an elastic regime followed by a plastic regime, both understood microscopically in terms of lattice distortion and dislocations. For amorphous solids the situation is instead less clear, and the microscopic understanding of the response to deformation and stress is a very active research topic. Several studies have revealed that even in the elastic regime the response is very jerky at low temperature, resembling very much the response of disordered magnetic materials(1-6). Here we show that in a very large class of amorphous solids this behaviour emerges upon decreasing temperature, as a phase transition, where standard elastic behaviour breaks down. At the transition all nonlinear elastic moduli diverge and standard elasticity theory no longer holds. Below the transition, the response to deformation becomes history- and time-dependent.
Multiple-length-scale elastic instability mimics parametric resonance of nonlinear oscillators - Brau, Fabian and Vandeparre, Hugues and Sabbah, Abbas and Poulard, Christophe and Boudaoud, Arezki and Damman, Pascal
NATURE PHYSICS 756-60 (2011)

Abstract : Spatially confined rigid membranes reorganize their morphology in response to the imposed constraints. A crumpled elastic sheet presents a complex pattern of random folds focusing the deformation energy(1), whereas compressing a membrane resting on a soft foundation creates a regular pattern of sinusoidal wrinkles with a broad distribution of energy(2-8). Here, we study the energy distribution for highly confined membranes and show the emergence of a new morphological instability triggered by a period-doubling bifurcation. A periodic self-organized focalization of the deformation energy is observed provided that an up-down symmetry breaking, induced by the intrinsic nonlinearity of the elasticity equations, occurs. The physical model, exhibiting an analogy with parametric resonance in a nonlinear oscillator, is a new theoretical toolkit to understand the morphology of various confined systems, such as coated materials or living tissues, for example wrinkled skin(3), internal structure of lungs(9), internal elastica of an artery(10), brain convolutions(11,12) or formation of fingerprints(13). Moreover, it opens the way to a new kind of microfabrication design of multiperiodic or chaotic (aperiodic) surface topography through self-organization.
Stiffer but flowing - Balibar, Sebastien
NATURE PHYSICS 5534-535 (2009)