Abstract:
Gels of semi-flexible polymers, network glasses made of low valence elements, softly compressed ellipses and dense suspensions under flow are examples of floppy materials. These systems present collective motions with almost no restoring force. We study theoretically and numerically the frequency-dependence of the response of these materials, and the length scales that characterize their elasticity. We show that isotropic floppy elastic networks present a phonon gap for frequencies smaller than a frequency $\omega^*$ governed by coordination, and that the elastic response is localized on a length scale $l_c\sim 1/\sqrt{\omega^*}$ that diverges as the phonon gap vanishes (with a logarithmic correction in the two dimensional case). $l_c$ also characterizes velocity correlations under shear, whereas another length scale $l^*\sim 1/\omega^*$ characterizes the effect of pinning boundaries on elasticity. We discuss the implications of our findings for suspensions flows, and the correspondence between floppy materials and amorphous solids near unjamming, where $l_c$ and $l^*$ have also been identified but where their roles are not fully understood.

Abstract:
We report theoretical and experimental results on 4-wave capillary wave turbulence. A system consisting of two inmiscible and incompressible fluids of the same density can be written in a Hamiltonian way for the conjugated pair $(\eta,\Psi)$. When given the symmetry $z\to-z$, the set of weakly non-linear interacting waves display a Kolmogorov-Zakharov (KZ) spectrum $n_k\sim k^{-4}$ in wave vector space. The wave system was studied experimentally with two inmiscible fluids of almost equal densities (water and silicon oil) where the capillary surface waves are excited by a low frequency random forcing. The power spectral density (PSD) and probability density function (PDF) of the local wave amplitude are studied. Both theoretical and experimental results are in fairly good agreement with each other.

Abstract:
We introduce an event-driven simulation scheme for overdamped dynamics of frictionless hard spheres subjected to external forces, neglecting hydrodynamic interactions. Our event-driven approach is based on an exact equation of motion which relates the driving force to the resulting velocities through the geometric information characterizing the underlying network of contacts between the hard spheres. Our method allows for a robust extraction of the instantaneous coordination of the particles as well as contact force statistics and dynamics, under any chosen driving force, in addition to shear flow and compression. It can also be used for generating high-precision jammed packings under shear, compression, or both. We present a number of additional applications of our method.

Abstract:
Super-cooled liquids are characterized by their fragility: the slowing down of the dynamics under cooling is more sudden and the jump of specific heat at the glass transition is generally larger in fragile liquids than in strong ones. Despite the importance of this quantity in classifying liquids, explaining what aspects of the microscopic structure controls fragility remains a challenge. Surprisingly, experiments indicate that the linear elasticity of the glass -- a purely local property of the free energy landscape -- is a good predictor of fragility. In particular, materials presenting a large excess of soft elastic modes, the so-called boson peak, are strong. This is also the case for network liquids near the rigidity percolation, known to affect elasticity. Here we introduce a model of the glass transition based on the assumption that particles can organize locally into distinct configurations, which are coupled spatially via elasticity. The model captures the mentioned observations connecting elasticity and fragility. We find that materials presenting an abundance of soft elastic modes have little elastic frustration: energy is insensitive to most directions in phase space, leading to a small jump of specific heat. In this framework strong liquids turn out to lie the closest to a critical point associated with a rigidity or jamming transition, and their thermodynamic properties are related to the problem of number partitioning and to Hopfield nets in the limit of small memory.

Abstract:
We study theoretically and numerically how hard frictionless particles in random packings can rearrange. We demonstrate the existence of two distinct unstable non-linear modes of rearrangement, both associated with the opening and the closing of contacts. Mode one, whose density is characterized by some exponent {\theta}', corresponds to motions of particles extending throughout the entire system. Mode two, whose density is characterized by an exponent {\theta} != {\theta}', corresponds to the local buckling of a few particles. Mode one is shown to yield at a much higher rate than mode two when a stress is applied. We show that the distribution of contact forces follows P(f) f^{min({\theta}',{\theta})}, and that imposing that the packing cannot be densified further leads to the bounds {\gamma} >= 1/(2+{\theta}') and {\gamma} >= (1-{\theta})/2, where {\gamma} characterizes the singularity of the pair distribution function g(r) at contact. These results extend the theoretical analysis of [M. Wyart, Phys. Rev. Lett 109, 125502 (2012)] where the existence of mode two was not considered. We perform numerics that support that these bounds are saturated with {\gamma} \approx 0.38, {\theta} \approx 0.17 and {\theta}' \approx 0.44. We measure systematically the stability of all such modes in packings, and confirm their marginal stability. The principle of marginal stability thus allows to make clearcut predictions on the ensemble of configurations visited in these out-of-equilibrium systems, and on the contact forces and pair distribution functions. It also reveals the excitations that need to be included in a description of plasticity or flow near jamming, and suggests a new path to study two-level systems and soft spots in simple amorphous solids of repulsive particles.

Abstract:
The dynamics of random weakly nonlinear waves is studied in the framework of vibrating thin elastic plates. Although it has been previously predicted that no stationary inverse cascade of constant wave action flux could exist in the framework of wave turbulence for elastic plates, we present substantial evidence of the existence of {\gr a time dependent} inverse cascade, opening up the possibility of self organization for a larger class of systems. This inverse cascade transports the spectral density of the amplitude of the waves from short up to large scales, increasing the distribution of long waves despite the short wave fluctuations. This dynamics appears to be self-similar and possesses a power law behaviour in the short wavelength limit which is significantly different from the exponent obtained via a Kolmogorov dimensional analysis argument. Finally, we show explicitly a tendency to build a long wave coherent structure in finite time.

Abstract:
We study the relationship between microscopic structure and viscosity in non-Brownian suspensions. We argue that the formation and opening of contacts between particles in flow effectively leads to a negative selection of the contacts carrying weak forces. We show that an analytically tractable model capturing this negative selection correctly reproduces scaling properties of flows near the jamming transition. In particular, we predict that (i) the viscosity {\eta} diverges with the coordination z as {\eta} ~ (z_c-z)^{-(3+{\theta})/(1+{\theta})}, (ii) the operator that governs flow displays a low-frequency mode that controls the divergence of viscosity, at a frequency {\omega}_min\sim(z_c-z)^{(3+{\theta})/(2+2{\theta})}, and (iii) the distribution of forces displays a scale f* that vanishes near jamming as f*/\sim(z_c-z)^{1/(1+{\theta})} where {\theta} characterizes the distribution of contact forces P(f)\simf^{\theta} at jamming, and where z_c is the Maxwell threshold for rigidity.

Abstract:
While the rheology of non-Brownian suspensions in the dilute regime is well-understood, their behavior in the dense limit remains mystifying. As the packing fraction of particles increases, particle motion becomes more collective, leading to a growing length scale and scaling properties in the rheology as the material approaches the jamming transition. There is no accepted microscopic description of this phenomenon. However, in recent years it has been understood that the elasticity of simple amorphous solids is governed by a critical point, the unjamming transition where the pressure vanishes, and where elastic properties display scaling and a diverging length scale. The correspondence between these two transitions is at present unclear. Here we show that for a simple model of dense flow, which we argue captures the essential physics near the jamming threshold, a formal analogy can be made between the rheology of the flow and the elasticity of simple networks. This analogy leads to a new conceptual framework to relate microscopic structure to rheology. It enables us to define and compute numerically normal modes and a density of states. We find striking similarities between the density of states in flow, and that of amorphous solids near unjamming: both display a plateau above some frequency scale \omega* ~ |z_c-z|, where z is the coordination of the network of particles in contact, z_c = 2D where D is the spatial dimension. However, a spectacular difference appears: the density of states in flow displays a single mode at another frequency scale \omega_{min} << \omega* governing the divergence of the viscosity.

Abstract:
Dense non-Brownian suspension flows of hard particles display mystifying properties: as the jamming threshold is approached, the viscosity diverges, as well as a length scale that can be identified from velocity correlations. To unravel the microscopic mechanism governing dissipation and its connection to the observed long-range correlations, we develop an analogy between suspension flows and the rigidity transition occurring when floppy networks are pulled -- a transition believed to be associated to the stress-stiffening of certain gels. After deriving the critical properties near the rigidity transition, we show numerically that suspensions flows lie close to it. We find that this proximity causes a decoupling between viscosity and the correlation length of velocities \xi, which scales as the length l_c characterizing the response of the velocity in flow to a local perturbation, previously predicted to follow l_c\sim 1/\sqrt{z_c-z}\sim p^{0.18} where p is the dimensionless particle pressure, z the coordination of the contact network made by the particles and z_c is twice the spatial dimension. We confirm these predictions numerically, predict the existence of a larger length scale l_r\sim 1/\sqrt{p} with mild effects on velocity correlation and the existence of a vanishing strain \delta \gamma\sim 1/p that characterizes de-correlation in flow.

Abstract:
We show numerically that the response of simple amorphous solids (elastic networks and particle packings) to a local force dipole is characterized by a lengthscale $\ell_c$ that diverges as unjamming is approached as $\ell_c \sim (z - 2d)^{-1/2}$, where $z \ge 2d$ is the mean coordination, and $d$ is the spatial dimension, at odds with previous numerical claims. We also show how the magnitude of the lengthscale $\ell_c$ is amplified by the presence of internal stresses in the disordered solid. Our data suggests a divergence of $\ell_c\sim (p_c-p)^{-1/4}$ with proximity to a critical internal stress $p_c$ at which soft elastic modes become unstable.