Claude Laroche

Subharmonic Instabilities and Defects in a Granular Layer under Vertical Vibrations

We report an experimental study of subharmonic instabilities observed in a horizontal layer of glass spheres under vertical vibrations. Above a critical value of the acceleration, the layer undergoes a period-doubling instability. When the acceleration is increased further, the spatially homogeneous temporal oscillation at half the excitation frequency becomes unstable and a regular pattern of defects is generated. Each defect separates two regions oscillating out of phase, and is thus connected with the discrete broken symmetry at the period-doubling bifurcation.

Observation of gravity-capillary wave turbulence

We report the observation of the crossover between gravity and capillary wave turbulence on the surface of mercury. The probability density functions of the turbulent wave height are found to be asymmetric and thus non-Gaussian. The surface wave height displays power-law spectra in both regimes. In the capillary region, the exponent is in fair agreement with weak turbulence theory. In the gravity region, it depends on the forcing parameters. This can be related to the finite size of the container. In addition, the scaling of those spectra with the mean energy flux is found in disagreement with weak turbulence theory for both regimes.

Violation of the fluctuation-dissipation relation during the formation of a colloidal glass

The relationship between the conductivity and the polarization noise is measured in a colloidal glass as a function of frequency in the range 1 Hz-40 Hz. It is found that at the beginning of the transition from a fluid-like sol to a solid-like colloidal glass the fluctuation-dissipation relation is strongly violated. The amplitude and the persistence time of this violation are decreasing functions of frequency. At the lowest frequencies of the measuring range it persists for times which are about 5% of the time needed to form the colloidal glass. This phenomenology is quite close to the recent theoretical predictions done for the violation of the fluctuation-dissipation relation in glassy systems.

Observation of intermittency in wave turbulence.

We report the observation of intermittency in gravity-capillary wave turbulence on the surface of mercury. We measure the temporal fluctuations of surface wave amplitude at a given location. We show that the shape of the probability density function of the local slope increments of the surface waves strongly changes across the time scales. The related structure functions and the flatness are found to be power laws of the time scale on more than one decade. The exponents of these power laws increase nonlinearly with the order of the structure function. All these observations show the intermittent nature of the increments of the local slope in wave turbulence. We discuss the possible origin of this intermittency.

“Turbulent” electrical transport in copper powders

Compressed copper powder has a very large electrical resistance (1 MΩ), due to the oxide layer on grains (100 μm). We observe that its voltage-current U-I characteristics are nonlinear, and undergo an instability, from an insulating to a conductive state at relatively small applied voltages. Current through the powder is then noisy, and the noise has interesting self-similar properties, including intermittency and scale invariance. We show that heat dissipation plays an essential role in the physics of the system. One piece of evidence is that the instability threshold always corresponds to the same Joule dissipated power whatever the applied stress. In addition, we observe long-time correlations which suggest that thermal expansion locally creates or destroys contacts, and is the driving mechanism behind the instability and noise observed in this granular system.

Advanced memory effects in the aging of a polymer glass

Abstract:A new kind of memory effect on low frequency dielectric measurements on plexiglass (PMMA) is described. These measurements show that cooling and heating the sample at constant rate give an hysteretic dependence on temperature of the dielectric constant ε. A temporary stop of cooling produces a downward relaxation of ε. Two main features are observed (i) when cooling is resumed ε goes back to the values obtained without the cooling stop (i.e. the low temperature state is independent of the cooling history) (ii) upon reheating ε keeps the memory of all the cooling stops (Advanced memory). The dependence of this effect on frequency and on the cooling rate is analyzed. The memory deletion is studied too. Finally the results are compared with those of similar experiments done in spin glasses and with the famous experiments of Kovacs.

Dynamics of Avalanches in a Rotating Cylinder

We consider the problem of avalanches along the surface of a heap of grains, the slope of which is increased at a constant rate in time. We study the distribution of irregular avalanches occuring at small rate of increase, and explain the transitions to a regular avalanche regime and then to a continuous flow regime at higher rates. We also report the observation of a cellular instability of the granular flow. We show that the dynamics of large avalanches is independent of the existence of small ones, and can be described with a simple macroscopic model which relates the slope of the heap and the local flow-rate. The irregular behaviour of avalanches results from the decrease of the dynamic friction coefficient with increasing flow-rate, together with the coupling between neighbouring avalanches.

Fluctuations of energy flux in wave turbulence.

We report that the power driving gravity and capillary wave turbulence in a statistically stationary regime displays fluctuations much stronger than its mean value. We show that its probability density function (PDF) has a most probable value close to zero and involves two asymmetric roughly exponential tails. We understand the qualitative features of the PDF using a simple Langevin-type model.

On the origin of intermittency in wave turbulence

Using standard signal processing tools, we experimentally report that intermittency of wave turbulence on the surface of a fluid occurs even when two typical large-scale coherent structures (gravity wave breakings and bursts of capillary waves on steep gravity waves) are not taken into account. We also show that intermittency depends on the power injected into the waves. The dependence of the power law exponent of the gravity-wave spectrum on the forcing amplitude cannot also be ascribed to these coherent structures. Statistics of these two events are studied.

Sound Propagation in a Liquid with Vapour Bubbles

We report experimental measurements of sound velocity in diethyl-ether containing vapour bubbles. We observe no effect of liquid-vapour transition on the sound velocity, which is in good agreement with the prediction of effective medium theory.