Manipulating bubbles with secondary Bjerknes forces

Gas bubbles in a sound field are submitted to a radiative force, known as the secondary Bjerknes force. We propose an original experimental setup that allows us to investigate in details this force between two bubbles, as a function of the sonication frequency, as well as the bubbles radii and distance. We report the observation of both attractive and, more interestingly, repulsive Bjerknes force, when the two bubbles are driven in antiphase. Our experiments show the importance of taking multiple scattering into account, which leads to a strong acoustic coupling of the bubbles when their radii are similar. Our setup demonstrates the accuracy of secondary Bjerknes forces for attracting or repealing a bubble, and could lead to new acoustic tools for non contact manipulation in microfluidic devices.Comment: 4 pages, 6 figure

History Memorized and Recalled upon Glass Transition

The memory effect upon glassification is studied in the glass to rubber transition of vulcanized rubber with the strain as a controlling parameter. A phenomenological model is proposed taking the history of the temperature and the strain into account, by which the experimental results are interpreted. The data and the model demonstrate that the glassy state memorizes the time-course of strain upon glassification, not as a single parameter but as the history itself. The data also show that the effect of irreversible deformation in the glassy state is beyond the scope of the present model. Authors' remark: The title of the paper in the accepted version is above. The title appeared in PRL is the one changed by a Senior Assistant Editor after acceptance of the paper. The recovery of the title was rejected in the correction process.Comment: 4 pages, 4 figure

Rejuvenation and overaging in a colloidal glass under shear

We report the modifications of the microscopic dynamics of a colloidal glass submitted to shear. We use multispeckle diffusing wave spectroscopy to monitor the evolution of the spontaneous slow relaxation processes after the sample have been submitted to various straining. We show that high shear rejuvenates the system and accelerates its dynamics whereas moderate shear overage the system. We analyze this phenomena within the frame of the Bouchaud's trap model.Comment: 4 pages, 4 figures, to be published in PR

Averaging rheological quantities in descriptions of soft glassy materials

Many mean-field models have been introduced to describe the mechanical behavior of glassy materials. They often rely on averages performed over distributions of elements or states. We here underline that averaging is a more intricate procedure in mechanics than in more classical situations such as phase transitions in magnetic systems. This leads us to modify the predictions of the recently proposed SGR model for soft glassy materials, for which we suggest that the viscosity should diverge at the glass transition temperature $T_g$ with an exponential form $\eta \sim \exp(\frac{A}{T-T_g})$.Comment: 4 pages, Latex, 1 eps figur

Finite-size effects and intermittency in a simple aging system

We study the intermittent dynamics and the fluctuations of the dynamic correlation function of a simple aging system. Given its size $L$ and its coherence length $\xi$, the system can be divided into $N$ independent subsystems, where $N=(\frac{L}{\xi})^d$, and $d$ is the dimension of space. Each of them is considered as an aging subsystem which evolves according to an activated dynamics between energy levels. We compute analytically the distribution of trapping times for the global system, which can take power-law, stretched-exponential or exponential forms according to the values of $N$ and the regime of times considered. An effective number of subsystems at age $t_w$, $N_{eff}(t_w)$, can be defined, which decreases as $t_w$ increases, as well as an effective coherence length, $\xi(t_w) \sim t_w^{(1-\mu)/d}$, where $\mu <1$ characterizes the trapping times distribution of a single subsystem. We also compute the probability distribution functions of the time intervals between large decorrelations, which exhibit different power-law behaviours as $t_w$ increases (or $N$ decreases), and which should be accessible experimentally. Finally, we calculate the probability distribution function of the two-time correlator. We show that in a phenomenological approach, where $N$ is replaced by the effective number of subsystems $N_{eff}(t_w)$, the same qualitative behaviour as in experiments and simulations of several glassy systems can be obtained.Comment: 15 pages, 6 figures, published versio

Effective Temperatures of a Driven System Near Jamming

Fluctuations in a model of a sheared, zero-temperature foam are studied numerically. Five different quantities that reduce to the true temperature in an equilibrium thermal system are calculated. All five have the same shear-rate dependence, and three have the same value. Near the onset of jamming, the relaxation time is the same function of these three temperatures in the sheared system as of the true temperature in an unsheared system. These results imply that statistical mechanics is useful for the system and provide strong support for the concept of jamming.Comment: 4 pages, 4 postscript figure

A Hydrodynamic model for a dynamical jammed-to-flowing transition in gravity driven granular media

Granular material on an inclined plane will flow like a fluid if the angle $\theta$ the plane makes with the horizontal is large enough. We employ a modification of a hydrodynamic model introduced previously to describe Couette flow experiments to describe chute flow down a plane. In this geometry, our model predicts a jammed-to-flowing transition as $\theta$ is increased even though it does not include solid friction, which might seem necessary to stabilize a state without flow. The transition is driven by coupling between mean and fluctuating velocity. In agreement with experiments and simulations, it predicts flow for layers with a thickness H larger than a critical value $H_{\rm stop}(\theta)$ and absence of flow for $H<H_{\rm stop}(\theta)$