Glass transition associated with the jamming of vibrated granular matter

We investigate a vibrated granular system composed of millimeter-size glass beads. When the system is submitted to a perturbation with decreasing intensity, below the fluidization limit, it evolves in a way similar to glass-forming liquids until it reaches an amorphous jammed state. This jamming transition is observed by the means of an immersed oscillator, either in the free or forced mode, while the granular medium is submitted to an external perturbation at a fixed frequency or within a frequency band. The complex susceptibility of the oscillator is measured as a function of the probe forcing frequency or as a function of the perturbation intensity. Data show that the jamming dynamics is "activated," similarly to thermal systems. The empirical control parameter is found proportional to the square root of the vibration intensity and inversely proportional to the vibration frequency. In the case of broadband external vibration, the average frequency of the power spectrum has to be considered.

Towards the Glass Transition in Vibrated Granular Matter

Alessandro Luigi Sellerio

Granular materials are large sets of macroscopic particles that interact solely via contact forces. The static behavior depends on the contact network and on the surface friction forces between grains; when they are set in motion (typically by vibrations) their dynamics is dominated by inelastic collisions. For these reasons granular media show an extremely rich phenomenology, ranging from fluid-like properties (if strongly vibrated), to "jamming", glassy, behavior (if weakly vibrated), to aging and hysteretical phenomena observed when they become trapped in frozen, amorphous states. The objective of this work is to study these states and transitions, and to characterize the analogies found between the dynamic behavior of vibrated granular media and the glass transition observed in thermal glass-formers. These analogies justify the interest in granular materials, because granular media can be seen as simplified model systems useful in the study of out of equilibrium thermodynamics, and, in general, to the larger framework known as "complexity". The granular materials considered here are composed of spheric, polished glass spheres. Since the surface state plays an important role in the grain-grain interaction, some measurements were also performed with acid etched beads, having different surface roughness. The samples are vertically vibrated to achieve vibrofluidization. Different kinds of vibration are used, to highlight different properties of the system. We first consider the transition between the fluid and the subcooled glassy phase, using different experimental techniques. The most important one is a torsion oscillator, that interacts with the granular media via immersed probes. The torsion oscillator can be used in forced mode. A torque is applied on the probe, and we measure the mechanical response function (complex susceptibility). In general, a relaxation is found and it is interpreted as the signature of the irreversible energy loss (damping) in granular collisions. This relaxation has an intrinsic time scale, and systematic analysis of it shows that a clear parallel can be traced to the behavior of "strong" glasses. In particular, it is found that (i) the relaxation time is a function of a unified control parameter, proportional to the square root of the average vibration, and phenomenologically equivalent to an effective temperature; (ii) the functional form with which the relaxation times approach the final "frozen" state has an Arrhenius, or Vögel-Fulcher-Tamman (VFT) behavior. The same torsion oscillator is employed in free mode. In this case, no external torque is applied, and the probe moves adapting its position under the effect of the continuous rearrangements in the sample. The system is studied by computing the power spectral density of the (angular position) time series. The resulting spectra represent a "configurational noise" as the system randomly hops from one configuration to the following. This allows to define, using a completely different approach, the same intrinsic time scale observed in forced mode measurements. The comparison of the two techniques allows to obtain a more complete and detailed picture of the dynamics in the jamming region. From this comparison, it was inferred that the system is also influenced by an effective vibration frequency, and that the relaxation time has indeed a non-Arrhenius behavior as a function of a control parameter defined as as = √ Γ/ωs. A model was developed combining rheological observations to a statistic approach describing extremal phenomena. This model justifies the appearance of both the control parameter and the VFT evolution of the relaxation. Furthermore, the model is predictive and exposes the effect of a few other rheologic properties of granular system. The effect of surface roughness are considered, showing that the static and dynamic surface friction coefficients are well described by the model. A second relevant part of this work is devoted to an explicit verification that macroscopic probes act as Brownian objects. This fact is often used to interpret experimental data (also in the present work) and to propose theoretical model. However, no explicit evidence has ever been discussed. This is hard to do, using a constrained system such as the torsion oscillator, because the restoring coefficient influences the dynamics of diffusion. To overcome the problem we built a different apparatus, called "Brownian motor", where the probes are mounted on ball bearings, so that they are free to turn without constraint. The properties of the time series of the position of the free turning probe and of the torsionally constrained oscillator can finally be analyzed and compared with simple simulations. The data show an overall diffusion-like behavior, that is influenced by the presence of constraints. Using fractal analysis we estimate the diffusion, or Hurst exponent. This allows to verify that a "macroscopic" object (the probe) immersed in the "microscopic" granular medium indeed behaves as a Brownian object, and that its dynamics can be studied in detail, showing that it undergoes anomalous diffusion. This work is concluded with a discussion on a few possible developments. The most promising idea is a novel approach to the study of the geometrical properties of the contact network of granular assemblies, that is responsible for many of the properties of the granular sample. By using Magnetic Resonance Imaging, the static 3-D structure of granular media can be reconstructed with unprecedented accuracy, resolution and ease of reproducibility. From the spatial information we can extract all the properties of static granular media: the compaction factor, the grain-grain correlation function, the free volume and other observables. Systematic studies could allow experimental confirmations of the many theoretical models that have been proposed in the last years and that still lack a thorough comparison with experiments. This idea does not conclude the perspectives of this work, that are vast and intriguing. A few promising subjects are reviewed more into detail in the corresponding Perspective section. To name a few we cite: measurements of induced aging in non-vibrated samples, the Brownian motor, stick and slip phenomena and their comparison with earthquakes.

EPFL2012

Fluid and glassy phases of vibrated granular matter studied with a torsion oscillator

Patrick Mayor

Granular matter submitted to external perturbations exhibits various behaviors depending on the vibration intensity: when strongly vibrated, the granular system has a fluid aspect, whereas under low intensity perturbations, it is in a quasi-solid phase. In this work, we clarify these analogies: we discuss to what extent a "granular fluid" is close to a standard liquid and, on the other hand, investigate the similarities between weakly perturbed granular materials and supercooled liquids undergoing a glass transition. We first consider the case where a granular medium undergoes vibrations of high intensity (with accelerations of about 1 to 10 times that of gravity). This vibrated system is investigated using an immersed torsion oscillator which is sensitive to the granular agitation and which, as a result, exhibits irregular angular deflections that can be analyzed to give information on the grains' motion. This oscillator can also be used in forced mode: applying a torque allows measures of the mechanical susceptibility, which displays a resonance peak similar to that of a damped oscillator. It is thus possible to introduce a viscosity parameter for the system, which is found to be inversely proportional to the vibration acceleration imposed to the container. Moreover, by considering both the fluctuations (given by the diffusive noise, with the oscillator in free mode) and the susceptibility (forced mode), the validity of the fluctuation-dissipation theorem can be tested. Surprisingly, it turns out that very complicated system, even though far from equilibrium, satisfies this basic law of equilibrium statistical mechanics in first approximation, and therefore behaves very much like a plain liquid. The immersed oscillator can thus be compared to a pollen particle exhibiting Brownian motion due to the continuous molecular – here, the granular – agitation. We can thus also introduce an "effective temperature" parameter for vibration-fluidized granular matter and discuss its properties. In particular, we observe that the temperature defined is inhomogeneous and anisotropic, contrary to usual liquids. An interesting issue is also studied carefully: when the damping becomes large (for when the imposed vibrations are lower, or when the oscillator is deeply immersed), a stiffening phenomenon is observed, in which the apparent elastic constant increases linearly with the friction. An elementary rheological model suggests that this may be caused by the appearance of a force chains fretwork resisting the probe rotation. Various granular materials are analyzed with this met hod. The grain mass seems to be an important parameter, as well as the grain surface state. Experiments in which the grains are etched with acid in order to modify the surface roughness are also discussed. While we know what happens to a liquid when its temperature is decreased one can wonder what happens to a granular system when the perturbations become critically low. By using vibrations of weak intensity (with accelerations below that of gravity), we study how the system reaches a "frozen" static configuration when the perturbation intensity is decreased. The observed diffusive noise appears to approach the final jammed state according to the Vogel-Fulcher-Tammann law, that also describes the temperature dependence of viscosity or diffusivity in supercooled liquids, thus showing strong analogies with the vitrification process. Here, the parameter playing the role of temperature in the equations is found to depend only on the vibration amplitude. Finally, we briefly discuss how a small modification of the experimental setup may allow to create a "Brownian motor" generating useful work out of the random agitation of the grains.

EPFL2005

Fluidized States of Vibrated Granular Media Studied by Mechanical Spectroscopy

Gérard Gremaud, Daniele Mari, Alessandro Luigi Sellerio

We investigate the jamming transition observed in vibrated granular systems composed of millimeter size glass beads. When a granular system is submitted to vibrations with decreasing intensity, it evolves in a way similar to glass-forming liquids: from a low viscosity, liquid-like state, it evolves into an amorphous jammed state. This evolution is observed by the means of an immersed oscillator acting as a torsion pendulum in forced mode. The complex susceptibility of the oscillator is measured as a function of the probe forcing frequency and of the vibration intensity. Focusing on the strongly vibrated states, we observe that there are two different dynamic regions. The first is a high fluidization regime, where the internal friction is found to be proportional to the ratio between the pulsation and the vibration intensity: tan(delta)=omega/Gamma. In this region, the system shows an apparent viscous friction eta=1/Gamma. In the second, low fluidization, regime, we observe a more complex behavior, and the measured internal friction appears to be well described by a relation of the form: tan(delta)=(omega/Gamma(2))(alpha) In this second case, the key role is played by a critical breakaway stress, sigma(cr), needed to break the network of chains of forces that form between the grains. Finally, if vibration intensities are still reduced, we also observe that onset of jamming is clearly distinguishable: we observe a sharp increase in the apparent dynamic modulus together with a peak in internal friction. This transition presents important similarities to those observed in glasses, and it leads to the second (low vibrations) regime, where the key role is played by the square root of the vibration intensity.