Granular State Effects on Wave Propagation

Sound and pressure wave propagation in a granular material is of interest not only for its intrinsic and practical value, but also because it provides a non-intrusive means of probing the state of a granular material. By examining wave speeds and attenuation, insight can be gained into the nature of the contacts between the particles. In the present paper, wave speeds and attenuation rates are first examined for a static granular bed for a variety of system parameters including particle size, composition and the overburden of the material above the measuring transducers. Agitation of the bed is then introduced by shaking the material vertically. This causes the bed to transition from a static granular state to a vibrofluidized state. The dilation of the bed allows for relative particle motion and this has a significant effect on the measured wave speeds and attenuation. Further, the fluid-like characteristics of the agitated bed distort the forcechain framework through which the waves are thought to travel. The consequences of bed consolidation, a natural result of shaking, are also examined

Chemical Equilibration and Transport Properties of Hadronic Matter near TcT_c

We discuss how the inclusion of Hagedorn states near $T_c$ leads to short chemical equilibration times of proton anti-proton pairs, $K\bar{K}$ pairs, and $\Lambda\bar{\Lambda}$ pairs, which indicates that hadrons do not need to be "born" into chemical equilibrium in ultrarelativistic heavy ion collisions. We show that the hadron ratios computed within our model match the experimental results at RHIC very well. Furthermore, estimates for $\eta/s$ near $T_c$ computed within our resonance gas model are comparable to the string theory viscosity bound $\eta/s=1/4\pi$. Our model provides a good description of the recent lattice results for the trace anomaly close to $T_c=196$ MeV.Comment: 4 pages, 3 figures, to appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse

Measurements and Simulations of Wave Propagation in Agitated Granular Beds

Wave propagation in a granular bed is a complicated, highly nonlinear phenomenon. Yet studies of wave propagation provide important information on the characteristics of these materials. Fundamental nonlinearities of the bed include those in the particle contact model and the fact that there exists zero applied force when grains are out of contact. The experimental work of Liu and Nagal showed the strong dependence of wave propagation on the forming and breaking of particle chains. As a result of the nonlinearities, anomalous behavior such as solitary waves and sonic vacuum have been predicted by Nesterenko. In the present work we examine wave propagation in a granular bed subjected to vertical agitation. The agitation produces continual adjustment of force chains in the bed. Wave propagation speed and attenuation measurements were made for such a system for a range of frequencies considerably higher than that used for the agitation. Both laboratory experiments and simulations (using a two-dimensional, discrete soft-particle model) have been used. The present paper is a progress report on the simulations

Pressure wave propagation in a shaken granular bed

Pressure waves in a granular material travel through particle contact points and are primarily transmitted by the "force chains" that carry most of the load in a granular medium. However, these force chains tend to be fragile and ephemeral and can be disrupted by very minor perturbations including the waves themselves. External vibration also disrupts the force chains and therefore also changes the wave propagation characteristics. In this paper we study the effects of vibration on wave propagation in a shaken granular bed

Hagedorn states and thermalization : XLIX International Winter Meeting on Nuclear Physics, 24 - 28 January 2011, Bormio, Italy

In recent years, Hagedorn states have been used to explain the equilibrium and transport properties of a hadron gas close to the QCD critical temperature. These massive resonances are shown to lower h/s to near the AdS/CFT limit close to the phase transition. A comparison of the Hagedorn model to recent lattice results is made and it is found that the hadrons can reach chemical equilibrium almost immediately, well before the chemical freeze-out temperatures found in thermal fits for a hadron gas without Hagedorn states