A quasi-elastic regime for vibrated granular gases

Using simple scaling arguments and two-dimensional numerical simulations of a granular gas excited by vibrating one of the container boundaries, we study a double limit of small $1-r$ and large $L$, where $r$ is the restitution coefficient and $L$ the size of the container. We show that if the particle density $n_0$ and $(1-r^2)(n_0 Ld)$ where $d$ is the particle diameter, are kept constant and small enough, the granular temperature, i.e. the mean value of the kinetic energy per particle, $/N$, tends to a constant whereas the mean dissipated power per particle, $/N$, decreases like $1/\sqrt{N}$ when $N$ increases, provided that $(1-r^2)(n_0 Ld)^2 < 1$. The relative fluctuations of $E$, $D$ and the power injected by the moving boundary, $I$, have simple properties in that regime. In addition, the granular temperature can be determined from the fluctuations of the power $I(t)$ injected by the moving boundary.

Velocity correlations in dense granular gases

We report the statistical properties of spherical steel particles rolling on an inclined surface being driven by an oscillating wall. Strong dissipation occurs due to collisions between the particles and rolling and can be tuned by changing the number density. The velocities of the particles are observed to be correlated over large distances comparable to the system size. The distribution of velocities deviates strongly from a Gaussian. The degree of the deviation, as measured by the kurtosis of the distribution, is observed to be as much as four times the value corresponding to a Gaussian, signaling a significant breakdown of the assumption of negligible velocity correlations in a granular system.Comment: 4 pages, 4 Figure

The energy flux into a fluidized granular medium at a vibrating wall

We study the power input of a vibrating wall into a fluidized granular medium, using event driven simulations of a model granular system. The system consists of inelastic hard disks contained between a stationary and a vibrating elastic wall, in the absence of gravity. Two scaling relations for the power input are found, both involving the pressure. The transition between the two occurs when waves generated at the moving wall can propagate across the system. Choosing an appropriate waveform for the vibrating wall removes one of these scalings and renders the second very simple.Comment: 5 pages, revtex, 7 postscript figure

Collision statistics of driven granular materials

We present an experimental investigation of the statistical properties of spherical granular particles on an inclined plane that are excited by an oscillating side-wall. The data is obtained by high-speed imaging and particle tracking techniques. We identify all particles in the system and link their positions to form trajectories over long times. Thus, we identify particle collisions to measure the effective coefficient of restitution and find a broad distribution of values for the same impact angles. We find that the energy inelasticity can take on values greater than one, which implies that the rotational degrees play an important role in energy transfer. We also measure the distance and the time between collision events in order to directly determine the distribution of path lengths and the free times. These distributions are shown to deviate from expected theoretical forms for elastic spheres, demonstrating the inherent clustering in this system. We describe the data with a two-parameter fitting function and use it to calculated the mean free path and collision time. We find that the ratio of these values is consistent with the average velocity. The velocity distribution are observed to be strongly non-Gaussian and do not demonstrate any apparent universal behavior. We report the scaling of the second moment, which corresponds to the granular temperature, and higher order moments as a function of distance from the driving wall. Additionally, we measure long time correlation functions in both space and in the velocities to probe diffusion in a dissipative gas.Comment: 12 pages, 4 figures, uses revtex

Temperature scaling in a dense vibro-fluidised granular material

The leading order "temperature" of a dense two dimensional granular material fluidised by external vibrations is determined. An asymptotic solution is obtained where the particles are considered to be elastic in the leading approximation. The velocity distribution is a Maxwell-Boltzmann distribution in the leading approximation. The density profile is determined by solving the momentum balance equation in the vertical direction, where the relation between the pressure and density is provided by the virial equation of state. The predictions of the present analysis show good agreement with simulation results at higher densities where theories for a dilute vibrated granular material, with the pressure-density relation provided by the ideal gas law, are in error. The theory also predicts the scaling relations of the total dissipation in the bed reported by McNamara and Luding (PRE v 58, p 813).Comment: ReVTeX (psfrag), 5 pages, 5 figures, Submitted to PR

Clustering, Order, and Collapse in a Driven Granular Monolayer

Steady state dynamics of clustering, long range order, and inelastic collapse are experimentally observed in vertically shaken granular monolayers. At large vibration amplitudes, particle correlations show only short range order like equilibrium 2D hard sphere gases. Lowering the amplitude "cools" the system, resulting in a dramatic increase in correlations leading either to clustering or an ordered state. Further cooling forms a collapse: a condensate of motionless balls co-existing with a less dense gas. Measured velocity distributions are non-Gaussian, showing nearly exponential tails.Comment: 9 pages of text in Revtex, 5 figures; references added, minor modifications Paper accepted to Phys Rev Letters. Tentatively scheduled for Nov. 9, 199

Near surface properties of mixtures of propylammonium nitrate with n-alkanols 1. Nanostructure

In situ amplitude modulated-atomic force microscopy (AM-AFM) has been used to probe the nanostructure of mixtures of propylammonium nitrate (PAN) with n-alkanols near a mica surface. PAN is a protic ionic liquid (IL) which has a bicontinuous sponge-like nanostructure of polar and apolar domains in the bulk, which becomes flatter near a solid surface. Mixtures of PAN with 1-butanol, 1-octanol, and 1-dodecanol at 10–70 vol% n-alkanol have been examined, along with each pure n-alkanol, to reveal the effect of composition and n-alkanol chain length. At low concentrations the butanol simply swells the PAN near-surface nanostructure, but at higher concentrations the nanostructure fragments. Octanol and dodecanol first lower the preferred curvature of the PAN near-surface nanostructure because, unlike n-butanol, their alkyl chains are too long to be accommodated alongside the PAN cations. At higher concentrations, octanol and dodecanol self-assemble into n-alkanol rich aggregates in a PAN rich matrix. The concentration at which aggregation first becomes apparent decreases with n-alkanol chain length

Grain Dynamics in a Two-dimensional Granular Flow

We have used particle tracking methods to study the dynamics of individual balls comprising a granular flow in a small-angle two-dimensional funnel. We statistically analyze many ball trajectories to examine the mechanisms of shock propagation. In particular, we study the creation of, and interactions between, shock waves. We also investigate the role of granular temperature and draw parallels to traffic flow dynamics.Comment: 17 pages, 24 figures. To appear in Phys.Rev.E. High res./color figures etc. on http://www.nbi.dk/CATS/Granular/GrainDyn.htm

Towards a middle-range theory of mental health and well-being effects of employment transitions: Findings from a qualitative study on unemployment during the 2009-2010 economic recession.

This article builds upon previous theoretical work on job loss as a status passage to help explain how people's experiences of involuntary unemployment affected their mental well-being during the 2009-2010 economic recession. It proposes a middle-range theory that interprets employment transitions as status passages and suggests that their health and well-being effects depend on the personal and social meanings that people give to them, which are called properties of the transitions. The analyses, which used a thematic approach, are based on the findings of a qualitative study undertaken in Bradford (North England) consisting of 73 people interviewed in 16 focus groups. The study found that the participants experienced their job losses as divestment passages characterised by three main properties: experiences of reduced agency, disruption of role-based identities, for example, personal identity crises, and experiences of 'spoiled identities', for example, experiences of stigma. The proposed middle-range theory allows us to federate these findings together in a coherent framework which makes a contribution to illuminating not just the intra-personal consequences of unemployment, that is, its impact on subjective well-being and common mental health problems, but also its inter-personal consequences, that is, the hidden and often overlooked social processes that affect unemployed people's social well-being. This article discusses how the study findings and the proposed middle-range theory can help to address the theoretical weaknesses and often contradictory empirical findings from studies that use alternative frameworks, for example, deprivation models and 'incentive theory' of unemployment