Tailoring the frictional properties of granular media

A method of modifying the roughness of soda-lime glass spheres is presented, with the purpose of tuning inter-particle friction. The effect of chemical etching on the surface topography and the bulk frictional properties of grains is systematically investigated. The surface roughness of the grains is measured using white light interferometry and characterised by the lateral and vertical roughness length scales. The underwater angle of repose is measured to characterise the bulk frictional behaviour. We observe that the co-efficient of friction depends on the vertical roughness length scale. We also demonstrate a bulk surface roughness measurement using a carbonated soft drink.Comment: 10 pages, 17 figures, submitted to Phys. Rev.

Gravity-driven Dense Granular Flows

We report and analyze the results of numerical studies of dense granular flows in two and three dimensions, using both linear damped springs and Hertzian force laws between particles. Chute flow generically produces a constant density profile that satisfies scaling relations suggestive of a Bagnold grain inertia regime. The type of force law has little impact on the behavior of the system. Bulk and surface flows differ in their failure criteria and flow rheology, as evidenced by the change in principal stress directions near the surface. Surface-only flows are not observed in this geometry.Comment: 4 pages, RevTeX 3.0, 4 PostScript figures (5 files) embedded with eps

Stress in frictionless granular material: Adaptive Network Simulations

We present a minimalistic approach to simulations of force transmission through granular systems. We start from a configuration containing cohesive (tensile) contact forces and use an adaptive procedure to find the stable configuration with no tensile contact forces. The procedure works by sequentially removing and adding individual contacts between adjacent beads, while the bead positions are not modified. In a series of two-dimensional realizations, the resulting force networks are shown to satisfy a linear constraint among the three components of average stress, as anticipated by recent theories. The coefficients in the linear constraint remain nearly constant for a range of shear loadings up to about .6 of the normal loading. The spatial distribution of contact forces shows strong concentration along ``force chains". The probability of contact forces of magnitude f shows an exponential falloff with f. The response to a local perturbing force is concentrated along two characteristic rays directed downward and laterally.Comment: 8 pages, 8 figure

Diphasic non-local model for granular surface flows

Considering recent results revealing the existence of multi-scale rigid clusters of grains embedded in granular surface flows, i.e. flows down an erodible bed, we describe here the surface flows rheology through a non-local constitutive law. The predictions of the resulting model are compared quantitatively to experimental results: The model succeeds to account for the counter-intuitive shape of the velocity profile observed in experiments, i.e. a velocity profile decreasing exponentially with depth in the static phase and remaining linear in the flowing layer with a velocity gradient independent of both the flowing layer thickness, the angle between the flow and the horizontal, and the coefficient of restitution of the grains. Moreover, the scalings observed in rotating drums are recovered, at least for small rotating speed.Comment: 7 pages, submitted to Europhys. Let

Finite-time fluctuations in the degree statistics of growing networks

This paper presents a comprehensive analysis of the degree statistics in models for growing networks where new nodes enter one at a time and attach to one earlier node according to a stochastic rule. The models with uniform attachment, linear attachment (the Barab\'asi-Albert model), and generalized preferential attachment with initial attractiveness are successively considered. The main emphasis is on finite-size (i.e., finite-time) effects, which are shown to exhibit different behaviors in three regimes of the size-degree plane: stationary, finite-size scaling, large deviations.Comment: 33 pages, 7 figures, 1 tabl

Static Friction Phenomena in Granular Materials: Coulomb Law vs. Particle Geometry

The static as well as the dynamic behaviour of granular material are determined by dynamic {\it and} static friction. There are well known methods to include static friction in molecular dynamics simulations using scarcely understood forces. We propose an Ansatz based on the geometrical shape of nonspherical particles which does not involve an explicit expression for static friction. It is shown that the simulations based on this model are close to experimental results.Comment: 11 pages, Revtex, HLRZ-33/9

Dissipation of vibration in rough contact

The relationship which links the normal vibration occurring during the sliding of rough surfaces and the nominal contact area is investigated. Two regimes are found. In the first one, the vibrational level does not depend on the contact area, while in the second one, it is propor- tional to the contact area. A theoretical model is proposed. It is based on the assumption that the vibrational level results from a competition between two processes of vibration damping, the internal damping of the material and the contact damping occurring at the interface

A model for collisions in granular gases

We propose a model for collisions between particles of a granular material and calculate the restitution coefficients for the normal and tangential motion as functions of the impact velocity from considerations of dissipative viscoelastic collisions. Existing models of impact with dissipation as well as the classical Hertz impact theory are included in the present model as special cases. We find that the type of collision (smooth, reflecting or sticky) is determined by the impact velocity and by the surface properties of the colliding grains. We observe a rather nontrivial dependence of the tangential restitution coefficient on the impact velocity.Comment: 11 pages, 2 figure

Is radiotherapy required in first-line treatment of stage I diffuse anaplastic Wilms tumor? A report of SIOP-RTSG, AIEOP, JWiTS, and UKCCSG

BACKGROUND: As a significant proportion of relapses occurred in the tumor bed or abdomen on patients with the fifth National Wilms Tumor Study stage I anaplastic Wilms tumor (WT), flank radiotherapy was added for stage I anaplastic WT in the subsequent study of the Children's Oncology Group (AREN0321). Preliminary results revealed reduction of relapse rate and improved survival. In cases treated with preoperative chemotherapy, such as in International Society of Pediatric Oncology (SIOP), the value of radiotherapy has never been studied. The aim of this observational study is to describe the pattern of recurrence and survival of patients with stage I diffuse anaplastic WT (DAWT) after induction chemotherapy. METHODS: Retrospective data analysis of the pattern of relapse and survival of all patients with stage I DAWT were included in recent SIOP, L'Associazone Italiana Ematologica Oncologia Pediatrica (AIEOP), Japan Wilms Tumor Study Group (JWiTS), United Kingdom Children's Cancer Study Group (UKCCSG) renal tumor registries. Postoperative treatment consisted of actinomycin D, vincristine, and doxorubicin for 28 weeks without local irradiation. RESULTS: One hundred nine cases with stage I DAWT were identified, of which 95 cases received preoperative chemotherapy. Of these, seven patients underwent preoperative true‐cut biopsy. Sixteen of the 95 patients relapsed (17%), six locally, four at distant site, and six combined, and all treated according to SIOP 2001 relapse protocol, which resulted in a 5‐year overall survival of 93%. CONCLUSION: Despite 13% locoregional relapse rate, an excellent rescue rate was achieved after salvage treatment, in patients with stage I DAWT whose first‐line treatment comprised three‐drug chemotherapy (including doxorubicin), without flank irradiation. Therefore, we continue not to advocate the use of radiotherapy in first‐line treatment after preoperative chemotherapy in stage I DAWT in the next SIOP protocol