15 research outputs found
Green's Function Measurements of Force Transmission in 2D Granular Materials
We describe experiments that probe the response to a point force of 2D
granular systems under a variety of conditions. Using photoelastic particles to
determine forces at the grain scale, we experimentally show that disorder,
packing structure, friction and texture significantly affect the average force
response in granular systems. For packings with weak disorder, the mean forces
propagate primarily along lattice directions. The width of the response along
these preferred directions grows with depth, increasingly so as the disorder of
the system grows. Also, as the disorder increases, the two propagation
directions of the mean force merge into a single direction. The response
function for the mean force in the most strongly disordered system is
quantitatively consistent with an elastic description for forces applied nearly
normally to a surface. These observations are consistent with recent
predictions of Bouchaud et al. and with the anisotropic elasticity models of
Goldenberg and Goldhirsch. At this time, it is not possible to distinguish
between these two models. The data do not support a diffusive picture, as in
the q-model. This system with shear deformation is characterized by stress
chains that are strongly oriented along an angle of 45 degrees, corresponding
to the compressive direction of the shear deformation. In this case, the
spatial correlation function for force has a range of only one particle size in
the direction transverse to the chains, and varies as a power law in the
direction of the chains, with an exponent of -0.81. The response to forces is
strongest along the direction of the force chains, as expected. Forces applied
in other directions are effectively refocused towards the strong force chain
direction.Comment: 38 pages, 26 figures, added references and content, to appear in
Physica
Footprints in Sand: The Response of a Granular Material to Local Perturbations
We experimentally determine ensemble-averaged responses of granular packings
to point forces, and we compare these results to recent models for force
propagation in a granular material. We used 2D granular arrays consisting of
photoelastic particles: either disks or pentagons, thus spanning the range from
ordered to disordered packings. A key finding is that spatial ordering of the
particles is a key factor in the force response. Ordered packings have a
propagative component that does not occur in disordered packings.Comment: 5 pages, 4 eps figures, Phys. Rev. Lett. 87, 035506 (2001
Science in the sandbox: Fluctuations, friction and instabilities
The study of granular materials is a novel and rapidly growing field. These materials are interest for a number of reasons, both practical and theoretical. They exhibit a rich of novel dyanamical states, and they exhibit 'phases'-solid, liquid, and gas-that resemble conventional thermodynamic phases. However, the presence of strong dissipation through friction and inelasticity places these systems well outside the usual class of systems that can be explained by equilibrium thermodynamics. Thus, there are important challenges to create new kinds of statistical physics and new analytical descriptions for the mean and fluctuating behavior of these materials. We explore recent work that focuses on several important issues. These include force propagation and fluctuations in static and driven systems. It is well known that forces propagate through granular structures along networks-force chains, whose structure is a function of history. It is much less clear how to describe this process, and even what kind of structures evolve in physical experiments. After a brief overview of the field, we consider models of force propagation and recent experiments to test these models. Among the latter are experiments that probe force profiles at the base of sandpiles and experiments that determine the Green's function response to point perturbations in granular systems. We also explore the nature of force fluctuations in slowly evolving systems, particulary sheared granular systems. These can be very strong-with rms fluctuations in the force that are as strong as the mean force. Finally, we pursue the analogy between conventional phases of matter, where we particularly focus on the transition between fluid and solid granular states in the presence of sustained horizontal shaking