(Georgia Institute of Technology, 2010-09-06)
Gravish, Nick; Umbanhowar, Paul B.; Goldman, Daniel I.
We use plate drag to study the response of granular media to localized forcing as a function of volume fractionϕ. A bifurcation in the force and flow occurs at the onset of dilatancy ϕ [subscript c]. Below ϕ [subscript c] rapid fluctuations in the drag force F [subscript D] are observed. Above ϕ [subscript c] fluctuations in F [subscript D] are periodic and increase in magnitude with ϕ. Velocity field measurements indicate that the bifurcation in F [subscript D] results from the formation of stable shear bands above ϕ [subscript c] which are created and destroyed periodically during drag. A friction-based wedge flow model captures the dynamics for ϕ >ϕ [subscript c].
(Georgia Institute of Technology, 2010-07-15)
Umbanhowar, Paul B.; Goldman, Daniel I.
Impact dynamics during collisions of spheres with granular media reveal a pronounced and nontrivial dependence on volume fraction ϕ. Postimpact crater morphology identifies the critical packing state ϕcps, where sheared grains neither dilate nor consolidate, and indicates an associated change in spatial response. Current phenomenological models fail to capture the observed impact force for most ϕ; only near ϕcps is force separable into additive terms linear in depth and quadratic in velocity. At fixed depth the quadratic drag coefficient decreases (increases) with depth for ϕ<ϕcps (ϕ>ϕcps). At fixed low velocity, depth dependence of force shows a Janssen-type exponential response with a length scale that decreases with increasing ϕ and is nearly constant for ϕ>ϕcps.