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W. G. Pritchard Lab Seminar: 4:00-5:00 PM, 320 Whitmore Laboratory

	            **Tuesday October 12, 2004**
	
Shocks and persistant holes in granular fluids

Erin Rericha

Center for Nonlinear Dynamics
University of Texas Austin 


Abstract:

Granular flows, such as moving sand, coal and sugar, are conceptually 
simple yet remain less understood than liquids and solids. The inelastic 
collisions between grains dissipate the random energy in the system, 
resulting in sound speeds much smaller than the average streaming motion of 
the grains. In this supersonic regime, shocks form when the granular flow 
encounters an obstacle. I will present experimental flow fields of granular 
shocks and compare results with molecular dynamics simulations and 
simulations of Navier-Stokes-like continuum equations.  

The interactions of the particles with an interstitial fluid introduces an 
additional complexity to granular flows. The free surface of a fluid at 
rest in a container is flat. Poke the surface, and the resulting 
indentation will be filled by the ensuing flow. In contrast, we have 
discovered that a vibrated aqueous suspension of cornstarch or glass 
microspheres can permanently support holes. With higher accelerations, the 
holes are unstable and nucleate a transition to a delocalized state where 
the surface of the fluid writhes on time and length scales well separated 
from the forcing of the system.  I will present experimental measurements 
of the holes and delocalized state. 

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