(Georgia Institute of Technology, 2004-12-23)
Heil, Patrick; Rericha, Erin C.; Goldman, Daniel I.; Swinney, Harry L.
We study the V-shaped wake (Mach cone) formed by a cylindrical rod moving through a thin, vertically vibrated granular layer. The wake, analogous to a shock (hydraulic jump) in shallow water, appears for rod velocities vR greater than a critical velocity c. We measure the half angle u of the wake as a function of vR and layer depth h. The angle satisfies the Mach relation, sin u =c/vR, where c=Îgh, even for h as small as one-particle diameter.
(Georgia Institute of Technology, 2004-05-07)
Merkt, Florian S.; Deegan, Robert D.; Goldman, Daniel I.; Rericha, Erin C.; Swinney, Harry L.
We observe stable holes in a vertically oscillated 0.5 cm deep aqueous suspension of cornstarch. Holes appear only if a finite perturbation is applied to the layer for accelerations α above 10g. Holes are circular and approximately 0.5 cm wide, and can persist for more than 10⁶ cycles. Above α ≃ 17g the rim of the hole becomes unstable, producing fingerlike protrusions or hole division. At higher acceleration, the hole delocalizes, growing to cover the entire surface with erratic undulations. We find similar behavior in an aqueous suspension of glass microspheres.