Persistent Holes in a Fluid

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 $a$ above $10g$. Holes are circular and approximately 0.5 cm wide, and can persist for more than ${10}^6$ cycles. Above $a\simeq 17g$ 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.