title: Penetration and blown air effect in granular media.
abstract: Sand is known to oppose an increasing resistance to penetration with depth. This is different from what happens in liquids since granular media, usually nonthermal systems, oppose solid friction to the motion. We report another striking and "counterintuitive" difference between the penetration dynamics observed in sand and in liquids. When pushing a top-closed shell (e.g., an upside down glass) into a liquid, the trapped air increases the buoyancy and opposes the penetration. It is more difficult to push a top capped cylinder than an opened one vertically into liquids. In contrast, the penetration is considerably easier in dense sand when cylinders are top capped. In this discrete and biphasic medium, the trapped air escapes from the shell, fluidizes the sand, and eases the motion.
authors.affiliation: Laboratoire Matière et Systèmes Complexes, UMR CNRS-Université Paris Diderot, France.
authors.forename: Raphaël
authors.initials: R
authors.lastname: Clément
authors.forename: Sylvain Courrech
authors.initials: SC
authors.lastname: du Pont
authors.forename: Mehdi
authors.initials: M
authors.lastname: Ould-Hamouda
authors.forename: Donald
authors.initials: D
authors.lastname: Duveau
authors.forename: Stéphane
authors.initials: S
authors.lastname: Douady