title: Forces encountered by a sphere during impact into sand.
abstract: We describe direct measurements of the acceleration of an object impacting on a loosely packed granular bed under various pressures, using an instrumented sphere. The sphere acts as a noninvasive probe that measures and continuously transmits the acceleration as it penetrates into the sand, using a radio signal. The time-resolved acceleration of the sphere reveals the detailed dynamics during the impact that cannot be resolved from the position information alone. Because of the unobstructed penetration, we see a downward acceleration of the sphere at the moment the air cavity collapses. The compressibility of the sand bed is observed through the oscillatory behavior of the acceleration curve for various ambient pressures; it shows the influence of interstitial air on the compaction of the sand as a function of time.
authors.affiliation: Laboratoire de Physique de l'École Normale Supérieure de Lyon, CNRS, Université de Lyon, F-69364 Lyon, France.
authors.forename: Sylvain
authors.initials: S
authors.lastname: Joubaud
authors.affiliation: Physics of Fluids Group, University of Twente, P.O. Box 217, 7500 AE Enschede, Netherlands.
authors.forename: Tess
authors.initials: T
authors.lastname: Homan
authors.affiliation: smartINST S.A.S., 213 rue de Gerland, 69007 Lyon, France.
authors.forename: Y
authors.initials: Y
authors.lastname: Gasteuil
authors.affiliation: Physics of Fluids Group, University of Twente, P.O. Box 217, 7500 AE Enschede, Netherlands.
authors.forename: Detlef
authors.initials: D
authors.lastname: Lohse
authors.affiliation: Physics of Fluids Group, University of Twente, P.O. Box 217, 7500 AE Enschede, Netherlands.
authors.forename: Devaraj
authors.initials: D
authors.lastname: van der Meer