  We have investigated experimentally the influence of a magnetic interaction
between the grains on the compaction dynamics of a granular pile submitted to a
series of taps. The granular material used to perform this study is a mixture
of metallic and glass grains. The packing is immersed in an homogeneous
external magnetic field. The magnetic field induces an interaction between the
metallic grains that constitutes the tunable cohesion. The compaction
characteristic time and the asymptotic packing fraction have been measured as a
function of the Bond number which is the ratio between the cohesive magnetic
force and the grain weight. These measurements have been performed for
different fractions of metallic beads in the pile. When the pile is only made
of metallic grains, the characteristic compaction time increases as the square
root of the Bond number. While the asymptotic packing fraction decreases as the
inverse of the Bond number. For mixtures, when the fraction of magnetized
grains in the pile is increased, the characteristic time increases while the
asymptotic packing fraction decreases. A simple mesoscopic model based on the
formation of granular chains along the magnetic field direction is proposed to
explain the observed macroscopic properties of the packings.
