Modes of response in horizontally vibrated granular matter

When forced by horizontal vibrations; granular material responds over a wid e range of time scales. These include rapid shearing, harmonic response, pe riod-doubling and long-time convection. Using high-speed video imaging and analysis, we map out the corresponding two-dimensional velocity fields. The presence of a free surface parallel to the shear direction and the fact th at gravity acts perpendicularly to that direction lead to a decoupling of t he system into two horizontal lavers of different character. We observe thi s decoupling on all time scales. implying that, in contrast to vertically e xcited systems, models without internal degrees of freedom are unable to de scribe the observed response modes.