Patterns in Flowing Sand
Patterns are ubiquitous in nature from convection rolls in thin layers
heated from below to the ripples of sand dunes shaped by the wind. Often
the formation of these patterns reveals the inner workings of the material
in which the patterns form. For granular systems, this is of particular
interest because complete descriptions of the equations of motion elude
us, making the behavior of granular flow decidedly unpredictable. Thus,
our experimental and numerical observations of robust stripe patterns of
sand flowing easily down a rough inclined plane may provide an important
ingredient for improved theory and practical modeling of this challenging
material. The stripes we observe are parallel to the downstream flow
direction, have alternating faster and slower regions, and appear to slide
along on top of highly-agitated "boiling" granular material - reminiscent
of the Leidenfrost effect where liquid is lifted by its vapor above a hot
surface.
***
BG11570
Magnet moves superconducting vortex like a hockey stick moves a puck
A movie created by magnetic force microscopy (MFM) shows the effect of its magnet (hockey stick) passing a vortex (puck) back and forth and pushing it between nanoholes in a superconductor. Analyzing such motion allows us to measure the tiny force (1 pN) that traps a vortex in a hole. The escape of vortices from traps limits the utility of superconductors because it can give rise to energy dissipation. Therefore, understanding vortex motion and trapping is important for technologies such as magnetic resonance imaging (MRI), power transmission and superconducting quantum interference devices (SQUIDs). In this work we use MFM at temperatures down to 5 K to image vortices in a superconducting thin film patterned with an array of 50 nm holes that are spaced 100 nm apart. Although the array is well ordered, we are sensitive to slight imperfections which enhance the trapping of vortices a specific sites and affect the way a vortex moves.
