Title: Wave patterns in shaken sand
Speaker: Dr. P. Umbanhower
Speaker Info: University of Pennsylvania
Special Note: Note changed location because of video
Experiments on vertically vibrated granular layers reveal, at a critical acceleration amplitude (about 2.5 g's), a well-defined hysteretic transition from a flat surface to standing wave patterns oscillating at half the drive frequency. At low frequencies, the patterns are squares, while at high frequencies, the patterns are stripes. A continuous transition between these states occurs when the vibration frequency is varied at constant acceleration amplitude. For frequencies in the middle of this transition regime, and at acceleration amplitudes below the onset of planar wave patterns, highly localized, non-propagating, standing waves appear. These localized excitations, which we term "oscillons", come in two varieties. Like oscillons repel, while opposites attract and bind to form diatoms, tetramers, and polymer-like chains. Finally, for constant frequency and varying acceleration amplitude, a series of bifurcations in the layer center of mass motion leads to the formation of hexagons, kinks, and other more complex patterns.Date: Friday, November 21, 1997
In addition to a qualitative discussion of these phenomena, I will also present a quantitative description of the dependence of pattern wavelength and pattern selection on various experimental parameters including the frequency, acceleration amplitude, particle size, and layer depth.