Comment on "Long-Lived Giant Number Fluctuations in a Swarming Granular Nematic"

doi:10.1126/science.1153456

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Comment on "Long-Lived Giant Number Fluctuations in a Swarming Granular Nematic"

I. S. Aranson¹^*, A. Snezhko¹, J. S. Olafsen² and J. S. Urbach³

¹ Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA.
² Department of Physics, Baylor University, One Bear Place, Box 97316, Waco, TX 76798–7316, USA.
³ Department of Physics, Georgetown University, Washington, DC 20057, USA.

Fig. 1. ${Delta}$ N/N^1/2 versus N in mechanically vibrated system N₀ = 1000, 1-mm steel spheres (top panel) and electrostatically vibrated system N₀ = 2800, 0.15-mm bronze balls (bottom panel). The triangles (procedure P1) and diamonds (procedure P2) show fluctuations for the lower value of the driving force when the systems separate into dense and dilute regions (a insets). The solid line indicates N^1/2 slope. Circles show fluctuations in the uniform state (b insets) corresponding to higher values of the driving. Dashed line shows dependence ${Delta}$ N/N^1/2 = (1 – N/N₀)^1/2. Squares show ${Delta}$ N/N^1/2 for the dilute regions of two-phase state shown in inset a. Plus symbols display ${Delta}$ N/N^1/2 for intermediate values of driving amplitude when the vibrated system shows localized transient high-density clusters. [View Larger Version of this Image (26K GIF file)]

Fig. 2. Density distribution averaged over entire duration of experiment reported by Narayan et al. (1) and taken along the slice depicted by dashed line in the inset. [View Larger Version of this Image (25K GIF file)]