The origin of granular convection in vertically vibrated particle beds: The differential shear flow fieldKun Xue, Yixin Zheng, Baolong Fan, Fangfang Li and Chunhua BaiEur. Phys. J. E, 36 1 (2013) 8DOI: https://doi.org/10.1140/epje/i2013-13008-1