The origin of granular convection in vertically vibrated particle beds: The differential shear flow field Kun Xue, Yixin Zheng, Baolong Fan, Fangfang Li and Chunhua Bai Eur. Phys. J. E, 36 1 (2013) 8 Published online: 29 January 2013 DOI: 10.1140/epje/i2013-13008-1