Coarse-Graining of a Physical Granular System

Results, including displacement, strain and stress fields, obtained by applying a resolution-controlled coarse-graining method to an experiment, comprised of a bidisperse system of photoelastic disks under pure shear, are presented. The paper reviews the experimental methods as well as the philosophical and technical bases of the coarse-graining methods employed in this study. Some fields reveal the emergence of a shear band while others do not. Correlations of the displacement fluctuations are shown to decay on a very small scale, of the order of a few particle diameters, even close to jamming. An unexpectedly simple relation between the particle rotation angles and the rotation field is reported. Implications of these and other findings are discussed.