What’s inside a crumpled ball?

A worm inside a head of cabbage can learn a lot by looking around. From the curvature of the surrounding leaves, it can get a good sense of how far it is from the center of the cabbage, and from their orientation, it can figure out which direction provides the shortest route to the surface. A worm within a crumpled ball of paper is not so lucky: No simple local measurement of the surrounding layers points the way out of the ball or suggests how far the journey might be. That’s what Dominique Cambou and Narayanan Menon , of the University of Massachusetts, found in their three-dimensional structural study of highly crumpled sheets. The researchers crumpled the sheets by hand and imaged them with a commercial x-ray computerized tomography (CT) scanner; one of their reconstructed cross-sectional slices is shown in the figure. Then they analyzed the 3D images to obtain statistical distributions for mass, orientation, and curvature and found them to be almost perfectly homogeneous and isotropic, despite the asymmetric nature of the crumpling of individual sheets. Ultimately, they’d like to understand not just the crumpled structure but the crumpling process. For that, they are abandoning x-ray tomography, which takes several hours to produce a single 3D image, in favor of optical tomography, which is much faster. (A. D. Cambou, N. Menon, Proc. Natl. Acad. Sci. USA, in press, doi: 10.1073/pnas.1019192108 .)—Johanna Miller