<< If you take a large collection of balls in contact and force some of them to spin, they can behave like a set of gears, with each ball rotating without slipping. Experiments and theory show that this synchronized motion can be predicted even when the initial motion involves seemingly random slipping between the spheres (like the skipping of gears). The researchers say that the ability to predict this collective motion could lead to three-dimensional gear systems in which manipulations of one or two spheres allows complete control of the rotational motion of the others >>
Michael Schirber. Focus: Balls as 3D Gears. June 24, 2016.
D. V. Stager, N. A. M. Araujo, and H. J. Herrmann. Prediction and Control of Slip-Free Rotation States in Sphere Assemblies. Phys. Rev. Lett. 116, 254301. Publ. 24 June 2016