Follow the bouncing balls! Three-dimensional imaging of flowing granular suspensions

Granular materials are difficult to study in three dimensions because of their opacity: Only their surface is directly visible. In close collaboration with Losert’s group, we have built an “index matched scanning” device, which allows us to study the full three-dimensional 3D structure and flow of grains suspended in a liquid. The device works by immersing transparent particles in a fluorescently dyed transparent fluid with the same refractive index. The resulting clear medium is imaged slice by slice by illuminating the medium with a laser sheet and recording the illuminated cross sections with a camera Fig. 1 a . We use this device to probe the motion of a very dense suspension, driven very slowly at =5 10−2 rps, by a rotating disk at the bottom of a box Fig. 1 b . The 3D particle positions of virtually all the particles in the dense suspension can be tracked. Particle trajectories, examples of which are shown in Fig. 1 c , can be traced over time. In Fig. 2 we show, from different angles, snapshots of the instantaneous 3D flow field. Close to the bottom the particles comove with the rotating disk as shown in Fig. 2 a . In Fig. 2 b half of all the particles are left out to reveal the 3D structure of the shearband inside the suspension. This work was financially supported by the Dutch physics foundation FOM.