Do you know how paper is made? It starts with trees that are harvested, turned into chips, and dissolved into cellulose fibers. The fibers are then carried by a turbulent water flow, which deposits them onto forming meshes, where the water is removed and the fibers form a paper sheet. While papermaking is among the oldest materials manufacturing processes developed by humans, many of its steps have been developed by trial and error over centuries, and we are only now developing a predictive scientific understanding that allows us to explain and optimize this process.
One important aspect of the papermaking process is the multiphase flow of solid fibers and liquid water. This flow is especially complex because the fibers are anisotropic—that is, their properties and behavior depend on how they are oriented in space, whereas spherical particles have the same properties and behave the same way regardless of how they are oriented.
The Voth research group at Wesleyan University has recently used multiple high-speed cameras to directly image these processes in three-dimensional space. A recent scientific paper describes their measurements of the rotation and alignment.
The turbulence of the flow helps randomize the fiber orientations, so the paper will be isotropic—the same in all directions—even though the individual fibers comprising it are not. This is important in producing high quality paper. You can easily observe the effects of this yourself: Take a piece of newspaper and try to tear it both horizontally and vertically. Which one tears more easily? Now try this with a piece of copy paper. Copy paper needs to be much more isotropic, so manufacturers must carefully control the turbulence just before the paper forms.
Fig. 1 (Click to enlarge) The plot above shows an experimentally measured trajectory of a fiber moving in intense turbulence, as it simultaneously moves, flips, and twists, much like a gymnast performing a floor routine. The individual rods show the orientation of the fiber at each point, with a red color indicating faster rotation speed.