Why Flour Sticks but Rice Falls Straight Through - The Science of Sifting When particulate materials such as rice or whole grains are passed through a sieve, their relatively large mass and smooth surface texture enable gravitational forces to dominate over any weak adhesive interactions with the mesh. In contrast, flour consists of fine particles with a low mass-to-surface-area ratio. The resulting high specific surface area increases the relative significance of surface forces, including cohes