Cited research Phys. Rev. Lett. 105, 034501 (2010)
The popular desktop toy Newton's cradle consists of a row of suspended metallic spheres. When the sphere at one end is pulled back and released, it strikes the row, causing the sphere at the other end to fly up with a similar velocity.
Christine Hrenya and her colleagues at the University of Colorado at Boulder wanted to study collisions in liquid, and so dipped the balls of a three-sphere system similar to a Newton's cradle into oil (pictured). The authors, like others before them, could not reproduce the characteristic cradle motion. Models revealed why: the narrow fluid bridge between the balls caused them to stick together after they collided, hindering the transfer of momentum. Reducing the volume of the fluid bridge restored the cradle action.
The team says the findings could be applicable to more complex systems involving wet particles, such as mixing during pharmaceutical processing.