# Synopsis: Fluid-Induced Orbital Motion in Zero Gravity

#### Spontaneous Orbiting of Two Spheres Levitated in a Vibrated Liquid

H. A. Pacheco-Martinez, L. Liao, R. J. A. Hill, Michael R. Swift, and R. M. Bowley

Published April 11, 2013

Particles floating “weightless” in a fluid would appear free to go where they please. However, observations show that suspended particles will self-organize because of fluid-mediated forces. A new experiment described in Physical Review Letters explores the hydrodynamic interactions between two small beads levitating in a gravity-canceling magnetic field. When the system is shaken, the beads come together in a mutual attraction and—as the shaking increases—begin an orbital dance. The authors believe this opens the door to further studies of particle-fluid mixtures in the absence of gravity.

On Earth, particles suspended in a fluid are largely confined to the depth where buoyancy and gravity forces balance out, so studies of their self-organization have often been limited to two dimensions. In real zero gravity, as on the International Space Station, suspended particles can assemble in three dimensions, but very few experiments have explored what influence hydrodynamic forces will have in this environment.

To recreate real zero gravity on Earth, Hector Pacheco-Martinez and colleagues from the University of Nottingham, UK, use a strongly varying magnetic field that can levitate small particles—as shown in previous studies. Inside their $17$-tesla magnetic system, the team placed a fluid-filled cell with two identical, millimeter-sized spheres. To induce fluid flow, they shook the cell up and down at a rate of around $20$ hertz. In response, the beads moved towards each other until they were side-by-side touching. As the amplitude of the shaking increased, the team observed the beads orbiting around each other in the horizontal plane (perpendicular to the shaking direction). The researchers also performed computer simulations that showed the orbiting arises from flow vortices sprouting out from the contact point between beads. – Michael Schirber