In 1998 we discovered that vertically vibrated rods of aspect ratio L/D = 12 and low volume fraction spontaneously stand up on their ends and organize into large rotating vortices. We gave our rods to the group of Arshad Kudrolli who followed up on this observation and reported their results in a fasinating paper. In the movies below, volume fraction is defined as the fraction of a monolayer of closed-packed rods aligned perpendicularly to the bottom of the container. So a volume fraction of 1 corresponds to a single closed packed layer of vertical rods.

A few minutes of vibration at 5 -7 g and 100 Hz causes the rods of L/D = 12 to stand up and form vortices. The movie is in real time with two 30 second cuts.

L/D = 24. Time lapse with 48 to 1 time compression. The entire pile slowly rotates at first, then once the rods reorient vertically they begin to rotate in vorticies. The time for vortex formation increases with increasing L/D. In this case the transition from horizontal to vertical orientation of the rods occurs after an hour of vibration at 50 Hz (hence the time lapse movie). Once aligned, the rods vortex surprisingly rapidly.

More Granular Movies:

The appearance of vorticies with increasing rod aspect ratio in a square container. The volume fraction of the transition to vorticing decreases with increasing aspect ratio.

L/D=3 no vorticies

L/D=5 small vorticies

L/D=6 vortexing

L/D=12 big vorticies (Aluminum or Nylon rods)
Aluminum:

Nylon:

L/D=36 still vortexing. Look into the eye of the hurricane. Low volume fraction vortices have open eyes.

L/D=48 large rods vortexing

At a fixed aspect ratio there is a minimun volume fraction below which vortices do not form. For L/D = 12 this threshold is about 30%. The series of movies is of vortex formation in suspensions of L/D = 12 as a function of volume fraction. All movies are in real time. Vortex formation takes several minutes with 100 Hz excitation of ampliltude of 5 - 7 g.

Aspect ratio L/D = 12, 100 Hz, 2D volume fraction 30%.
Too dilute to form vortices.

Aspect ratio L/D = 12, 100 Hz, 2D volume fraction 40%.
Vortices begin to form.

Aspect ratio L/D = 12, 100 Hz, 2D volume fraction 50%.

Aspect ratio L/D = 12, 100 Hz, 2D volume fraction 60%.

Aspect ratio L/D = 12, 100 Hz, 2D volume fraction 70%.

Aspect ratio L/D = 12, 100 Hz, 2D volume fraction 80%.