A Chiral Granular Gas of Rattlebacks(*)
-------------- J.-C. Tsai, Juan Rodriguez,
Fangfu Ye, J.P. Gollub and T.C. Lubensky
Inspired by rattleback toys,
we created small chiral wires that rotate in a preferred direction
on a vertically oscillating platform. We demonstrate experimentally
that angular momentum of rotation about particle centers of mass is
converted to angular momentum of center-of-mass motion in a granular
gas of these wires, and we introduce a continuum model that explains
our observations.
MPEG
Movies of Chiral Wires:
200
Wires ; 400 Wires. (~
Real-time speed, alpha = -135 degrees) [9.9MB
each]
Isoloated single wire of different
arm angles: alpha = +45
degrees ; alpha = -45
degrees ; alpha = +135 degrees
; alpha = -135 degrees.
(~30X fast forward) [2.5MB each]
There are very few materials to which angular
momentum can be delivered uniformly throughout the bulk
of a sample. One such material is a ferrofluid in which a rotating
or an oscillating magnetic field causes component magnetic
nanoparticles to spin. The rotating magnetic particles deliver angular
momentum to the fluid in which they are dispersed. Another is a colloidal
dispersion of anisotropic particles illuminated with circularly
polarized light.................
(* ) The Rattleback toy can be traced back to the ancient Egyptians
and Celts. It is an elongated object, shaped like the hull of a boat
with a curved bottom and a flat top, that has a preferred direction
of rotation. When placed on a hard surface and spun in its preferred
direction, it will rotate until its motion is damped out; when spun
in the opposite direction, it will wobble and then reverse its spin
to rotate in its preferred direction. If a vertical impulse is delivered
to one end of a rattleback, it
will wobble initially and then rotate in its preferred direction. Two
different types of rattlebacks are now readily available. The first
type is a piece of smooth plastic with an asymmetrically curved bottom
whose principal axes of curvature do not coincide with the symmetry
axes of the flat top surface. The second type, often referred to as
a Russian rattleback, consists of a hull with a symmetric bottom but
with two projections (turtles) on its flat deck whose horizontal orientation
can be changed at will. When the turtles face each other, the Russian
rattleback has no preferred direction of rotation; when they face in
opposite irections perpendicular to the long axis of the hull, it has
a preferred rotation direction defined by the direction the turtles
face.