When a rigid circular cylinder or sphere is placed on a
rough inclined plane it will roll down the plane. When the
experiment is repeated with a rigid cube it will slide down
the plane. If the object is deformable a variety of motions
become possible; the motion of elastic bodies and fluid drops
depends on the interfacial energies of the materials, the
roughness of the interfaces, the size of the objects, etc. This
is because a deformable body maintains contact with the surface over a finite area. For a viscous fluid droplet, two possible motions may ensue. If the droplet partially wets the
surface it slides along it, while if the droplet is nonwetting, it
can roll on the surface, much like an elastic body when
viewed from the exterior. Here we consider the motion of a
small nonwetting droplet forced by a weak gravitational
field. A classic example of this motion is exhibited by a
droplet of mercury on an inclined plane and is probably the
origin of the name quicksilver, after the Latin Argentum Vivum for the swiftly moving droplet of the silvery liquid.