Unravelling the mysteries of coiling ropes

Unravelling the mysteries of coiling ropes
http://physicsworld.com/cws/article/news/31564;jsessionid=F2B86CF9053C797422EE964D036D7FBB

If you carefully lower a rope onto the floor it will probably form a
neat coil. While most people wouldn't give this a second thought, an
international team of physicists has done a series of experiments and
numerical simulations to work out why. Their new insights into
coiling could shed light on the behaviour of an important class of
materials called "elastic ropes", which includes DNA molecules and
structural reinforcing rods in buildings (Phys. Rev. Lett. 99
154302).

Neil Ribe at the University of Paris-7 and colleagues in Iran and the
Netherlands used a reel powered by an electric motor to feed ordinary
rope or thread down through a hole and onto a glass or paper plate
below. The rate of descent and the distance between the reel and the
plate could be changed, allowing the team to study coiling over a
wide range of speeds and drop lengths. A second set of similar
experiments looked at the coiling of soft strands of spaghetti.

Ribe told physicsworld.com that the team is the first to perform
controlled lab experiments on coiling and their use of different
materials allowed them to build up a comprehensive understanding of
why some ropes coil and others don't.

According to Ribe, one surprise result is that the coiling always
occurred at several different "frequencies" for fixed values of the
feed rate and fall distance. These frequencies correspond to the
vibrational modes of the nearly vertical upper part of the falling
rope. They discovered that coiling occurs when any of these
frequencies matches the angular frequency at which bottom end of the
rope whirls into a coil.

The team were also able to describe their observations in a numerical
model that treated coiling as a fine balancing act between elastic,
gravitational and inertial forces acting on the rope. According to
Ribe the model was able to reproduce the observed multi-frequency
nature of coiling.

"This is an exciting paper, which details the many different coiling
patterns in an elastic rope," says Herbert Huppert, a geophysicist at
the UK's University of Cambridge who has an interest in such
materials. "The agreement [between experiment and numerical model]
gives confidence to the detailed and complex nature of this sub-field
of highly nonlinear dynamical systems, in contrast to many other
situations for which the description is at best qualitative. Many a
physicist is going to enjoy playing with his pasta after reading this
paper."

Everyday ropes are the simplest example of elastic ropes -- a class
of materials that includes encompassing electrical cables, plant
vines, DNA and steel rods. Elastic ropes can act as nonlinear
dynamical systems, the behaviour of which can be very difficult to
understand. Ribe and colleagues hope that their simple experiments
will cast light on the complex nature of these common materials.
.