Natural Science Forum / Physics / General Physics / October 2007

Unravelling the mysteries of coiling ropes
  http://physicsworld.com/cws/article/news/31564;jsessionid=F2B86CF9053C797422EE96
4D036D7FBB

  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.