The addition of high-molecular-weight polymers in very low
concentrations to a liquid can dramatically change its rheology,
e.g. 5 parts per million can halve the pressure drop of turbulent pipe
The traditional rheological model is based on several linearisations.
Important modifications in the nonlinear
regime, which I reviewed in 1988 with J.M. Rallison, include the novel
suggestion that hydrodynamic interactions lead to a hysteresis in which a large
deformation created by a strong flow can be maintained by a weaker
Recent computer simulations have demonstrated the need for a
new viscous contribution to the stress which increases rapidly with
This comes from the way a chain unfolds by merging straightened
A simplified one-dimensional `kinks' model of this unfolding
provides the scaling of the new term.
These ideas are now being explored in Stanford, Harvard and Monash.