Negative Normal Stress

Negative normal sterss, discovered by physicists a quarter-century ago, is a poorly understood attraction between two parallel plates that occurs when certain complex fluids flow between them. First demonstrated in liquid crystalline polymers---the main ingredients of such technologically important materials as Kevlar and Zylon---this rare phenomenon has now been observed in two very different systems. The two new reports (Lin-Gibson et al., Physical Review Letters, 30 January 2004, Montesi et al., Physical Review Letters, 6 February 2004) offer insight into solving the puzzle of what creates these "negative" stresses in nature. Such a solution would increase our understanding and control of these technologically important materials.

To visualize this phenomenon, imagine a liquid in between a pair of parallel plates. For classical "Newtonian" liquids like water, glycerin, and oil, "shearing" the plates, or sliding one with respect to the other, will only create a tangential stress, or a force parallel to the plates. However, some liquids, such as polymer melts and solutions, behave differently when sheared. In addition to the tangential stress, they react with a positive normal stress--a force that acts perpendicular to the plates. The net effect is to push the plates apart.

About 25 years ago, researchers studying liquid crystalline polymers (very stiff linear molecules dissolved in a low-molecular weight solvent) observed that subjecting these fluids to shear made the plates want to pull together. Such negative normal stress has proven rare and somewhat controversial. Occasionally reported in a few complex soft materials since then, rheologists are now starting to accept and study them.

In one new example of this phenomenon (contact Erik Hobbie, NIST, erik.hobbie@nist.gov), a semi-dilute suspension of carbon nanotubes is dispersed in a Newtonian polymer melt. When these suspensions are subjected to weak-to-modest shearing flows (the parallel plate experiment described above), the tubes entangle with each other and form diffuse aggregates. Because these aggregates are composed of incredibly strong carbon nanotubes, they have remarkable elasticity. This elasticity causes the aggregates to grow and roll like logs.

The other new system (Matteo Pasquali, Rice University, mp@rice.edu) is a concentrated suspension of soft water droplets in a Newtonian oil called an emulsion. In this case, there is an attractive force between the droplets. When this suspension is put in the same type of modest shearing flows, the droplets come together to form exactly the same type of log-rolling aggregates, and similarly exhibit negative normal stresses.

The striking difference between these two systems is that one is a fairly dilute suspension of nanofibers while the second is a concentrated suspension of soft spheres, and yet they show the same response, suggesting some underlying universal phenomenon which is now being further investigated.