Negative normal stress

Negative normal stress in sheared complex fluids has been observed in two different systems. Imagine a liquid between a pair of parallel plates. For classical Newtonian liquids like water, glycerin, and oil, sliding one plate with respect to the other creates only a tangential stress, a force parallel to the plates. But some complex fluids, such as polymer melts and solutions, also react with a positive normal stress, a force that pushes the plates apart. About 25 years ago, researchers found that shearing some liquid crystalline polymers made the plates want to pull together. Such negative normal stress has proven rare and somewhat controversial. In one of the new examples, a semi-dilute suspension of carbon nanotubes, dispersed in a Newtonian polymer melt, was subjected to modest shearing flows. The tubes formed diffuse aggregates that elongated into cylinders and rolled like strings of dough in the hands of a chef. The other new system is an emulsion of water droplets suspended in an oil. When subjected to the same type of modest shearing flows, the droplets came together to form exactly the same type of dough-rolling aggregates and similarly exhibited negative normal stresses. That the same response was seen in both the fairly dilute suspension of nanofibers and the concentrated suspension of soft spheres suggests some underlying universal rheological principle. (S. Lin-Gibson et al., Phys. Rev. Lett. 92, 048302, 2004 http://dx.doi.org/10.1103/PhysRevLett.92.048302 ; A. Montesi et al., Phys. Rev. Lett. 92, 058303, 2004 http://dx.doi.org/10.1103/PhysRevLett.92.058303 ).