Principal Investigator Gareth McKinley
The Extensional Rheology Experiment (ERE) is part of NASA's microgravity flight program, in which an experiment is flown in one of NASA's microgravity platforms.
Gravitational body forces cause appreciable sagging of fluid filaments when they are deformed in a uniaxial extensional flow. This sagging is most notable for low deformation rates where strain-hardening is not significant. Removing these forces will allow one to probe a wider parametric range of strain rates, while simultaneously measuring the total stress exhibited by the deforming fluid, the molecular level orientation of the polymer chains in the fluid, and the velocity field of the fluid.
First, an understand what rheology means is needed. In short, rheology is the study of the fluid response of a material to an imposed stress. Extensional rheology focuses on pulling on a piece of material in a purely extensional manner, i.e. no shear. This is the type of flow you would see if you stretched a rubber band. A shearing flow would be the response of the rubber band if you laid it on a flat surface and slid your hand over it. The rubber band extends slightly, then tumbles over and contracts, repeating this action over and over again. In an extensional flow, the rubber band will continue to elongate until you can not stretch it any further.
It is easy to see that the amount of force required to stretch the rubber band is considerably greater than than required to shear the rubber band. The stress, or the force/area, is higher in the extensional deformation due to extent of the deformation. Entropic forces, which want to randomize the configuration of the rubber band, resist the deformation, which aligns the macromolecules that make up the rubber band. Whence, the stress increases as the deformation increases.
How does this apply to polymer materials? A polymer material is anything that contains long molecules composed of repeat units. Take a small molecule, like ethane. Link several ethane units together, forming a chain. Keep doing this until you have roughly 10,000-100,000 ethanes, or repeat units. This is now a polymer molecule with the name polyethylene, the material from which milk jugs are made.
A polymer chain is like a string of spaghetti, which can wind around, double over on itself, and become entangled with other polymer chains. The more entangled the chains, the more difficult it is to deform them, in either shear or in extension. This resistance to deformation rate is called viscosity. The greater the resistance to deformation, the greater the viscosity.