SHERE Experiment on the International Space Station

The resistance of a fluid to an imposed flow is termed ‘viscosity’, and it is a fundamental material parameter by which manufacturers and end-users characterize a material. Normally, researchers place a material in a commercial instrument that imposes a simple rotational shearing flow and obtains a rate-dependent shear viscosity. While this level of characterization is sufficient for some processes, in typical industrial polymer processing operations the material experiences a complex flow history with both shear and extensional characteristics. For example, in fiber spinning, the fluid experiences a complex rotational shear flow as it flows through the spinneret head before entering a region of dominant axial elongation in the spinline.

Polymer behavior under these conditions is process-dependent and stems from their long chain structure. Polymers are typically hydrocarbon-based molecules composed of repeated molecular units and can contain hundreds to tens of thousands of these repeat units. The resulting long molecular chain is usually very flexible, allowing the polymer to coil, extend, and entangle with neighboring polymer chains. In its rest state, a typical polymer chain will assume a random coiled configuration. When exposed to a rotational shearing flow, this coil will align 45o to the flow direction and flip over and over again to coil the polymer chain. When exposed to an extensional flow, the coil extends axially and can be pulled taut if the flow is strong enough. Because polymers act like springs, more stress is required to stretch them to higher strains. This relationship between stress and extensional deformation rate (i.e., strain rate) is expressed as an extensional viscosity and is a fundamental material parameter independent of shear viscosity.