Extensional Rheology Basics

Rheology is a term coined by Eugene C. Bingham from the greek phrase "panta rhei" or "everything flows. It is the study of (i) how materials deform under a well-defined stress or (ii) what stress is required to deform a material a specified amount. There are two fundamental flow deformation fields that are most commonly used in rheological studies: (1) simple shear and (2) uniaxial extension. The simplicity of the flow field makes it easy to analytically determine material parameters such as the viscosity, modulus, relaxation times, and much more.

There is a fundamental difference between simple shear and uniaxial extension. Whereas in shear the cross-sectional area of the material is fixed in the presence of flow, typically extensional flow induces a change in cross-sectional area of the material with time. Therefore the definition of strain and stress requires accurate measurement of both force and cross-sectional area. For the working pricinple of the VADER 1000, known as filament stretching rheology the strain and stress are given by: