Acoustic rheometer

An acoustic rheometer employs a piezo-electric crystal that can easily launch a successive wave of extensions and contractions into the fluid. It applies an oscillating extensional stress to the system. System response can be interpreted in terms of extensional rheology.
It is well known that properties of viscoelastic fluid are characterised in shear rheology with a shear modulus "G", which links shear stress "Tij" and shear strain "Sij"
There is similar linear relationship in extensional rheology between extensional stress "P", extensional strain "S" and extensional modulus "K":
Detail theoretical analysis indicates that propagation of sound or ultrasound through a viscoelastic fluid depends on both shear modulus "G" and extensional modulus "K". It is convenient to introduce a combined longitudinal modulus "M":
There are simple equations that express longitudinal modulus in terms of acoustic properties, sound speed "V" and attenuation α
Acoustic rheometer measures sound speed and attenuation of ultrasound for a set of frequencies in the megahertz range. These measurable parameters can be converted into real and imaginary components of "longitudinal modulus". 
This type of rheometer works at much higher frequencies than others. It is suitable for studying effects with much shorter relaxation times than any other rheometer.