Sentence:  The concept of the strain <WSD>energy</WSD> as a Gibbs function difference * * f and exerting a force normal to the shearing face is compatible with the information obtained from optical birefringence studies of fluids undergoing shear . 
Meaning of the word marked with <WSD> tags:  (physics) a thermodynamic quantity equivalent to the capacity of a physical system to do work; the units of energy are joules or ergs