Sentence:  The concept of the strain energy as a Gibbs <WSD>function</WSD> 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:  (mathematics) a mathematical relation such that each element of a given set (the domain of the function) is associated with an element of another set (the range of the function)