We discuss the benefits of using the so-called implicit type constitutive relations introduced by K. R.
Rajagopal, J. Fluid Mech. 550, 243-249 (2006) and K.
R. Rajagopal, Appl.
Math. 48, 279-319 (2003) in the description of the behaviour of non-Newtonian fluids. In particular, we focus on the benefits of using the implicit type constitutive relations in the mathematical modelling of fluids in which the shear stress/shear rate dependence is given by an S-shaped curve, and in modelling of fluids that exhibit nonzero normal stress differences.
We also discuss a thermodynamical framework that allows one to cope with the implicit type constitutive relations.