POLYFLOW - How to define a Coulomb friction law?

Coulomb friction law is such that the tangential component of the effort is proportional to the normal component. How can this be defined in POLYFLOW by using the available laws?
The user can select an asymptotic law given by
Fs = -k.(1-exp(-V/Vc))
where k denotes the upper limit of the friction force, while Vc is a (small) velocity beyond which the friction will be equal to k. Through the mechanism of PMAT, a pressure dependence can be defined for k. As a result, the tangential force will be dependent on p.
Most probably, an appropriate evolution scheme will be necessary, for properly handling the non-linearities.
It is worth mentioning that, for an incompressible fluid, the pressure is defined with respect to an additive constant. Boundary conditions have to be selected in accordance.

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