SurfaceViscosity

trait SurfaceViscosity:
    .doc: """
Surface viscosity defines the viscous resistance in the tangential contact direction.
It has units

N/(m/s) = Ns/m = kg/s

an is a viscous damping coefficient acting on the relative slip velocity at the contact.

The viscous friction force is typically proportional to the slip velocity:

F_v = c_v * v_slip

where c_v is the surface viscosity.

This parameter is used together with a DryFriction model to regularize the friction response. A value of

c_v = 0

means that no viscous resistance is applied. In that case, the contact does not generate any viscous friction force.

Increasing c_v increases the viscous resistance, meaning that a larger tangential force is generated for a given slip velocity. Equivalently, for a given tangential force, the resulting slip velocity becomes smaller.

In this sense, the surface viscosity controls how strongly the dry-friction model is regularized by viscosity.
The inverse of this viscosity is called friction compliance.

This model works with Physics.Interactions.Dissipation.DefaultDryFriction
"""

    viscosity is Real