I don't quite understand the nature of friction forces, in simple mechanics and also in fluid mechanics. Is the nature of these forces, intermolecular forces ie. chemical nature? Or is it microscopical mechanical forces, ie actual mechanical contact nature? Or both? And which nature is for which case, for the various situations, eg tyre on road or fluid flowing in pipe?
The nature of frictional forces is primarily electromagnetic forces, in case of both rough surfaces and in case of adhesive surfaces. A rough surface acts like it has teeth on it, which goes into the grooves of another surface sliding on it, and the teeth apply a contact force (which is fundamentally electromagnetic) on the groove walls, which we see as frictional force.
In case of adhesive surfaces like a wide cellotape, if you try to rub your finger on the adhesive part, it resists your movement. This is partially due to "chemical forces", that is attractive interactions between glue material and your skin. But even that is primarily an electromagnetic force, like all chemical bonds.
Easiest way to think about it is as an electrostatic force between the atoms/molecules of two different materials causing repulsion. This is based purely on the EM field of the atoms/object, and their proximity to one another. This is the case for both "simple mechanics and fluid mechanics" so for both a "tyre on a road" and "fluid flowing in pipe."
Is the nature of these forces, intermolecular forces ie. chemical nature? Or is it microscopical mechanical forces, ie actual mechanical contact nature? Or both?
It's both in the sense that "contact" occurs when the electrostatic forces between two objects stop them from passing through one another.
Hope that helps!