# Stiction, friction and sticking

If we have two flat surfaces touching each other and apply a force parallel to the surfaces, one may move relative to the other.
An example would be a polymer block gliding on another.

Dependent on the surface properties, they could smoothly and easily glide,
or be hard to move, or just stick together, with no relative movement.
The first is an example for low friction,
the second shows high friction, and in the third case, there is very high friction, so high that the force we apply is not enough to move it, called static friction. Alternatively, one could say they stick together.

But with some surface properties, there are further variants:
The surfaces can at first stick together, and then begin to move.

And, different from that, there is the case when the surfaces intermittently stick together and move with high friction. I think this case is called stiction, but I have seen this term being used for the previous case also.

I would like to know what the term stiction is referring to,
and ideally, understand how the last two cases are related.

First, there's a lot of friction models out there. Search on "modelling friction" or for a deep dive, "tribology friction".

Friction is one of those behaviors where the useful models are often inadequate, and the adequate models are often very difficult to formulate and apply.

The three most prevalent models (because they're workable and pretty good) are:

Coloumb friction. This is the friction with constant magnitude, in the direction of relative motion. It's a not-bad model for friction between two surfaces once they're moving relative to one another.

Viscous friction. This is friction with a magnitude that rises with rising relative velocity (truly viscous friction would have magnitude equal to the square of the magnitude of the relative velocity).

Static friction. This is your "sticktion". This is the phenomenon where two surfaces will "bind" up until some force is reached, then will let go at which point Coloumb and possibly viscous friction will take over.

According to Webster, Stiction is the force required to cause one body in contact with another to begin to move.

And from Wikipedia we get: Any solid objects pressing against each other (but not sliding) will require some threshold of force parallel to the surface of contact in order to overcome static cohesion. Stiction is a threshold, not a continuous force.

It sounds like both your cases are examples of stiction. In the 2nd case the area of the blocks encounter a large static friction area and stop sliding. Then stiction comes into play again. But ideally it means that two blocks in contact must experience stiction to start sliding.