Writing a friction-based pair style
In this section, a frictional force is implemented on spherical atoms that depends on the tangential time of contact and the tangential velocity difference of two interacting atoms, via the Cundall-Strack scheme (https://doi.org/10.1007/978-94-017-2653-5_22).
In the Cundall-Strack scheme, also known as Coulomb friction, a tangential frictional force, 
, acts on two spheres in contact that increases with the relative tangential velocities of the spheres up to an upper bound value. It is assumed that, during prolonged contact between the spheres, an imaginary spring of spring constant 
is extended by a length 
owing to the relative tangential velocity (
) and it exerts an opposing force on the spheres to mimic the frictional force. In addition, a drag force is exerted based on a drag coefficient (A) and 
that also acts tangentially to impede rotational motion.
The frictional and drag forces are pegged at an upper limit defined by 
, where 
is the...
