shearing sheet: particle pulled away to avoid aggregation

In the original implementation of collision resolving, particles may be submerged into themselves due to self-gravity. One solution is to pull the contacting particles away artificially.
See the movie for example, with surfacedensity= 840kg/m2 and particledensity=900kg/m3, 
original: https://www.youtube.com/watch?v=lWkNq1kLjtk
revised: https://www.youtube.com/watch?v=SfLj8-MWGTE
There should be several ways to do this, such as conserving total momentum or conserving angular momentum.