I just recently pick up houdini and trying to achieve the effect that the particles in the red part of the cube have smaller targetstiffness than the others but without success. Although I have study the ‘KeyframedGrain’ example and other similar projects, I still quite confused about how some of the parameters works. I already create ‘targetstiffness’ attribute in Attribute wrangle but it doesn't seem to have any effect at all.

I attach my project file below. I hope someone can help me out with this.
I don't know if my approach is valid or not, feel free to modify it