the antonym oF "slippery" is surely grippy or maybe sticky . There is a perfectly valid word in engineering - "stiction" which is friction without motion cf the friction during motion. Stiction is why it takes more force to get something started than when it is moving slowly and why gentle pushes can result in surges. But I ain't never see a word like stictious.

I think words like torpor and turgid would suffice for human relationships (or volatile at the other end of the continuum).