Rigidity of acute angled corners for one phase Muskat interfaces

We consider the one-phase Muskat problem modeling the dynamics of the free boundary of a single fluid in porous media. In the stable regime, we prove local well-posedness for fluid interfaces that are general curves and can have singularities. In particular, the free boundary can have acute angle corners or cusps. Moreover, we show that isolated corners/cusps on the interface must be rigid, meaning the angle of the corner is preserved for a finite time, there is no rotation at the tip, the particle at the tip remains at the tip and the velocity of that particle at the tip points vertically downward.