“…Besides the fundamental well-posedness issue also other important features like the stability of stationary solutions [13,14,16,20,26,28,33,36], parabolic smoothing properties [24][25][26], the zero surface tension limit [8,31], and the degenerate limit when the thickness of the fluid layers (or a certain nondimensional parameter) vanishes [15,22,37] have been investigated in this context. We also refer to [19,21] for results on the Hele-Shaw problem with surface tension effects, which is the one-phase version of the Muskat problem, and to [34,35,38] for results on the related Verigin problem with surface tension.…”