Asymptotic Behavior of Solutions of a Free Boundary Problem Modelling   the Growth of Tumors with Stokes Equations
  We study a free boundary problem modelling the growth of non-necrotic tumors with fluid-like tissues. The fluid velocity satisfies Stokes equations with a source determined by the proliferation rate of tumor cells which depends on the concentration of nutrients, subject to a boundary condition with stress tensor effected by surface tension. It is easy to prove that this problem has a unique radially symmetric stationary solution. By using a functional approach, we prove that there exists a threshold value $\gamma_*>0$ for the surface tension coefficient $\gamma$, such that in the case $\gamma>\gamma_*$ this radially symmetric stationary solution is asymptotically stable under small non-radial perturbations, whereas in the opposite case it is unstable. 
Junde Wu and Shangbin Cui