A sphere moving down the surface of a static sphere and a simple phase   diagram
  A small sphere placed on the top of a big static frictionless sphere, slips until it leaves the surface at an angle $\theta_{l}=\cos^{-1}{2/3}$. On the other extreme, if the surface of the big sphere has coefficient of static friction, $\mu_s\to\infty$, the small sphere starts rolling and continues to do so until it leaves the surface at an angle $\theta_{l} =\cos^{-1}{10/17}$. In the case where, $0\leq\mu_s<\infty$, we get a simple phase diagram. The three phases are pure rolling, rolling with slipping and detached state. One phase line separates pure rolling from rolling with slipping. This diagram is obtained when stopping angles for pure rolling are plotted against static friction coefficients $\mu_s$. Study in this article is restricted to the case when the mobile sphere starts at the top of the static sphere with infinitesimal kinetic energy. 
V. Jayanth (Mechanical Engineering), C. Raghunandan (EEE), Anindya   Kumar Biswas (Physics BITS-Pilani Goa Campus, Vasco, Goa)