We study the dynamics of a spinning sphere whose south pole is in touch with the surface of a still body of liquid. When the sphere is turning fast enough, the fluid rises up the sphere, reaches the equator and is flung out as a fountain of droplets. Although the fountain forms for water a weakly viscous fluid, and for propylene glycol a much more viscous fluid, the dynamical situation is different for each. For flows at mid-latitudes on the sphere, we formulate the dynamical equations for the two liquids in terms of Newton's law in a rotating frame, noting that the Coriolis force plays an essential role in both liquids, and obtain qualitative agreement with observations. We also discuss the possible roles played by other forces.

A simplified theoretical model for the linear Rayleigh–Taylor instability of finite thickness elastic–plastic solid constantly accelerated by finite thickness viscous fluid is performed. With the irrotational assumption, it is possible to consider viscosity, surface tension, elasticity or plasticity effects simultaneously. The model considers thicknesses at rigid wall boundary conditions with the velocity potentials, and deals with solid elastic–plastic transition and fluid viscosity based on the velocity continuity and force equilibrium at contact interface. The complete analytical expressions of the amplitude motion equation, the growth rate, and the instability boundary are obtained for arbitrary Atwood number, viscosity, thicknesses of solid and fluid. The thicknesses effects of two materials on the growth rate and the instability boundary are discussed.