Free Surface Deformation and Cusp Formation During the Drainage of a   Very Viscous Fluid
  We report an experimental study of the deformation of the free surface of a very viscous fluid when drained out of a container through a circular orifice. At some critical height of the fluid, the free surface deforms and forms a dimple. At later times, this dimple becomes a cusp. We found that the height of the dimple measured from the bottom of the tank as well as the curvature of the dimple before the cusp formation, follows power laws behaviors. We found that this behavior was due to an interplay between viscous pressure and surface tension 
Sahraoui Chaieb