  We study the instability development during a viscous liquid drop impacting a
smooth substrate, using high speed photography. The onset time of the
instability highly depends on the surrounding air pressure and the liquid
viscosity: it decreases with air pressure with the power of minus two, and
increases linearly with the liquid viscosity. From the real-time dynamics
measurements, we construct a model which compares the destabilizing stress from
air with the stabilizing stress from liquid viscosity. Under this model, our
experimental results indicate that at the instability onset time, the two
stresses balance each other. This model also illustrates the different
mechanisms for the inviscid and viscous regimes previously observed: the
inviscid regime is stabilized by the surface tension and the viscous regime is
stabilized by the liquid viscosity.
