We study the instability development during a viscous liquid drop impacting asmooth substrate, using high speed photography. The onset time of theinstability highly depends on the surrounding air pressure and the liquidviscosity: it decreases with air pressure with the power of minus two, andincreases linearly with the liquid viscosity. From the real-time dynamicsmeasurements, we construct a model which compares the destabilizing stress fromair with the stabilizing stress from liquid viscosity. Under this model, ourexperimental results indicate that at the instability onset time, the twostresses balance each other. This model also illustrates the differentmechanisms for the inviscid and viscous regimes previously observed: theinviscid regime is stabilized by the surface tension and the viscous regime isstabilized by the liquid viscosity.
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