  Making use of experimental and theoretical considerations, in this Letter we
deduce a criterion to determine the critical velocity for which a drop
impacting a smooth dry surface either spreads over the substrate or
disintegrates into smaller droplets. The derived equation, which expresses the
splash threshold velocity as a function of the material properties of the two
fluids involved, the drop radius and the mean free path of the molecules
composing the surrounding gaseous atmosphere, has been thoroughly validated
experimentally at normal atmospheric conditions using eight different liquids,
with viscosities ranging from $3\times 10^{-4}$ to $10^{-2}$ Pa$\cdot$s and
interfacial tension coefficients which vary between $17$ and $72$
mN$\cdot$m$^{-1}$.
