Japanese researchers have succeeded in deriving a theoretical formula that quantitatively predicts the wetting and spreading behavior of droplets that collide with the flat surface of a solid material. Although the behavior of droplets colliding with a solid surface looks simple superficially, it is actually quite complicated due to interrelated factors such as surface roughness, fluid motion, and wettability (ease of liquid adherence) of the solid surface by the liquid. In the past, researchers from all over the world have attempted to make quantitative predictions about the extent of wetted areas through experimentation, theory and numerical analysis, but prediction, particularly during slow collisions, have not yet been realized.