  When a fast-moving drop impacts onto a smooth substrate, splashing will be
produced at the edge of the expanding liquid sheet. This ubiquitous phenomenon
lacks a fundamental understanding. Combining experiment with model, we
illustrate that the ultrathin air film trapped under the expanding liquid front
triggers splashing. Because this film is thinner than the mean free path of air
molecules, the interior airflow transfers momentum with an unusually high
velocity comparable to the speed of sound and generates a stress 10 times
stronger than the airflow in common situations. Such a large stress initiates
Kelvin Helmholtz instabilities at small length scales and effectively produces
splashing. Our model agrees quantitatively with experimental verifications and
brings a fundamental understanding to the ubiquitous phenomenon of drop
splashing on smooth surfaces.
