title: Kelvin-Helmholtz instability in an ultrathin air film causes drop splashing on smooth surfaces.
abstract: 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.
authors.affiliation: Department of Physics, The Chinese University of Hong Kong, Hong Kong, China; and.
authors.forename: Yuan
authors.initials: Y
authors.lastname: Liu
authors.affiliation: Department of Physics, The Chinese University of Hong Kong, Hong Kong, China; and State Key Laboratory of Surface Physics, Department of Physics, Fudan University, Shanghai 200433, China xulei@phy.cuhk.edu.hk tanpeng@fudan.edu.cn.
authors.forename: Peng
authors.initials: P
authors.lastname: Tan
authors.affiliation: Department of Physics, The Chinese University of Hong Kong, Hong Kong, China; and xulei@phy.cuhk.edu.hk tanpeng@fudan.edu.cn.
authors.forename: Lei
authors.initials: L
authors.lastname: Xu
keywords: Kelvin–Helmholtz instability; drop impact; splash; thin air film