title: Airflows generated by an impacting drop.
abstract: A drop impacting a solid surface with sufficient velocity will splash and emit many small droplets. However, lowering the ambient air pressure suppresses splashing completely. This effect, robustly found for different liquid and substrate properties, raises the fundamental question of how air affects a spreading drop. In a combined experimental and numerical study we characterize the flow of air induced by the drop after it hits the substrate, using a modified Schlieren optics technique combined with high-speed video imaging and Lattice-Boltzmann simulations. Our experiments reveal the emergence of air structures on different length scales. On large scales, the airflow induced in the drop's wake leads to vortex structures due to interaction with the substrate. On smaller scales, we visualize a ring structure above the outer edge of the spreading liquid generated by the spreading of the drop. Our simulations reveal the interaction between the wake vorticity and the flows originating from the rapidly escaping air from below the impacting drop. We show that the vorticity is governed by a balance between inertial and viscous forces in the air, and is unrelated to the splashing threshold.
authors.affiliation: The James Franck and Enrico Fermi Institutes and The Department of Physics, The University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Irmgard
authors.initials: I
authors.lastname: Bischofberger
authors.affiliation: Department of Mechanical Engineering, City College of City University of New York, New York, 10031, USA.
authors.forename: Bahni
authors.initials: B
authors.lastname: Ray
authors.affiliation: Department of Chemical Engineering, City College of City University of New York, New York, 10031, USA and Benjamin Levich Institute, City College of City University of New York, New York, NY 10031, USA. irmgard.bischofberger@gmail.com.
authors.forename: Jeffrey F
authors.initials: JF
authors.lastname: Morris
authors.affiliation: Department of Mechanical Engineering, City College of City University of New York, New York, 10031, USA.
authors.forename: Taehun
authors.initials: T
authors.lastname: Lee
authors.affiliation: The James Franck and Enrico Fermi Institutes and The Department of Physics, The University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Sidney R
authors.initials: SR
authors.lastname: Nagel