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Early azimuthal instability during drop impact

Published online by Cambridge University Press:  13 June 2018

E. Q. Li
Affiliation:
Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900, Saudi Arabia Department of Modern Mechanics, University of Science and Technology of China, Hefei 230027, PR China
M.-J. Thoraval
Affiliation:
Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900, Saudi Arabia State Key Laboratory for Strength and Vibration of Mechanical Structures, Shaanxi Key Laboratory of Environment and Control for Flight Vehicle, International Center for Applied Mechanics, School of Aerospace, Xi’an Jiaotong University, Xi’an 710049, PR China
J. O. Marston
Affiliation:
Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900, Saudi Arabia Department of Chemical Engineering, Texas Tech University, Lubbock, TX 79409-3121, USA
S. T. Thoroddsen*
Affiliation:
Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900, Saudi Arabia
*
Email address for correspondence: [email protected]

Abstract

When a drop impacts on a liquid surface its bottom is deformed by lubrication pressure and it entraps a thin disc of air, thereby making contact along a ring at a finite distance from the centreline. The outer edge of this contact moves radially at high speed, governed by the impact velocity and bottom radius of the drop. Then at a certain radial location an ejecta sheet emerges from the neck connecting the two liquid masses. Herein, we show the formation of an azimuthal instability at the base of this ejecta, in the sharp corners at the two sides of the ejecta. They promote regular radial vorticity, thereby breaking the axisymmetry of the motions on the finest scales. The azimuthal wavenumber grows with the impact Weber number, based on the bottom curvature of the drop, reaching over 400 streamwise streaks around the periphery. This instability occurs first at Reynolds numbers ($Re$) of ${\sim}7000$, but for larger $Re$ is overtaken by the subsequent axisymmetric vortex shedding and their interactions can form intricate tangles, loops or chains.

Type
JFM Papers
Copyright
© 2018 Cambridge University Press 

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Li et al. supplementary movie 1

Movie 1: Video corresponding to Figure 4(a). The frame rate is 500 kfps.

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Supplementary material: PDF

Li et al. supplementary material

Supplementary material

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Li et al. supplementary movie 2

Movie 2: Video corresponding to Figure 4(b). The frame rate is 2 million fps.

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Li et al. supplementary movie 3

Movie 3: Video corresponding to Figure 5(a). The frame rate is 2 million fps.

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Li et al. supplementary movie 4

Movie 4: Video corresponding to Figure 5(b). The frame rate is 2 million fps.

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Li et al. supplementary movie 5

Movie 5: Video corresponding to Figure 8(a). The frame rate is 1 million fps.

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Li et al. supplementary movie 6

Movie 6: Video showing close-up of radial and axial vortices, with entrapment of bubble rings. Impact height is 59 cm and pixel resolution is 1 micron/px. The frame rate is 1 million fps.

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Video 16.9 MB