Abstract
Droplet impact on a miscible liquid film finds several applications in scientific and engineering problems. Here, we report that how pre-existing perturbations of a target liquid film, in the form of surface waves, change the impact dynamics and splashing phenomena. For this purpose, we release dyed water droplets and study droplet impact dynamics on a wavy water film, where the waves atop the liquid film are generated by imposing low-frequency two-dimensional (2D) lateral (horizontal) vibrations on glass substrates. Methylene blue-dyed droplet was employed to facilitate the observation of the details of mixing phenomenon given its importance in some applications such as casting homogeneous thin-film coatings. The results are compared with the impact on a quiescent/smooth liquid surface. Splashing is found to be more pronounced for the wavy surface given the additional energy imparted by vibrations and waves. An increase in the crown diameter is also observed for the vibrated wavy liquid film. It is observed that mixing is enhanced significantly in the presence of vibrations and surface waves. The interaction between the pre-existing surface waves and the waves formed after the droplet impact is also studied.
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Abbreviations
- \({D}_{0}\) :
-
Droplet initial diameter
- \({D}_{ic}\) :
-
Crown inner diameter
- f :
-
Vibration frequency
- h :
-
Mean film thickness
- h* :
-
Non-dimensional film thickness = h/D0
- H c :
-
Crown height
- K sp :
-
Splashing threshold
- Oh :
-
Ohnesorge number
- Re :
-
Reynolds number
- t :
-
Time
- U 0 :
-
Droplet velocity at touchdown
- We :
-
Weber number
- σ :
-
Surface tension
- ρ :
-
Density
- μ :
-
Dynamic viscosity
- η :
-
Wave amplitude
- λ :
-
Wavelength
- τ :
-
Non-dimensional time = U0t/D0
- Π c :
-
Non-dimensional crown height = Hc/D0
- Φ ic :
-
Non-dimensional crown diameter = Dic/D0
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Acknowledgements
Authors acknowledge financial support from the Shanghai Municipal Education Commission via the Oriental Scholar fund and the funding from the National Natural Science Foundation of China (NSFC, Grant No. 51550110229). TK acknowledges the scholarship from the Higher Education Commission (HEC), Pakistan.
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TK and NEE designed the experiments based on the research direction outlined by ME. TK and NEE performed the experiments. TK analyzed the data, prepared the figures and drafted the manuscript. ME revised the manuscript and improved the interpretation and presentation of the results. ME secured research funding for the project.
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Khan, T., Ersoy, N.E. & Eslamian, M. Droplet impact on a wavy liquid film under multi-axis lateral vibrations. Exp Fluids 61, 173 (2020). https://doi.org/10.1007/s00348-020-03010-9
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DOI: https://doi.org/10.1007/s00348-020-03010-9