Abstract
When high-velocity droplets make grazing impact with a superhydrophobic surface, the droplets undergo significant deformation before recoiling and rebounding from the surface. Two distinct operating regimes describe the response of the reflected droplet stream after impact. In the first regime, the droplets remain discrete and uniform after the impact, but exhibit rotation and significant oscillations. This regime dominates if each droplet can clear the impact region before the next droplet arrives. In the second regime, droplets cannot avoid coalescing into a puddle at the surface. A secondary jet is ejected from the puddle which breaks up into a random droplet stream after traveling a short distance due to the lack of a forced unstable perturbation. The droplet-to-droplet spacing in the incoming stream determines which regime rules, with the critical value correlated by a Weber number. In the first regime, a detailed accounting of the kinetic and potential energies reveals that neither droplet oscillation nor rotation can fully account for the loss of translational kinetic energy, indicating significant internal circulation must occur in the droplets at impact. An application of droplet rebound from a superhydrophobic surface is proposed.
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Notes
3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-1-decanethiol.
While it is non-zero for the shallow-angle impacts we have investigated, \( E_{K}^{*} = 0 \) for the case of large, perpendicularly impacting droplets (Clanet et al. 2004).
In our experiments, even if all of the initial kinetic energy of a droplet was instantaneously converted to heat, the droplet temperature would rise only ~0.01°.
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Acknowledgments
The authors acknowledge the financial and technical support of Eastman Kodak Co. We particularly thank J. Gao, Y. Xie, and G. Hawkins. The Center for Electronic Imaging Systems (CEIS) at the University of Rochester also contributed financially. A. Tucker-Schwartz of UCLA suggested use of the HDFT-based electroless process for creating superhydrophobic surfaces, and P. Osborne fabricated components for the experimental test fixture.
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Chiarot, P.R., Jones, T.B. Grazing impact of continuous droplet streams with a superhydrophobic surface. Exp Fluids 49, 1109–1119 (2010). https://doi.org/10.1007/s00348-010-0860-x
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DOI: https://doi.org/10.1007/s00348-010-0860-x