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Drops bouncing off macro-textured superhydrophobic surfaces
Published online by Cambridge University Press: 13 July 2017
Abstract
Recent experiments with droplets impacting macro-textured superhydrophobic surfaces revealed new regimes of bouncing with a remarkable reduction of the contact time. Here we present a comprehensive numerical study that reveals the physics behind these new bouncing regimes and quantifies the roles played by various external and internal forces. For the first time, accurate three-dimensional simulations involving realistic macro-textured surfaces are performed. After demonstrating that simulations reproduce experiments in a quantitative manner, the study is focused on analysing the flow situations beyond current experiments. We show that the experimentally observed reduction of contact time extends to higher Weber numbers, and analyse the role played by the texture density. Moreover, we report a nonlinear behaviour of the contact time with the increase of the Weber number for imperfectly coated textures, and study the impact on tilted surfaces in a wide range of Weber numbers. Finally, we present novel energy analysis techniques that elaborate and quantify the interplay between the kinetic and surface energy, and the role played by the dissipation for various Weber numbers.
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- © 2017 Cambridge University Press
References
Mazloomi Moqaddam et al. supplementary movie 1
Impact of a liquid drop on a flat superhydrophobic surface at We=26.25. The drop continues to spread after it hits the solid, after that it recoils and finally detaches from the surface at t*contact=tcontact⁄τ=2.26.
Mazloomi Moqaddam et al. supplementary movie 2
Impact of a liquid drop on tapered posts at We=6. The drop exhibits conventional bouncing (spreading, retracting and then leaving the substrate) with tcontact ≫t↑.
Mazloomi Moqaddam et al. supplementary movie 3
Bouncing off a tapered macrotexture at We=30. The drop rebounds with a pancake shape resulting in a fourfold reduction of contact time as compared with conventional bouncing.
Mazloomi Moqaddam et al. supplementary movie 4
Drop impact on a perfectly coated tapered macrotexture at a higher Weber number, We = 80. Liquid hits the base of the macrotexture, experiences a quick lateral extension and then detaches from the base, returns to the top of the posts, and finally the drop bounces off the surface.
Mazloomi Moqaddam et al. supplementary movie 5
Drop bouncing off a perfect coated tapered macrotexture at high Weber number, We = 120.
Mazloomi Moqaddam et al. supplementary movie 6
Impact on imperfectly coated posts at We = 80. Contact angle at the base of the posts and 10% above it set to θbottom=140°; for the rest of the solid the contact angle is θ=165°.
Mazloomi Moqaddam et al. supplementary movie 7
Impact on imperfectly coated posts at We = 120. Contact angle at the base of the posts and 10% above it set to θbottom=140°; for the rest of the solid the contact angle is θ=165°.
Mazloomi Moqaddam et al. supplementary movie 8
History of various components of the energy balance. Circle: Normalized kinetic energy K ̃; Downward triangle: Normalized surface energy S ̃; Upward triangle: Normalized dissipated energy Ξ ̃. Squares: Normalized energy balance K ̃+S ̃+Ξ ̃. Diamond: Normalized center-of-mass kinetic energy K cm. Impact on a perfectly coated SHS θ=165° at We = 30.
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