Surfaces designed so that drops do not adhere to them but instead bounce off have received substantial attention because of their ability to stay dry1,2,3,4, self-clean5,6,7 and resist icing8,9,10. A drop striking a non-wetting surface of this type will spread out to a maximum diameter11,12,13,14 and then recoil to such an extent that it completely rebounds and leaves the solid material15,16,17,18. The amount of time that the drop is in contact with the solid—the ‘contact time’—depends on the inertia and capillarity of the drop1, internal dissipation19 and surface–liquid interactions20,21,22. And because contact time controls the extent to which mass, momentum and energy are exchanged between drop and surface23, it is often advantageous to minimize it. The conventional approach has been to minimize surface–liquid interactions that can lead to contact line pinning20,21,22; but even in the absence of any surface interactions, drop hydrodynamics imposes a minimum contact time that was conventionally assumed to be attained with axisymmetrically spreading and recoiling drops21,24. Here we demonstrate that it is possible to reduce the contact time below this theoretical limit by using superhydrophobic surfaces with a morphology that redistributes the liquid mass and thereby alters the drop hydrodynamics. We show theoretically and experimentally that this approach allows us to reduce the overall contact time between a bouncing drop and a surface below what was previously thought possible.
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K.K.V. was supported by a DARPA Young Faculty Award, the MIT Energy Initiative, an NSF Career Award (0952564) and the MIT-Deshpande Center. J.C.B. was supported by an NSF Postdoctoral Research Fellowship (DMS1004678). We thank T. Buonassisi, Y. Cui, A. Paxson and J. Bales at MIT for the use of equipment, and D. Quéré, G. McKinley, J. Bush, K. Corriveau and P. Barbone for reading and commenting on the manuscript.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Impact of molten tin droplets (250 °C) on microscopically textured silicon substrates without (top row) and with (bottom row) macroscopic ridges.
The substrate temperature is 150 °C, 82 °C below the droplet freezing point. In both cases, the droplets are able to bounce off the substrate.
Extended Data Figure 2 Impact of molten tin droplets (250 °C) on microscopically textured silicon substrates without contacting (top row) and contacting (bottom row) a macroscopic ridge.
Here the substrate is maintained at 125 °C (a subcooling of 107 °C). When the droplet hits the macroscopic ridge, it is able to bounce off in 6.8 ms, whereas when impact is not on the ridge, the droplet is arrested owing to solidification. For more details, see Supplementary Video 5.
Extended Data Figure 3 Impact of molten tin droplets (250 °C) on microscopically textured silicon substrates without (top row) and with (bottom row) ridges.
Droplets impacting the ridge surface continued to bounce off until the substrate was cooled to about 50 °C, indicating that a significantly large subcooling (∼182 °C) is needed to arrest the droplets on the ridge surface. Droplets impacting the surface without ridges (maintained at 50 °C) is arrested owing to solidification.
a, Top view of the anodized aluminium oxide (AAO) surface showing the macro-scale ridges (height ∼100 μm, width ∼200 μm); scale bar, 5 mm. b, Magnified SEM image of a single ridge showing micropits; scale bar, 100 μm. c, Further magnified SEM image showing nanoscale pores; scale bar, 1 μm.
a, SEM image of the copper oxide nano-textured macro-ridge (height ∼100 μm, width ∼200 μm); scale bar, 100 μm. b, A magnified image, showing spiky nano-textures; scale bar, 1 μm.
a, A vein on the wing of a Morpho butterfly (M. didius); b, a vein on a nasturtium leaf (T. majus L.). Scale bars in a left to right; 200 μm, 50 μm and 1 μm: scale bars in b left to right; 200 μm, 10 μm and 2 μm.
a–c, Diagrams of the ridge case (a), the simplistic case where a droplet splits before impact (b), and the generalized (n-split parts) ridge case (c).
This file contains Supplementary Discussions on the Freezing of Impacting Droplets and Droplet Splitting and Contact Time. (PDF 425 kb)
This video corresponds to Figure1b,e, showing equal-sized water droplets impacting two different superhydrophobic surfaces: the left one (control) without any macrotexture giving axisymmetric recoil (the typical scenario), and the right one with designed macrotexture giving non-axisymmetric recoil in which the drop centre actively assists in retraction. Both surfaces are superhydrophobic, and made of silicon, textured by laser-ablation. (MOV 8348 kb)
This video shows simultaneous side and top views of water droplet impingement on both the control and macrotextured surface, shown in Figure1a, b and Figure 2c, d, respectively. On the control surface, the droplet spreads and recoils axisymmetrically; whereas on the macrotextured surface, the radial symmetry of droplet is broken, creating a “zipping” effect that reduces the overall contact time. Both surfaces are superhydrophobic, and made of silicon, textured by laser-ablation. (MOV 9137 kb)
This video shows the contact time differences on three different superhydrophobic surfaces (micropillar array, control surface, and macrotextured surface) with the side views of water droplet impingement. All surfaces were made of silicon. While the micropillar array takes the longest (18.0 ms) to repel the impinging water droplet, the control surface takes much shorter contact time (12.4 ms) due to minimal pinning, representing the theoretical minimum contact time under these conditions. The surface with designed macrotexture however goes even beyond the theoretical limit by repelling the impinging droplet in a mere 7.8 ms. (MOV 6386 kb)
This video contains the videos of a water drop impacting on artificial (AAO, and nano-textured copper oxide) and natural superhydrophobic surfaces (a leaf of Nasturtium plant, wing of a Morpho butterfly, and lotus leaf), shown in Figure 4 Both the Nasturtium leaf and butterfly wing have multiple ridges or veins that trigger non-axisymmetric recoil and reduce the contact time of the bouncing drop, as well as both the artificially made surfaces. In contrast, the impinging droplet on the lotus leaf is nearly axisymmetric throughout impact resulting in a contact time close to the theoretical limit. (MOV 5405 kb)
This video corresponds to Extended Data Figure 2, showing the impact of molten tin droplets (250 °C) onto laser-ablated silicon without and with ridge, maintained at 125 °C (subcooling = 107 °C). When the droplet impacts the ridge, it is able to bounce off the surface in 6.8 ms. In contrast, droplet impacting the substrate without ridge sticks to the surface due to solidification at the contact area. (MOV 3874 kb)
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Bird, J., Dhiman, R., Kwon, HM. et al. Reducing the contact time of a bouncing drop. Nature 503, 385–388 (2013). https://doi.org/10.1038/nature12740
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