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Probing the nanoscale: the first contact of an impacting drop

Published online by Cambridge University Press:  16 November 2015

E. Q. Li Open the ORCID record for E. Q. Li [Opens in a new window] ,
I. U. Vakarelski Open the ORCID record for I. U. Vakarelski [Opens in a new window]  and
S. T. Thoroddsen Open the ORCID record for S. T. Thoroddsen [Opens in a new window]
E. Q. Li
Affiliation:
Division of Physical Sciences and Engineering and Clean Combustion Research Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
I. U. Vakarelski
Affiliation:
Division of Physical Sciences and Engineering and Clean Combustion Research Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
S. T. Thoroddsen*
Affiliation:
Division of Physical Sciences and Engineering and Clean Combustion Research Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
*
†Email address for correspondence: [email protected]
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Abstract

When a drop impacts onto a solid surface, the lubrication pressure in the air deforms its bottom into a dimple. This makes the initial contact with the substrate occur not at a point but along a ring, thereby entrapping a central disc of air. We use ultra-high-speed imaging, with 200 ns time resolution, to observe the structure of this first contact between the liquid and a smooth solid surface. For a water drop impacting onto regular glass we observe a ring of microbubbles, due to multiple initial contacts just before the formation of the fully wetted outer section. These contacts are spaced by a few microns and quickly grow in size until they meet, thereby leaving behind a ring of microbubbles marking the original air-disc diameter. On the other hand, no microbubbles are left behind when the drop impacts onto molecularly smooth mica sheets. We thereby conclude that the localized contacts are due to nanometric roughness of the glass surface, and the presence of the microbubbles can therefore distinguish between glass with 10 nm roughness and perfectly smooth glass. We contrast this entrapment topology with the initial contact of a drop impacting onto a film of extremely viscous immiscible liquid, where the initial contact appears to be continuous along the ring. Here, an azimuthal instability occurs during the rapid contraction at the triple line, also leaving behind microbubbles. For low impact velocities the nature of the initial contact changes to one initiated by ruptures of a thin lubricating air film.

JFM classification

Interfacial Flows (free surface): Contact lines Drops and Bubbles: Drops and Bubbles Interfacial Flows (free surface): Fingering instability
Type
Rapids
Information
Journal of Fluid Mechanics , Volume 785 , 25 December 2015 , R2
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Copyright
© 2015 Cambridge University Press 

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Supplementary material: Image

Li et al. supplementary material

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Li et al. supplementary material

Supplementary material

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Li et al. supplementary movie

Ring of microbubbles produced by a water drop impacting on Corning microscope glass slide, for conditions in Figure 3(a). Recording frame rate is 5 million fps. Horizontal extent: 0.96 mm. Vertical extent: 0.80 mm.

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Video 5.8 MB

Li et al. supplementary movie

Ring of microbubbles produced by a water drop impacting on Corning microscope glass slide, for conditions in Figure 3(a). Recording frame rate is 5 million fps. Horizontal extent: 0.96 mm. Vertical extent: 0.80 mm.

Download Li et al. supplementary movie(Video)
Video 7.3 MB

Li et al. supplementary movie

Drop impact on mica surface for conditions in Figure 3(b). Recording frame rate is 5 million fps. Horizontal extent: 0.67 mm. Vertical extent: 0.67 mm.

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Video 4.2 MB

Li et al. supplementary movie

Drop impact on mica surface for conditions in Figure 3(b). Recording frame rate is 5 million fps. Horizontal extent: 0.67 mm. Vertical extent: 0.67 mm.

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Video 4.4 MB

Li et al. supplementary movie

Drop impact on Fisher microscope slide, for conditions in Figure 3(c). Recording frame rate is 5 million fps. Horizontal extent: 0.32 mm. Vertical extent: 0.43 mm.

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Video 4.5 MB

Li et al. supplementary movie

Drop impact on Fisher microscope slide, for conditions in Figure 3(c). Recording frame rate is 5 million fps. Horizontal extent: 0.32 mm. Vertical extent: 0.43 mm.

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Video 4.3 MB

Li et al. supplementary movie

Drop impact on 20 million cSt silicone oil, for conditions in Figure 4(a). Recording frame rate is 5 million fps. Horizontal extent: 0.86 mm. Vertical extent: 0.72 mm.

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Video 3.7 MB

Li et al. supplementary movie

Drop impact on 20 million cSt silicone oil, for conditions in Figure 4(a). Recording frame rate is 5 million fps. Horizontal extent: 0.86 mm. Vertical extent: 0.72 mm.

Download Li et al. supplementary movie(Video)
Video 4.9 MB

Li et al. supplementary movie

Drop impact on 20 million cSt silicone oil, for conditions in Figure 4(b). Recording frame rate is 2 million fps. Horizontal extent: 0.67 mm. Vertical extent: 0.58 mm.

Download Li et al. supplementary movie(Video)
Video 6.1 MB

Li et al. supplementary movie

Drop impact on 20 million cSt silicone oil, for conditions in Figure 4(b). Recording frame rate is 2 million fps. Horizontal extent: 0.67 mm. Vertical extent: 0.58 mm.

Download Li et al. supplementary movie(Video)
Video 6 MB

Li et al. supplementary movie

Rupture of the thin air-film between a sliding water drop impacting on 20 million cSt silicone oil, for conditions in Figure 5. Recording frame rate is 5 million fps. Horizontal extent: 0.52 mm. Vertical extent: 0.43 mm.

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Video 5.4 MB

Li et al. supplementary movie

Rupture of the thin air-film between a sliding water drop impacting on 20 million cSt silicone oil, for conditions in Figure 5. Recording frame rate is 5 million fps. Horizontal extent: 0.52 mm. Vertical extent: 0.43 mm.

Download Li et al. supplementary movie(Video)
Video 6 MB