Pancake bouncing on superhydrophobic surfaces

Liu, Yahua; Moevius, Lisa; Xu, Xinpeng; Qian, Tiezheng; Yeomans, J. M.; Wang, Zuankai

doi:10.1038/nphys2980

Cited by 839 publications

(761 citation statements)

References 30 publications

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“…Regular arrays of vertical pillars are commonly used in experiments in order to generate superhydrophobic surfaces [1,24,27,42]. For the experiments of Maitra et al [26] in which entrapped gas bubbles are observed in the substrate, the height of the pillars forming the substrateh is of the order of 10 µm.…”

Section: Gas Behaviour In the Substratementioning

confidence: 99%

Gas-cushioned droplet impacts with a thin layer of porous media

Hicks

Purvis

2015

J Eng Math

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The pre-impact gas cushioning behaviour of a droplet approaching touchdown onto a thin layer of porous substrate is investigated. Although the model is applicable to droplet impacts with any porous substrate of limited height, a thin layer of porous medium is used as an idealized approximation of a regular array of pillars, which are frequently used to produced superhydrophobic and superhydrophilic textured surface. Bubble entrainment is predicted across a range of permeabilities and substrate heights, as a result of a gas pressure build-up in the viscous-gas squeeze film decelerating the droplet free-surface immediately below the centre of the droplet. For a droplet of water of radius 1 mm and impact approach speed 0.5 m s −1 the change from a flat rigid impermeable plate to a porous substrate of height 5 µm and permeability 2.5 µm 2 reduces the initial horizontal extent of the trapped air pocket by 48%, as the porous substrate provides additional pathways through which the gas can escape. Further increases in either the substrate permeability or substrate height can entirely eliminate the formation of a trapped gas pocket in the initial touchdown phase, with the droplet then initially hitting the top surface of the porous media at a single point.Droplet impacts with a porous substrate are qualitatively compared to droplet impacts with a rough impermeable surface, which provides a second approximation for a textured surface. This indicates that only small pillars can be successfully modelled by the porous media approximation. The effect of surface tension on gas-cushioned droplet impacts with porous substrates is also investigated. In contrast to the numerical predictions of a droplet free-surface above flat plate, when a porous substrate is included the droplet free-surface touches down in finite time. Mathematically this is due to the regularization of the parabolic degeneracy associated with the small gas-film-height limit the gas squeeze film equation, by non-zero substrate permeability and height, and physically suggests that the level of surface roughness is a critical parameter in determining the initial touchdown characteristics.

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Section: Gas Behaviour In the Substratementioning

confidence: 99%

Gas-cushioned droplet impacts with a thin layer of porous media

Hicks

Purvis

2015

J Eng Math

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“…A rich body of literature already exists considering perpendicular drop impacts, with a droplet falling from above onto a flat surface [1][2][3][4][5][6][7][8]. These studies have revealed the underlying mechanics and possible outcomes of droplet splashing and are industrially relevant to processes such as spray coating, spray cooling and inkjet printing.…”

Section: Introductionmentioning

confidence: 99%

“…Droplet impacts on superhydrophobic (SHP) surfaces have gained recent attention because the non-wetting nature of such surfaces provokes entirely new impact behaviours such as rebounding, partial rebounding, and pancake bouncing [2,3]. Superhydrophobicity is a surface's ability to easily repel water droplets and resist wetting.…”

Section: Introductionmentioning

confidence: 99%

Splashing Threshold of Oblique Droplet Impacts on Surfaces of Various Wettability

Aboud

Kietzig

2015

Langmuir

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Oblique drop impacts were performed at high speeds (up to 27 m/s, We>9000) with millimetric water droplets, and a linear model was applied to define the oblique splashing threshold. Six different sample surfaces were tested: two substrate materials of different inherent surface wettability (PTFE and aluminum), each prepared with three different surface finishes (smooth, rough, and textured to support superhydrophobicity). Our choice of surfaces has allowed us to make several novel comparisons. Considering the inherent surface wettability, we discovered that PTFE, as the more hydrophobic surface, exhibits lower splashing thresholds than the hydrophilic surface of aluminum of comparable roughness. Furthermore, comparing oblique impacts on smooth and textured surfaces, we found that asymmetrical spreading and splashing behaviours occurred under a wide range of experimental conditions on our smooth surfaces, however impacts 2 occurring on textured surfaces were much more symmetrical, and one-sided splashing occurred only under very specific conditions. We attribute this difference to the air-trapping nature of textured superhydrophobic surfaces, which lowers the drag between the spreading lamella and the surface. The reduced drag affects oblique drop impacts by diminishing the effect of the tangential component of the impact velocity, causing the impact behaviour to be governed almost exclusively by the normal velocity. Finally, by comparing oblique impacts on superhydrophobic surfaces at different impact angles, we discovered that although the pinning transition between rebounding and partial rebounding is governed primarily by the normal impact velocity, there is also a weak dependence on the tangential velocity. As a result, pinning is inhibited in oblique impacts. This led to the observation of a new behaviour in highly oblique impacts on our superhydrophobic surfaces which we named the stretched rebound, where the droplet is extended into an elongated pancake shape and rebounds while still outstretched, without exhibiting a recession phase.

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“…Whether an impacting droplet 1 sticks or not to a solid surface has been conventionally controlled by functionalizing the target surface [2][3][4][5][6][7][8] or by using additives in the drop 9,10 . Here we report on an unexpected self-peeling phenomenon that can happen even on smooth untreated surfaces by taking advantage of the solidification of the impacting drop and the thermal properties of the substrate.…”

mentioning

confidence: 99%

Self-peeling of impacting droplets

2017

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Whether an impacting droplet 1 sticks or not to a solid surface has been conventionally controlled by functionalizing the target surface 2-8 or by using additives in the drop 9,10 . Here we report on an unexpected self-peeling phenomenon that can happen even on smooth untreated surfaces by taking advantage of the solidification of the impacting drop and the thermal properties of the substrate. We control this phenomenon by tuning the coupling of the shorttimescale fluid dynamics-leading to interfacial defects upon local freezing-and the longer-timescale thermo-mechanical stresses-leading to global deformation. We establish a regime map that predicts whether a molten metal drop impacting onto a colder substrate 11-14 will bounce, stick or self-peel. In many applications, avoiding adhesion of impacting droplets around designated target surfaces can be as crucial as bonding onto them to minimize waste or cleaning. These insights have broad applicability in processes ranging from thermal spraying 12,15 and additive manufacturing [16][17][18][19] to extreme ultraviolet lithography 20 .We release molten tin droplets of millimetric size (R = 0.95 ± 0.03 mm) from a nozzle, such that they hit a target surface with moderate velocity (v = 1.9 ± 0.1 m s −1 , see Methods). Figure 1a shows a comparison between the deposition and subsequent solidification of a droplet (initial temperature T d,0 = 240 • C) onto a silicon wafer (top) and a glass slide with ∼200 times lower thermal conductivity (bottom). While the solidified droplet (or splat) sticks to the glass surface, it unexpectedly detaches from the silicon surface: we call this phenomenon 'self-peeling' (see Fig. 1a(top) at 3 and 100 ms, the increasing air gap between droplet and surface being a manifestation of this self-peeling). Two main differences can be observed at this stage. First, interferometric profilometry (see Fig. 1b) confirms that the bottom of the droplet impacting on glass is perfectly flat, while it has a nearly spherical bending curvature κ = 16 m −1 when deposited on silicon.Second, both bottom interfaces show a texture 21 formed by annular regions spaced by air ridges (insets of Fig. 1b) that act as interfacial defects. Their number is much higher for silicon than for glass. The striking contrasts in bending and defect density suggest that a balance between thermally induced stress and adhesion determines whether or not a droplet can self-peel upon solidification.To unravel the competing phenomena, we study the dynamics of the impact using high-speed photography through a transparent surface (see Supplementary Movie 2). Immediately upon contact, micrometre-sized annular air ridges are being trapped. Along the splat radius, their height increases up to a few micrometres, and their width up to tens of micrometres (as measured by profilometry). We also extract the distance d between ridges and the contact line velocity v CL from Supplementary Movie 2, and find that the characteristic timescale τ = d/v CL to form these ridges increases radially. Figure 2a ...

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Pancake bouncing on superhydrophobic surfaces

Cited by 839 publications

References 30 publications

Gas-cushioned droplet impacts with a thin layer of porous media

Gas-cushioned droplet impacts with a thin layer of porous media

Splashing Threshold of Oblique Droplet Impacts on Surfaces of Various Wettability

Self-peeling of impacting droplets

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