Air entrapment under an impacting drop

Thoroddsen, Sigurður T.; Etoh, Takeharu; Takehara, Kohsei

doi:10.1017/s0022112002003427

Cited by 177 publications

(195 citation statements)

References 20 publications

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“…The behaviour here matches exactly that observed above a smooth flat impermeable plate, which is to be expected as the squeeze film equation (37). In this case there is an initial pressure build-up underneath the descending droplet causing the descending droplet free-surface to decelerate.…”

Section: Gas Behaviour In the Substratesupporting

confidence: 82%

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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 Substratesupporting

confidence: 82%

“…An iteration is then performed between these two equations; convergence of the solution to within 10 −6 typically takes a limited number of iterations. This method offers a considerable saving in simulation time over previous work [15,16] that discretised (37), particularly when surface tension is included.…”

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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“…During drop impacts the lubrication pressure in the air creates a dimple below the drop, thereby entrapping a bottom disk of air, as shown by Thoroddsen et al (2003). Similar film thickness distributions are observed in numerous drainage films, as reviewed by Chan, Klaseboer & Manica (2011), and more recently by high-speed observations by van der Veen et al (2012) for a water drop impacting a solid surface.…”

Section: Film Thickness and Speed Of Rupturementioning

confidence: 59%

“…Figure 3(b) shows how the depth of the ring of ruptures moves progressively closer to the original free surface, as the drop impact velocity increases. This approaches the early entrapment of an air disk as studied by Thoroddsen et al (2003).…”

Section: Bubble Morphology: Hanging Necklaces and Bubble Chandeliersmentioning

confidence: 95%

“…470 S. T. Thoroddsen, M.-J. Thoraval, K. Takehara and T. G Thoroddsen, Etoh & Takehara (2003) imaged detailed dynamics of the entrapment of a central bubble, along with a handful of realizations for film breakup for water drops. However, Mills, Saylor & Testik (2012) showed convincingly that the water case suffers from highly random film ruptures and is not repeatable, even if surfactants are included.…”

Section: Introductionmentioning

confidence: 99%

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Micro-bubble morphologies following drop impacts onto a pool surface

et al. 2012

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When a drop impacts at low velocity onto a pool surface, a hemispheric air layer cushions and can delay direct contact. Herein we use ultra-high-speed video to study the rupture of this layer, to explain the resulting variety of observed distribution of bubbles. The size and distribution of micro-bubbles is determined by the number and location of the primary punctures. Isolated holes lead to the formation of bubble necklaces when the edges of two growing holes meet, whereas bubble nets are produced by regular shedding of micro-bubbles from a sawtooth edge instability. For the most viscous liquids the air film contracts more rapidly than the capillary–viscous velocity through repeated spontaneous ruptures of the edge. From the speed of hole opening and the total volume of micro-bubbles we conclude that the air sheet ruptures when its thickness approaches ${\ensuremath{\sim} }100~\mathrm{nm} $.

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Uniform Spread of High‐Speed Drops on Superhydrophobic Surface by Live‐Oligomeric Surfactant Jamming

et al. 2019

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self-cleaning and antifouling ability for repelling the deposition of other materials and liquid confining properties for enhancing printing resolution and avoiding coffee-ring effects. [13] However, inertial water drops impacting superhydrophobic surfaces can bounce off quickly or splash violently. [14][15][16][17][18][19][20][21][22][23] Undesired rebound and splash cause material waste [24] and weaken the related performance and efficiency. Many attempts have been conducted to promote water drop spreading on hydrophobic surfaces by using polymers [1,23,[25][26][27][28] or surfactants. [22,[29][30][31][32] However, these two methods still have drawbacks for achieving drop deposition, not to mention uniform spreading: 1) Polymer additives can delay drop retraction but leave drops with hemispherical shape and nonuniform material distribution on the hydrophobic substrate.2) The poor wettability and large mole cular weight of polymer additives restrict the ejecting process during inkjet printing. 3) Surfactant additives can promote drop spreading in a static state owing to the reduced surface tension (γ); [33] however, the low surface tension increases the instability of the impacting drop and leads to drop splashing with satellite droplets, according to the Kelvin-Helmholtz instability, [34] k max ∼ 2ρ a U r 2 /3γ (ρ a is the air density). It is therefore a great challenge for uniform shape spreading on superhydrophobic surfaces without any loss of the drops. Here, we show a new and simple strategy for uniform round-shape drop spreading on superhydrophobic surfaces after high-speed impact, up to 5.0 m s −1 , by utilizing live-oligomeric surfactant jamming, diethylenetriamine/sodium dodecyl sulfate (triamine/SDS). The live-oligomeric surfactant, which noncovalently constructed by SDS and triamine through electrostatic interaction, has a dynamic equilibrium between monomer surfactant and oligomeric surfactant. Figure 1 shows the contrast spread dynamics of a liveoligomeric surfactant drop and other drops impacting superhydrophobic surfaces at an impacting velocity (U ) of 2.42 m s −1 from side and bottom views (Movie S1, Supporting Information). The diameter (D 0 ) of pure water and the surfactant drops is ≈2.25 and 1.90-2.00 mm, respectively (Figure S1, Supporting Information for experimental setup). The Weber number (We), We = ρDV 2 /γ, of water, SDS, N2C3/SDS, triamine/SDS, and 12-3-12-3-12 is 182. 68, 295.29, 358.29, 383.00, and 292.29, respectively. The superhydrophobic surface [35] composed of random micro-nanostructures of typical size and spacing of Inkjet printing of water-based inks on superhydrophobic surfaces is important in high-resolution bioarray detection, chemical analysis, and highperformance electronic circuits and devices. Obtaining uniform spreading of a drop on a superhydrophobic surface is still a challenge. Uniform round drop spreading and high-resolution inkjet printing patterns are demonstrated on superhydrophobic surfaces without splash or rebound after high-speed impacting by introducing...

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Air entrapment under an impacting drop

Cited by 177 publications

References 20 publications

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

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

Micro-bubble morphologies following drop impacts onto a pool surface

Uniform Spread of High‐Speed Drops on Superhydrophobic Surface by Live‐Oligomeric Surfactant Jamming

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