The air bubble entrapped under a drop impacting on a solid surface

Thoroddsen, Sigurður T.; Etoh, Takeharu; Takehara, Kohsei; Ootsuka, N.; Hatsuki, Yuya

doi:10.1017/s0022112005006919

Cited by 196 publications

(240 citation statements)

References 13 publications

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“…Eventually the decelerated droplet free-surface directly below the centre of the droplet is overtaken by the surrounding free surface, with the pressure profiles similarly bifurcating to produce maxima where the separation between free surface and substrate are least. Touchdown then goes on to occur around a ring some horizontal distance away from the point directly below the centre of an air bubble, leading to a trapped pocket of gas, which subsequently may evolve to form a bubble, as observed in experiments [4,6,38]. In a vacuum, the initial touchdown would occur at time t = 0.…”

Section: Gas Behaviour In the Substratementioning

confidence: 95%

“…The free-surface profiles for k > 0 actually reach touchdown, while for the impermeable plate case the numerical solution of the parabolic equation (38) fails to converge just prior to touchdown. Numerical convergence is harder to achieve for the flat plate case as the pressures and pressure gradients just before touchdown are larger than their counterparts with a permeable substrate.…”

Section: Gas Behaviour In the Substratementioning

confidence: 97%

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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 Substratementioning

confidence: 95%

Section: Gas Behaviour In the Substratementioning

confidence: 97%

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

Hicks

Purvis

2015

J Eng Math

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“…It can have detrimental effect on the uniformity in coating processes and affect conduction or conductivity through the interface between a solidified drop and the substrate. This central bubble was first observed in snapshot photographs (Chandra & Avedisian (1991), Thoroddsen & Sakakibara (1998)), but Thoroddsen et al (2005) used high-speed video to image the initial entrapment of the air-disk and modeled its subsequent contraction into the central bubble. They also identified a dark ring, which often marked the initial diameter of the air disk, proposing it was due to the trapping of micro-bubbles.…”

Section: Introductionmentioning

confidence: 99%

“…This drop was imaged through the bottom glass by Thoroddsen et al (2005) and later verified by Lee et al (2012) using X-rays. Liu et al (2013) and Driscoll & Nagel (2011) used interferometry with approximately 7 µs time-resolution to observe the compression of the air-disc for high impact velocities.…”

Section: Introductionmentioning

confidence: 99%

“…This formula is in perfect correspondence with the recent experiments of , if one uses the bottom radius of curvature of the drop R b for R. This is mandated by the significant shape oscillations of the large water drops used in those experiments. Thoroddsen et al (2005) measured the size of the central bubble for high impact velocities, but for low impact velocities, Bouwhuis et al (2012) identified a local parameter, where the size of the entrapped bubble is maximum. Mandre et al (2009) investigated the compressibility of the air, which occurs for higher impact velocities.…”

Section: Introductionmentioning

confidence: 99%

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Probing the nanoscale: the first contact of an impacting drop

Vakarelski

Thoroddsen

2015

J. Fluid Mech.

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When a drop impacts 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-highspeed 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 micro-bubbles, owing 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 micro-bubbles 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 micro-bubbles can therefore distinguish between glass with 10 nm roughness from 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 continuous along the ring. Here an azimuthal instability occurs during the rapid contraction at the triple line, also leaving behind micro-bubbles. For low impact velocities the nature of the initial contact changes to one initiated by ruptures of a thin lubricating air film.

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Macrodrop‐Impact‐Mediated Fluid Microdispensing

et al. 2021

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High‐resolution fluid dispensing techniques play a critical role in modern digital microfluidics, micro‐biosensing, and advanced fabrication. Though most of existing dispensers can achieve precise and high‐throughput fluid dispensing, they suffer from some inherent problems, such as specially fabricated dispensing micronozzles/microtips, large operating systems, low volume tunability, and poor performance for low surface tension liquids and liquids containing solid/liquid additives. Herein, the authors propose a facile, low‐frequency micro dispensing technique based on the Rayleigh–Plateau instability of singular liquid jets, which are stimulated by the air cavity collapse arising in the impact of microliter drops on non‐wetting surfaces. This novel dispensing strategy is capable to produce single microdrops of low‐viscosity liquids with a tunable volume from picoliters to nanoliters, and the operational surface tension range covers most laboratory solvents. The dispensing function is implemented without using small‐dimension nozzles/tips and enables handling diverse complex liquids. Moreover, the rather simple operating platform allows the integration of the whole dispensing function into a handy portable device with a low cost. Employing this microdispensing technique, the authors have controlled microchemical reactions, handled liquid samples in biological analysis, and fabricated smart materials and devices. The authors envision that this rational microdrop generator would find applications in various research areas.

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The air bubble entrapped under a drop impacting on a solid surface

Cited by 196 publications

References 13 publications

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

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

Probing the nanoscale: the first contact of an impacting drop

Macrodrop‐Impact‐Mediated Fluid Microdispensing

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