Rebounding suppression of droplet impact on hot surfaces: effect of surface temperature and concaveness.
TL;DR: The morphology of drop impact on the concave surface was captured and the influence of its asymmetric deformation on extensive suppression of drop bouncing was discussed and the amount of dissipated energy due to drop deformation was obtained based on an energy balance analysis.
Abstract: When a droplet impinges on a hot surface it is crucial to increase the contact time or decrease the rebounding distance if the heat transfer between the droplet and the surface is important. This will be more sensitive when the temperature regime is above the Leidenfrost values. The focus of the present experimental study is on the maximum height of drop bouncing after impinging on flat and semi-cylindrical concave surfaces, in particular in terms of surface temperature. It is shown that the behavior of the lamella during the spreading to its maximum diameter has a considerable impact on the maximum height of the drop bouncing. For different impact Weber numbers the map of thermal versus inertia effects is extracted for both the flat and concave surfaces for rebound and thermal atomization mid-regimes. It was shown that the thermal atomization mid-regime was eliminated in the case of drop impact on the concave surface in a wide range of impact Weber numbers and surface temperatures. The variations in rebounding and maximum heights at different regions of the maps are quantified and discussed. The morphology of drop impact on the concave surface was captured and the influence of its asymmetric deformation on extensive suppression of drop bouncing was discussed. Finally, the amount of dissipated energy due to drop deformation was obtained based on an energy balance analysis.
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TL;DR: In this article, the existence of a new regime termed explosive boiling for impact of binary (composed of two miscible liquids) droplets on superheated substrates at temperatures between the respective Leidenfrost temperatures of the two liquid constituents.
20 citations
TL;DR: It is shown that the delicate gas thin film can be fundamentally altered for even weakly charged droplets, providing new insights into a range of applications such as mitigating pinhole defects in additive manufacturing.
Abstract: Electric charges are often found in naturally or artificially formed droplets, such as raindrops and those generated by Kelvin's water dropper. In contrast to the impact of neutral droplets on a flat solid surface upon which a thin convex lens shape layer of the gas film is typically formed, we show that the delicate gas thin film can be fundamentally altered for even weakly charged droplets. As the charge level is raised above a critical level of ∼1% of the Rayleigh limit for representative impact conditions, the Maxwell stress overcomes the gas pressure buildup to deform the droplet bottom surface. A conical liquid tip forms and pierces through the gas film, leading to a circular contact line moving outwards that does not trap any gas. The critical charge level only depends on the capillary number based on the gas viscosity. This finding applies to common liquids and molten alloy droplets, providing new insights into a range of applications such as mitigating pinhole defects in additive manufacturing.
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14 citations
TL;DR: In this paper, the performance of electrospray cooling with a hemispherical cap was evaluated for high volumetric flow rates (80 mL/h) stabilized by a novel Hemispherical nozzle.
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TL;DR: In this paper, the collision dynamics of a liquid droplet on a solid metallic surface were studied using a flash photographic method, which provided clear images of the droplet structure during the deformation process.
Abstract: The collision dynamics of a liquid droplet on a solid metallic surface were studied using a flash photographic method. The intent was to provide clear images of the droplet structure during the deformation process. The ambient pressure (0.101 MPa), surface material (polished stainless steel), initial droplet diameter (about 1.5 mm), liquid (n-heptane) and impact Weber number (43) were fixed. The primary parameter was the surface temperature, which ranged from 24 degrees C to above the Leidenfrost temperature of the liquid. Experiments were also performed on a droplet impacting a surface on which there existed a liquid film created by deposition of a prior droplet. The evolution of wetted area and spreading rate, both of a droplet on a stainless steel surface and of a droplet spreading over a thin liquid film, were found to be independent of surface temperature during the early period of impact. This result was attributed to negligible surface tension and viscous effects, and in consequence the measurements made during the early period of the impact process were in good agreement with previously published analyses which neglected these effects. A single bubble was observed to form within the droplet during impact at low temperatures. As surface temperature was increased the population of bubbles within the droplet also increased because of progressive activation of nucleation sites on the stainless steel surface. At surface temperatures near to the boiling point of heptane, a spoke-like cellular structure in the liquid was created during the spreading process by coalescence of a ring of bubbles that had formed within the droplet. At higher temperatures, but below the Leidenfrost point, numerous bubbles appeared within the droplet, yet the overall droplet shape, particularly in the early stages of impact (< 0.8 ms), was unaffected by the presence of these bubbles. The maximum value of the diameter of liquid which spreads on the surface is shown to agree with predictions from a simplified model.
1,032 citations
TL;DR: In this article, the authors focus on recent experimental and theoretical studies, which aim at unraveling the underlying physics, characterized by the delicate interplay of liquid inertia, viscosity, and surface tension, but also the surrounding gas.
Abstract: A drop hitting a solid surface can deposit, bounce, or splash. Splashing arises from the breakup of a fine liquid sheet that is ejected radially along the substrate. Bouncing and deposition depend crucially on the wetting properties of the substrate. In this review, we focus on recent experimental and theoretical studies, which aim at unraveling the underlying physics, characterized by the delicate interplay of not only liquid inertia, viscosity, and surface tension, but also the surrounding gas. The gas cushions the initial contact; it is entrapped in a central microbubble on the substrate; and it promotes the so-called corona splash, by lifting the lamella away from the solid. Particular attention is paid to the influence of surface roughness, natural or engineered to enhance repellency, relevant in many applications.
994 citations
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TL;DR: In this article, a liquid drop impacts a solid, it spreads (with possibly beautiful fingering patterns) up to the point when kinetic energy is dissipated by viscosity, then it can retract (if the solid is partially wetted by the liquid), or not.
Abstract: When a liquid drop impacts a solid, it spreads (with possibly beautiful fingering patterns) up to the point when kinetic energy is dissipated by viscosity. Then, it can retract (if the solid is partially wetted by the liquid), or not. A very different behaviour can be observed on highly hydrophobous solids. On such solids, the contact angle is close to 180°, so that the kinetic energy of the impinging drop can be transferred to surface energy, without spreading. Thus, the drop can fully bounce. However, the liquid nature of this kind of spring imposes a limit for the restitution coefficient, which is due to the fact that the drop, after the lift-off, oscillates.
459 citations
TL;DR: In this paper, the dimensionless maximum spreading γ of impacting droplets on the heated surfaces in both gentle and spraying film boiling regimes was shown to have a universal scaling with the Weber number We (γ~We(2/5)), which is much steeper than for the impact on nonheated (hydrophilic or hydrophobic) surfaces (γ ~We(1/4)).
Abstract: At the impact of a liquid droplet on a smooth surface heated above the liquid's boiling point, the droplet either immediately boils when it contacts the surface ("contact boiling"), or without any surface contact forms a Leidenfrost vapor layer towards the hot surface and bounces back ("gentle film boiling"), or both forms the Leidenfrost layer and ejects tiny droplets upward ("spraying film boiling"). We experimentally determine conditions under which impact behaviors in each regime can be realized. We show that the dimensionless maximum spreading γ of impacting droplets on the heated surfaces in both gentle and spraying film boiling regimes shows a universal scaling with the Weber number We (γ~We(2/5)), which is much steeper than for the impact on nonheated (hydrophilic or hydrophobic) surfaces (γ~We(1/4)). We also interferometrically measure the vapor thickness under the droplet.
358 citations
TL;DR: In this paper, a comprehensive review of published literatures concerning the fluid mechanics and heat transfer mechanisms of liquid drop impact on a heated wall is provided, divided into four parts, each centered on one of the main heat transfer regimes: film evaporation, nucleate boiling, transition boiling, and film boiling.
357 citations