Dynamics of the vapor layer below a Leidenfrost drop

Caswell, Thomas A

doi:10.1103/physreve.90.013014

Cited by 31 publications

(22 citation statements)

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“…In the case of Leidenfrost drops, there is no obvious frequency or wavelength selection mechanism generated by the flow and evaporation of vapor beneath the drop. It is possible that a "breathing mode" of the drop would cause the radius to vary with time, however, recent measurements of the breathing mode in both low and high-viscosity levitated drops show that the frequency rapidly decreases with R [6,22,30], in contrast to the data shown in Fig. 3c.…”

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“…The drop will levitate on a thermally-insulating vapor layer and survive for minutes [1][2][3][4]. For small drops, the geometry and dynamics of the vapor layer have been recently characterized [5,6]. The complex interactions between the liquid, vapor, and solid interfaces have led to a broad range of applications such as turbulent dragreduction [7], self-propulsion of drops on ratcheted surfaces [8,9], green nanofabrication [10], fuel combustion [11], and thermal control of nuclear reactors [12].…”

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confidence: 99%

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Star-shaped oscillations of Leidenfrost drops

Liétor-Santos

Burton

2017

Phys. Rev. Fluids

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We experimentally investigate the self-sustained, star-shaped oscillations of Leidenfrost drops. The drops levitate on a cushion of evaporated vapor over a heated, curved surface. We observe modes with n = 2 − 13 lobes around the drop periphery. We find that the wavelength of the oscillations depends only on the capillary length of the liquid, and is independent of the drop radius and substrate temperature. However, the number of observed modes depends sensitively on the liquid viscosity. The dominant frequency of pressure variations in the vapor layer is approximately twice the drop oscillation frequency, consistent with a parametric forcing mechanism. Our results show that the star-shaped oscillations are driven by capillary waves of a characteristic wavelength beneath the drop, and that the waves are generated by a large shear stress at the liquid-vapor interface.PACS numbers: 47.35.Pq, 47.15.gm, 47.85.mf The Leidenfrost effect can be easily observed by placing a millimeter-scale water drop onto a sufficiently hot pan. The drop will levitate on a thermally-insulating vapor layer and survive for minutes [1][2][3][4]. For small drops, the geometry and dynamics of the vapor layer have been recently characterized [5,6]. The complex interactions between the liquid, vapor, and solid interfaces have led to a broad range of applications such as turbulent dragreduction [7], self-propulsion of drops on ratcheted surfaces [8,9], green nanofabrication [10], fuel combustion [11], and thermal control of nuclear reactors [12].Large Leidenfrost drops are well-known to form selfsustained, star-shaped oscillations (Fig. 1a). Since the 1950's, a number of studies have investigated these oscillations, often with different conclusions as to their physical origin based on the complicated interplay between thermal and hydrodynamic effects in both the liquid and gas phases [13][14][15][16][17][18][19]. A simple underlying mechanism for the onset of star oscillations remains unknown. Drops subjected to external, periodic excitations can form star oscillations with a frequency half that of the external excitation due to a parametric coupling mechanism [20,21]. However, if a parametric mechanism causes Leidenfrost stars, then the source of the periodic excitation is unclear. Recently, Bouwhuis et al. investigated the star oscillations of drops levitated by an air flow over a porous surface [22]. They showed that the onset of star oscillations occurs when the flow rate of air beneath the drop reaches a threshold, suggesting that a hydrodynamic coupling between the gas flow and liquid interface initiates the oscillations.Here we report measurements of star-shaped oscillations of six different liquids on a hot, curved surface. We observe stars with n = 2 − 13 lobes around the drop periphery. Although the number of observed modes depends on the liquid viscosity and substrate temperature, we find that the wavelength and frequency of the modes only depend on the capillary length, l c = γ/ρ l g, where γ and ρ l are the surface tension and ...

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Star-shaped oscillations of Leidenfrost drops

Liétor-Santos

Burton

2017

Phys. Rev. Fluids

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“…Side view During the bouncing series, the drop retains its radial symmetry with respect to the vertical axis. This is in contrast to larger drops (R 0 > 2 mm) levitated on a steady, but relatively fast, ascending air flow (Bouwhuis et al 2013), or in the Leidenfrost state (Caswell 2014), in which experiments the radial symmetry was broken, leading to star-shaped drop oscillations. Moreover, their air film is non-axisymmetric (Caswell 2014), while radial symmetry is clearly present in the current experiment, as shown in the bottom line of figure 1(b).…”

Section: Methodsmentioning

confidence: 92%

Bouncing on thin air: how squeeze forces in the air film during non-wetting droplet bouncing lead to momentum transfer and dissipation

et al. 2015

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Millimetre-sized droplets are able to bounce multiple times on flat solid substrates irrespective of their wettability, provided that a micrometre-thick air layer is sustained below the droplet, limiting We to 4. We study the energy conversion during a bounce series by analysing the droplet motion and its shape (decomposed into eigenmodes). Internal modes are excited during the bounce, yet the viscous dissipation associated with the in-flight oscillations accounts for less than 20 % of the total energy loss. This suggests a significant contribution from the bouncing process itself, despite the continuous presence of a lubricating air film below the droplet. To study the role of this air film we visualize it using reflection interference microscopy. We quantify its thickness (typically a few micrometres) with sub-millisecond time resolution and ∼30 nm height resolution. Our measurements reveal strong asymmetry in the air film shape between the spreading and receding phases of the bouncing process. This asymmetry is crucial for effective momentum reversal of the droplet: lubrication theory shows that the dissipative force is repulsive throughout each bounce, even near lift-off, which leads to a high restitution coefficient. After multiple bounces the droplet eventually hovers on the air film, while continuously experiencing a lift force to sustain its weight. Only after a long time does the droplet finally wet the substrate. The observed bounce mechanism can be described with a single oscillation mode model that successfully captures the asymmetry of the air film evolution.

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“…This reduction in solid-liquid contact area means that the heat flux required to generate the supporting vapour layer required for a Leidenfrost state has to be transferred through the reduced contact area between the liquid and the solid wires. This would mean that a higher temperature is needed for the required heat flux to generate a continuous vapour and so produce a Leidenfrost state [21][22][23][24]. Figure 2(b), degrades the fit, suggesting that this area is not a significant factor.…”

Section: Meshmentioning

confidence: 99%

Leidenfrost transition temperature for stainless steel meshes

Geraldi

McHale

et al. 2016

Materials Letters

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(2016) 'Leidenfrost transition temperature for stainless steel meshes. ', Materials letters., Further information on publisher's website: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbstractOn surfaces well above 100°C water does not simply boil away. When there is a sufficient heat transfer between the solid and the liquid a continuous vapour layer instantaneous forms under a droplet of water and the drop sits on a cushion of vapour, highly mobile and insulated from the solid surface. This is known as the Leidenfrost effect and the temperature at which this occurs if known as the Leidenfrost transition temperature. In this report, an investigation of discontinuous surfaces, stainless steel meshes, have been tested to determine the effect of the woven material on the Leidenfrost phenomenon. . This allows suppression of the Leidenfrost effect as it can be increase to over 300°C from 185°C for a stainless steel surface. Graphical Abstract Introduction

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Dynamics of the vapor layer below a Leidenfrost drop

Cited by 31 publications

References 21 publications

Star-shaped oscillations of Leidenfrost drops

Star-shaped oscillations of Leidenfrost drops

Bouncing on thin air: how squeeze forces in the air film during non-wetting droplet bouncing lead to momentum transfer and dissipation

Leidenfrost transition temperature for stainless steel meshes

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