The many faces of a Leidenfrost drop

Ma, Xiaolei; Liétor-Santos, Juan-José; Burton, Justin

doi:10.1063/1.4930913

Cited by 17 publications

(14 citation statements)

References 9 publications

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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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confidence: 56%

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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confidence: 56%

Star-shaped oscillations of Leidenfrost drops

Liétor-Santos

Burton

2017

Phys. Rev. Fluids

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“…Large Leidenfrost drops form liquid puddles whose thickness is approximately 2l c . These puddles are known to spontaneously form large-amplitude, star-shaped oscillations (Holter & Glasscock 1952;Adachi & Takaki 1984;Strier et al 2000;Snezhko et al 2008;Strier et al 2000;Ma et al 2015Ma et al , 2017. Similar oscillations have been observed in large drops on periodically-shaken, hydrophobic surfaces (Noblin et al 2005(Noblin et al , 2009, drops levitated by an underlying airflow (Bouwhuis et al 2013), and drops excited by an external acoustic or electric field (Shen et al 2010a,b;Mampallil et al 2013).…”

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confidence: 78%

Self-organized oscillations of Leidenfrost drops

Burton

2018

J. Fluid Mech.

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In the Leidenfrost effect, a thin layer of evaporated vapor forms between a liquid and a hot solid. The complex interactions between the solid, liquid, and vapor phases can lead to rich dynamics even in a single Leidenfrost drop. Here we investigate the self-organized oscillations of Leidenfrost drops that are excited by a constant flow of evaporated vapor beneath the drop. We show that for small Leidenfrost drops, the frequency of a recently reported "breathing mode" (Caswell, Phys. Rev. E, vol. 90, 2014, 013014) can be explained by a simple balance of gravitational and surface tension forces. For large Leidenfrost drops, azimuthal star-shaped oscillations are observed. Our previous work showed how the coupling between the rapid evaporated vapor flow and the vapor-liquid interface excites the star oscillations (Ma et al., Phys. Rev. Fluids, vol. 2, 2017, 031602).In our experiments, star-shaped oscillation modes of n = 2 to 13 are observed in different liquids, and the number of observed modes depends sensitively on the viscosity of the liquid. Here we expand on this work by directly comparing the oscillations with theoretical predictions, as well as show how the oscillations are initiated by a parametric forcing mechanism through pressure oscillations in the vapor layer. The pressure oscillations are driven by the capillary waves of a characteristic wavelength beneath the drop. These capillary waves can be generated by a large shear stress at the liquid-vapor interface due to the rapid flow of evaporated vapor. We also explore potential effects of thermal convection in the liquid. Although the measured Rayleigh number is significantly larger than the critical Rayleigh number, the frequency (wavelength) of the oscillations depends only on the capillary length of the liquid, and is independent of the drop radius and substrate temperature. Thus convection seems to play a minor role in Leidenfrost drop oscillations, which are mostly hydrodynamic in origin.

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“…Our findings unleash a powerful and widely applicable strategy for injecting mechanical energy into soft materials, with relevance to fields ranging from soft robotics and metamaterials to microfluidics and active matter.The Leidenfrost effect is commonly observed in the kitchen-splash a droplet of water onto a hot pan and, rather than boiling, it counterintuitively floats above the surface 1 .Far beyond a curiosity, this effect plays a critical role in industrial settings ranging from 2 alloy production plants 4 to nuclear reactors 20 and provides a mechanism to reduce drag in fluid 4,6 and solid 19 transport. Although first described more than two centuries ago 1 , issues as fundamental as droplet shape 13,14 , the dynamics during impact 8,11,21 , and the effects of substrate texturing 3,7,12,16,17 are only recently becoming understood. One issue that has remained unquestioned is the potential importance of the mechanical properties of the object itself.…”

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Coupling the Leidenfrost effect and elastic deformations to power sustained bouncing

et al. 2017

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The Leidenfrost e ect occurs when an object near a hot surface vaporizes rapidly enough to lift itself up and hover 1,2 . Although well understood for liquids [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and sti sublimable solids [15][16][17][18] , nothing is known about the e ect with materials whose sti ness lies between these extremes. Here we introduce a new phenomenon that occurs with vaporizable soft solids-the elastic Leidenfrost e ect. By dropping hydrogel spheres onto hot surfaces we find that, rather than hovering, they energetically bounce several times their diameter for minutes at a time. With high-speed video during a single impact, we uncover highfrequency microscopic gap dynamics at the sphere/substrate interface. We show how these otherwise-hidden agitations constitute work cycles that harvest mechanical energy from the vapour and sustain the bouncing. Our findings suggest a new strategy for injecting mechanical energy into a widely used class of soft materials, with potential relevance to fields such as active matter, soft robotics and microfluidics.The Leidenfrost effect is commonly observed in the kitchensplash a droplet of water onto a hot pan and, rather than boiling, it counterintuitively floats above the surface 1 . Far beyond a curiosity, this effect plays a critical role in industrial settings ranging from alloy production plants 4 to nuclear reactors 19 and provides a mechanism to reduce drag in fluid 4,6 and solid 18 transport. Although first described more than two centuries ago, issues as fundamental as droplet shape 12,13 , the dynamics during impact 8,10,20 , and the effects of substrate texturing 3,7,11,15,16 are only recently becoming understood. One issue that has remained unquestioned is the potential importance of the mechanical properties of the object itself. For sublimable solids such as dry ice, the Young's modulus is far too large (∼10 GPa) for mechanical deformations to be relevant [15][16][17] . In liquids, surface tension can lead to quasi-elasticity for tiny droplets 14 , but otherwise its influence is limited to capillary oscillations 13 . Here we introduce a new type of Leidenfrost effect that occurs with vaporizable soft solids-in our experiments, water-saturated hydrogel spheres (diameters 1.49 ± 0.01 cm, masses 1.75 ± 0.03 g). Despite consisting of ∼99% water, these behave like linear elastic solids (Young's moduli Y = 50 ± 4 kPa; see the Methods and Supplementary Fig. 2). The effect is illustrated in Fig. 1a, where we show top-down tracks of five dyed hydrogel spheres cast onto a ceramiccoated aluminium surface at 215• C. Immediately on contact the spheres exhibit energetic activation, frenetically travelling around the surface at speeds of up to 0.5 m s −1 and emitting high-pitched screeching noises (see Supplementary Movie 1). This demonstrates the potential usefulness of the effect as an energy injection strategy, particularly to create macroscopic active matter 21,22 . While the tracks convey horizontal motion, this is achieved through sustained vertical...

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The many faces of a Leidenfrost drop

Cited by 17 publications

References 9 publications

Star-shaped oscillations of Leidenfrost drops

Star-shaped oscillations of Leidenfrost drops

Self-organized oscillations of Leidenfrost drops

Coupling the Leidenfrost effect and elastic deformations to power sustained bouncing

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