Star-shaped oscillations of Leidenfrost drops

Abstract

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 ...

Self-organized oscillations of Leidenfrost drops

“…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).…”

“…This may be due to inherent viscous damping that prevents the sustained excitation of large-amplitude oscillations. Following the analysis in Ma et al (2017), the role of damping can be characterized by the Reynolds number associated with the liquid oscillation. For a star oscillation, the characteristic length scale and time scale are l c and l 3 c ρ l /γ, respectively.…”

“…For simplicity, we have assumed that the normal velocity is zero at the upper surface of the drop. For a large Leidenfrost water drop whose characteristic pressure oscillation frequency in the vapor layer is f p ≈ 26 Hz (see figure 7), the corresponding capillary wavelength is calculated to be λ c ≈ 3.03l c , so that k c ≈ 8.3 cm −1 (Ma et al 2017). Similarly, for the Leidenfrost acetone and ethanol drops shown in figures 13(a), 13(c), and 13(e), the corresponding k c ≈ 13 cm −1 , which is slightly larger than the positions of the peaks indicated in figures 13(b), 13(d ), and 13(f ), where λ c ≈ 4l c .…”

The crackling sound of Leidenfrost stars

“…Hence, given the general expression of the drop eigenfrequency f stated above, it becomes clear that drop oscillations are induced by a parametric forcing. But the underlying mechanism of the instability remains ill-understood, as shown by several seemingly contradictory results on the dependence of frequency on drop radius (Bouwhuis et al 2013;Caswell 2014;Ma, Lietor-Santos & Burton 2017).…”

Acoustic detection of electrostatic suppression of the Leidenfrost state

“…This well-known phenomenon, known as the Leidenfrost effect, has been widely studied [1][2][3][4][5][6][7][8], with the goal of elevating the Leidenfrost temperature to prevent surface dryout. There is significant literature on different aspects of the Leidenfrost effect including geometry of the droplet [9,10], droplet oscillations [11][12][13][14][15], self-propulsion of Leidenfrost droplets and Leidenfrost state-based drag reduction [16][17][18][19]. Recent studies show that an externally applied electric field in the vapor gap fundamentally eliminates [20][21][22][23][24][25][26][27] the Leidenfrost state by electrostatically attracting the droplet towards the surface.…”