We here present a simple fitting-parameter-free theory of the Leidenfrost effect (droplet levitation above a superheated plate) covering the full range of stable shapes, i.e., from small quasispherical droplets to larger puddles floating on a pocketlike vapor film. The geometry of this film is found to be in excellent quantitative agreement with the interferometric measurements of Burton et al. [Phys. Rev. Lett. 109, 074301 (2012)]. We also obtain new scalings generalizing classical ones derived by Biance et al. [Phys. Fluids 15, 1632(2003] as far as the effect of plate superheat is concerned and highlight the relative role of evaporation, gravity, and capillarity in the vapor film. To further substantiate these findings, a treatment of the problem by matched asymptotic expansions is also presented.

We show that a volatile liquid drop placed at the surface of a nonvolatile liquid pool warmer than the boiling point of the drop can be held in a Leidenfrost state even for vanishingly small superheats. Such an observation points to the importance of the substrate roughness, negligible in the case considered here, in determining the threshold Leidenfrost temperature. A theoretical model based on the one proposed by Sobac et al. [Phys. Rev. E 90, 053011 (2014)] is developed in order to rationalize the experimental data. The shapes of the drop and of the liquid substrate are analyzed. The model notably provides scalings for the vapor film thickness profile. For small drops, these scalings appear to be identical to the case of a Leidenfrost drop on a solid substrate. For large drops, in contrast, they are different, and no evidence of chimney formation has been observed either experimentally or theoretically in the range of drop sizes considered in this study. Concerning the evaporation dynamics, the radius is shown to decrease linearly with time whatever the drop size, which differs from the case of a Leidenfrost drop on a solid substrate. For high superheats, the characteristic lifetime of the drops versus the superheat follows a scaling law that is derived from the model, but, at low superheats, it deviates from this scaling by rather saturating.

Low viscosity (<100 cSt) silicon oil droplets are placed on a high viscosity (1000 cSt) oil bath that vibrates vertically. The viscosity difference ensures that the droplet is more deformed than the bath interface. Droplets bounce periodically on the bath when the acceleration of its sinusoidal motion is larger than a threshold value. The threshold is minimum for a particular frequency of excitation: droplet and bath motions are in resonance. The bouncing droplet has been modeled by considering the deformation of the droplet and the lubrication force exerted by the air layer between the droplet and the bath. Threshold values are predicted and found to be in good agreement with our measurements.

We investigate the dynamics of a dimer bouncing on a vertically oscillated plate. The dimer, composed of two spheres rigidly connected by a light rod, exhibits several modes depending on initial and driving conditions. The first excited mode has a novel horizontal drift in which one end of the dimer stays on the plate during most of the cycle, while the other end bounces in phase with the plate. The speed and direction of the drift depend on the aspect ratio of the dimer. We employ event-driven simulations based on a detailed treatment of frictional interactions between the dimer and the plate in order to elucidate the nature of the transport mechanism in the drift mode.

The crater formation due to the impact of a water droplet onto a granular bed has been experimentally investigated. Three parameters were tuned: the impact velocity, the size of the droplet, and the size of the grains. The aim is to determine the influence of the kinetic energy on the droplet pattern. The shape of the crater depends on the kinetic energy at the moment the droplet starts to impact the bed. The spreading and recession of the liquid during the impact were carefully analyzed from the dynamical point of view, using image analysis of high-speed video recordings. The different observed regimes are characterized by the balance between the impregnation time of the water by the granular bed by the water and the capillary time responsible for the recession of the drop.

When an oil droplet is placed on a quiescent oil bath, it eventually collapses into the bath due to gravity. The resulting coalescence may be eliminated when the bath is vertically vibrated. The droplet bounces periodically on the bath, and the air layer between the droplet and the bath is replenished at each bounce. This sustained bouncing motion is achieved when the forcing acceleration is higher than a threshold value. When the droplet has a sufficiently low viscosity, it significantly deforms : spherical harmonic Y m ℓ modes are excited, resulting in resonant effects on the threshold acceleration curve. Indeed, a lower acceleration is needed when ℓ modes with m = 0 are excited. Modes m = 0 are found to decrease the bouncing ability of the droplet. When the mode ℓ = 2 and m = 1 is excited, the droplet rolls on the vibrated surface without touching it, leading to a new self-propulsion mode.

Antibubbles are unusual fluid objects consisting of a thin spherical air shell surrounding a liquid globule. Here we study and analyze the aging of these inverted bubbles. The lifetime is found to be distributed along an exponential law. Moreover, the breakdown of the air film is observed to be the analogue of dewetting by spinodal decomposition. We interpret the long lifetime of the antibubbles as resulting from the slow drainage of the air until the film reaches a critical thickness. Then, van der Waals forces act, leading to the collapse of the film. * * * SD would like to thank FNRS for financial support. This work has been also supported by the contract ARC 02/07-293. P.-G. de Gennes and F. Brochard-Wyart are gratefully thanked for their fruitful comments and encouragements. Special thanks are also due to H. Caps (ULg) and J. Magnaudet (IMFT, Toulouse, France) for very valuable discussions.

1Cellular structures are very common in nature [1]. Each cell of the cellular structure can be a bubble in a beer, a biological cell in a tissue [2], a grain in a polycrystal [3] or a magnetic domain in a solid [4]. Foams have becomed paradigms of disordered cellular systems. Among the physical properties of interest, one is the longterm behaviour of a froth driven by topological rearrangements. In the present work, we report acoustic experiments on foam systems.We have recorded the sound emitted by crackling cells during the collapsing of foams. The sound pattern is then analyzed using classical methods of statistical physics. Fundamental processes at the surface of the collapsing foam are found. In particular, size is not a relevant parameter for exploding bubbles. In addition, many topological rearrangements such as the cell side switching or the vertex disappearance take also place in the foam [1]. The combination of bubble area growth/decay and topological rearrangements induces a complex dynamics [7] in which subtle correlations are found as e.g. described by the so-called Aboav-Weaire law [5]. After noise filtering, the power P dissipated within the small interval ∆t ≈ τ 0 is then calculated for each peak, i.e.(1)The dissipated power P is given in arbitrary units; nevertheless it is assumed to be proportional to the energy dissipated during the explosion of the bubble membrane, i.e. to be proportional to the surface area of the disappearing cell.Figure 3 presents a typical histogram h(P ) of the frequency of peak occurence as a function of the peak intensity P . This distribution presents a maximum and a long "tail". The inset of Figure 3 shows a log-log plot of the long tail.For large P values, h(P ) behaves like a power lawThis power law behaviour of the tail holds over 1.5 decades for best cases. In summary, our acoustic experiments have put into evidence the intermittent and correlated character of popping bubbles in a collapsing dry foam. In other words, the dynamics of a collapsing foam is discontinuous and evolves by sudden bursts of activity separated by periods of stasis. These avalanches are correlated. Moreover, we have discovered that a wide variety of bubble sizes participate to the phenomenon.