When a liquid droplet impacts a hot solid surface, enough vapor may be generated under it to prevent its contact with the solid. The minimum solid temperature for this so-called Leidenfrost effect to occur is termed the Leidenfrost temperature, or the dynamic Leidenfrost temperature when the droplet velocity is non-negligible. We observe the wetting or drying and the levitation dynamics of the droplet impacting on an (isothermal) smooth sapphire surface using high-speed total internal reflection imaging, which enables us to observe the droplet base up to about 100 nm above the substrate surface. By this method we are able to reveal the processes responsible for the transitional regime between the fully wetting and the fully levitated droplet as the solid temperature increases, thus shedding light on the characteristic time and length scales setting the dynamic Leidenfrost temperature for droplet impact on an isothermal substrate. DOI: 10.1103/PhysRevLett.116.064501 Boiling and spreading of droplets impacting on hot substrates have been extensively studied since both phenomena strongly affect the heat transfer between the liquid and the solid. Applications include spray cooling [1], spray combustion [2], and others [3].At room temperature, an impacting droplet spreads on a solid surface and entraps a bubble under it [4][5][6]. At temperatures higher than the boiling temperature T b , vapor bubbles appear which disturb and finally rupture the free surface, resulting in the violent spattering of tiny droplets [7,8]. On even hotter surfaces, however, beyond the socalled Leidenfrost temperature T L , the droplet interface becomes smooth again without any bubbles inside it. In this regime the droplet lives much longer, as now it levitates on its own vapor layer: the well-known Leidenfrost effect [9,10].In order to determine the Leidenfrost temperature T L and its dependence on the impact velocity U, phase diagrams have been experimentally produced for various impacting droplets with many combinations of substrates and liquids: water on smooth silicon [8], water on microstructured silicon [11], FC-72 on carbon nanofiber [12], water on aluminium [13], and ethanol on sapphire [14]. All of these phase diagrams show a weakly increasing behavior of T L with U. When theoretically deriving T L , one needs to determine the vapor thickness profile. In the case of a gently deposited droplet, this can be accomplished since the shape of the droplet is fixed except for the bottom surface, which reduces the problem to a lubrication flow of vapor in the gap between the substrate and the free surface [15][16][17][18][19][20]. For impacting droplets on an unheated surface at high Weber number We ≡ ρU 2 D 0 =σ (here, D 0 is the equivalent diameter of the droplet and ρ and σ are the density and the surface tension of the liquid, respectively), it is known that the neck around the dimple beneath the impacting droplet rams the surface. In this cold impact case, the neck propagates outwards like a wave [21]. For impact on a superheated s...

We experimentally investigate the nucleation of surface nanobubbles on PFDTS-coated silicon as a function of the specific gas dissolved in the water. In each case we restrict ourselves to equilibrium conditions (c = 100 %, T liquid = T substrate ). Not only is nanobubble nucleation a strong function of gas type, but there also exists an optimal system temperature of ∼ 35 − 40 o C where nucleation is maximized, which is weakly dependent on gas type. We also find that contact angle is a function of nanobubble radius of curvature for all gas types investigated. Fitting this data allows us to describe a line tension which is dependent on the type of gas, indicating that the nanobubbles are sat on top of adsorbed gas molecules. The average line tension was τ ∼ −0.8nN.

No abstract

Freezing or solidification of impacting droplets is omnipresent in nature and technology, be it a rain droplet falling on a supercooled surface, be it in inkjet printing where often molten wax is used, be it in added manufacturing or in metal production processes or in extreme ultraviolet lithography (EUV) for the chip production where molten tin is used to generate the EUV radiation. For many of these industrial applications, a detailed understanding of the solidification process is essential. Here, by adopting a totally new optical technique in the context of freezing, namely TIR (Total-Internal-Reflection), we elucidate the freezing kinetics during the solidification of a droplet while it impacts on an undercooled surface. We show for the first time that at sufficiently high undercooling a peculiar freezing morphology exists that involves sequential advection of frozen fronts from the centre of the droplet to its boundaries. This phenomenon is examined by combining elements of classical nucleation theory to the large scale hydrodynamics on the droplet scale, bringing together two subfields which traditionally have been quite separated. Furthermore, we report a peculiar self-peeling phenomenon of a frozen splat that is driven by the existence of a transient crystalline state during solidification.Solidification | Phase change | Droplet Impact | Classical nucleation theory | Crystal growth I mpact of a droplet on an undercooled solid surface instigates a number of physical processes simultaneously, including drop scale fluid motion, heat transfer between the liquid and the substrate, and the related phase transition. Whereas a large number of studies have investigated the corresponding interface deformations and the spreading of a droplet after it impinges onto an undercooled surface (1-9), the kinetics of phase transition within the impacting droplet has been addressed only in a few (10-12). Moreover, among the studies concerning solidification kinetics, only the regimes where phase transition effects are slower than the fast dynamics of droplet impact have been investigated. Here, we explore freezing behaviours that arise due to the rapid solidification of an impacting droplet at a sufficiently high substrate undercooling. Such scenarios are encountered in a number of industrial processes ranging from additive manufacturing (13,14) to thermal plasma spraying of ceramics and metallic materials (15-17), extreme ultraviolet lithography (18,19) etc.In the present work, we adapt the total-internal-reflection (TIR) technique (20-22) to visualise the phase transition in the vicinity of the liquid-substrate interface after a droplet impacts onto an undercooled transparent surface. This unique technique allows temporally and spatially resolved insight into the nucleation events and crystal growth occurring next to the cold surface on an evanescent length scale (∼ 100 nm), which is otherwise inaccessible through any other optical technique. Moreover, it allows us to monitor the delamination of the frozen-splat from the ...