Phase diagram for droplet impact on superheated surfaces

Abstract

We experimentally determine the phase diagram for impacting ethanol droplets on a smooth, sapphire surface in the parameter space of Weber number We versus surface temperature T. We observe two transitions, namely the one towards splashing (disintegration of the droplet) with increasing We, and the one towards the Leidenfrost state (no contact between the droplet and the plate due to a lasting vapour film) with increasing T. Consequently, there are four regimes: contact and no splashing (deposition regime), contact and splashing (contact-splash regime), neither contact nor splashing (bounce regime), and finally no contact, but splashing (film-splash regime). While the transition temperature T L to the Leidenfrost state depends weakly, at most, on We in the parameter regime of the present study, the transition Weber number We C towards splashing shows a strong dependence on T and a discontinuity at T L . We quantitatively explain the splashing transition for T < T L by incorporating the temperature dependence of the physical properties in the theory by Riboux & Gordillo (Phys.

The dynamics of Leidenfrost drops

“…Since the surface to cools down as a result of the presence of the drop, the wetted areas with their entrapped bubbles can survive on the surface, resulting in the spray formation by the bursting of the bubbles at the top surface of the drop, in agreement with previous studies on spray formation [66,97]. Furthermore, we find the boundary between contact boiling and transition boiling to be weakly dependent on the impact velocity, similarly as reported for ethanol drops impacting on glass [57] and on sapphire [45,69]. Since no cooling was found in the latter two studies, it is not surprising that no spray is formed:…”

“…Because of the absence of the spray, Staat et al [69] identified all impacts above this boundary to be film boiling, though in later studies the lower part of this regime was named and identified as the transition boiling regime [57,57].…”

“…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 micro-structured silicon [50], FC-72 on carbon-nanofiber [67], water on aluminium [68], and ethanol on sapphire [69]. All these phase diagrams show a weakly increasing behaviour of T L with U .…”

“…When the impacting drop has an initial downward velocity U = 0, the phenomenon is referred to as dynamic Leidenfrost effect. Due to its widespread applications, this dynamic case has drawn much interest [50,56,67,69,73,[83][84][85]. By the dynamic pressure of the impact, the drop partly overcomes the cushioning of the vapour layer and only touches down at plate temperatures higher than the static T L .…”

Dynamic Leidenfrost Effect: Relevant Time and Length Scales

“…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.…”