Fingering patterns during droplet impact on heated surfaces

Abstract

A droplet impinging on a sufficiently heated surface may be cushioned by its own vapor and never touch the surface. In previous work, the transition to this so-called Leidenfrost regime was only qualitatively described as an abrupt change between the "contact-boiling" regime, which is characterized by violent boiling behaviors, and the Leidenfrost state. We reveal that the wetted area can be used as a quantity that quantitatively characterizes this transition and it is a continuous function of surface temperature up to the Leidenfrost regime. The wetted area exhibits fingering patterns caused by vapor flow under the liquid. This underlines the crucial role of vapor transport in the Leidenfrost transition and unveils the physical mechanism of the transition to the Leidenfrost regime.

Dynamic Leidenfrost Effect: Relevant Time and Length Scales

“…This can be explained by the cooling by the vapor from the droplet base located at about 50 nm from the substrate. We also found that there exists no fingering pattern, in contrast to what was observed on glass substrates [24]. These results indicate that the fingering pattern is related to the cooling of the substrate by radially flowing vapor and the subsequent rewetting of the substrate.…”

“…In order to explore how these scales change when undergoing the transition from contact to the Leidenfrost regime, we employed total internal reflection (TIR) imaging (see Fig. 1), which is a powerful technique to quantitatively evaluate the approach of impacting droplets on an evanescent length scale, typically 100 nm [5,22,23], and to clearly distinguish the wetted area from vapor bubbles or patches on heated substrates [1,24]. Next to the impact velocity U, a key process that significantly affects T L is the cooling of the substrate due to its exposure to the cold liquid.…”

The dynamics of Leidenfrost drops

“…Previous models [53][54][55][56] neglect multiple transmissions and reflections at both top and bottom interfaces of the thin film and also the light polarization. Those assumptions imply that the measured evanescent light intensity decays exponentially with increasing film thickness.…”

“…As a consequence, Kolinski, Rubinstein and co-workers [53][54][55] recently developed frustrated total internal reflection (FTIR) imaging for their study of thinner films. We have adopted this powerful technique [45,56,57]. Here, a transparent wall is illuminated at an angle larger than the critical one.…”

“…The energy loss of the light beam is thus a measure for the distance between the wall and the other medium. Further applications of this technique other than the film thickness measurement include particle image velocimetry in a near-wall flow [58], studies of moving contact lines and wetting/drying behaviour of heated surfaces in nucleate boiling studies [13,45,56,57].…”