Thermosuperrepellency of a hot substrate caused by vapour percolation

Abstract

Drop rebound after collision with a very hot substrate is usually attributed to the Leidenfrost effect, characterized by intensive film boiling in a thin vapour gap between the liquid and substrate. Similarly, drop impact onto a cold superhydrophobic substrate leads to a complete drop rebound, despite partial wetting of the substrate. Here we study the repellent properties of hot smooth hydrophilic substrates in the nucleate boiling, non-Leidenfrost regime and discover that the thermally induced repellency is associated with vapour percolation on the substrate. The wetting structure in the presence of the percolating vapour rivulets is analogous to the Cassie-Baxter wetting mode, which is a necessary condition for the repellency in the isothermal case. The theoretical predictions for the threshold temperature for vapour percolation agree well with the experimental data for drop rebound and correspond to the minimum heat flux when spray cooling.

Interaction of Drops and Sprays with a Heated Wall

“…The occurrence of large vapour clusters can be described by the percolation of single bubbles within the lamella, as shown in Fig. 14c and described in [25]. The remaining wetted area, which is not covered by vapour, is described by c ≈ 0.32.…”

Impact of a suspension drop onto a hot substrate: diminution of splash and prevention of film boiling

“…The accompanied phenomena, often wrongly associated with the film boiling regime, are described in detail in the recent experimental and theoretical study. 17 This regime includes some phenomena typical to the nucleate boiling regime and to the film boiling regime. It is characterized by the drop rebound.…”

“…Comparison of the drop spreading time and the time required for bubble percolation, leading to the formation of the vapor channels at the substrate surface, allows to estimate the temperature at which the drop rebounds from the substrate after its spreading and receding. 17 The estimated temperatures for the drop rebound…”

“…• at higher temperatures, the relative area covered by the bubbles increases and, under certain conditions, leads to the percolation of vapor channels 17,18 in the transition boiling regime (Fig. 1b);…”

Collaborative Research on Droplet Dynamics Under Extreme Ambient Conditions

“…The novel insight gained is related to the importance of the substrate material, in particular the thermal effusivity of the substrate. The heat transfer at these high temperatures is strongly a conjugate heat transfer problem; hence, the rate at which the thermal boundary in the substrate develops becomes an important factor in defining this point [50].…”