Thermosuperrepellency of a hot substrate caused by vapour percolation

Droplet impact on surfaces has wide applications regardless of the discipline area and several hypotheses have been put forward to explain the mechanism of film boiling. Here, the authors combine theory and experiment to investigate liquid drop impact off hot hydrophilic substrates, and explain the transition between deposition and rebound in terms of vapour percolation. 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.

Automated summary

Droplet impact and rebound on hot hydrophilic substrates can be explained by vapor percolation, while complete rebound of droplets on cold superhydrophobic substrates can still occur despite partial wetting. The repellent properties of hot smooth hydrophilic substrates during nucleate boiling are associated with vapor percolation, similar to the Cassie-Baxter wetting mode.