Drag reduction on drop during impact on multiscale superhydrophobic surfaces

Droplet hurdles race

“…Water on superhydrophobic materials can reach a velocity on the order of 1 m/s on substrates tilted by a few degrees only, 3,4 a remarkably high speed that qualifies drops as suitable athletes. The very low friction of water on such surfaces offers this extreme mobility, [5][6][7][8] which makes drops hard to control. [9][10][11][12][13] However, macroscopic decorations were found to provide solutions to master the liquid dynamics.…”

Slippery bounces

“…The second surface (figure 2b) is covered with a unique texture at a smaller scale, associated with a moderate slip (b = 8 µm). An originality of the work of Martouzet et al (2020) is to focus on the spreading time τ , which is essential in impact dynamics. Their experiment, presented in figure 2, gives a counter-intuitive result: the spreading lasts longer on the hierarchical surface than on the nanotextured one.…”

“…The work of Martouzet et al (2020) shows that the subtle interplay between wetting and slip generates unexpected effects, such as an effective reduction of waterrepellency by hierarchical textures, which are the most lubricated superhydrophobic materials. Is all then lost for the hierarchical lotus leaf?…”

“…One might then wonder: What role does the slip play when the leaf is lashed by rain? This original question is the starting point of the work of Martouzet et al (2020), whose approach also sheds new light onto a much wider and debated issue on the role of friction on the impact dynamics of droplets on both wetting and non-wetting substrates.…”

Competition between shear and biaxial extensional viscous dissipation in the expansion dynamics of Newtonian and rheo-thinning liquid sheets

“…15 The other distinguishable configuration, albeit sparsely studied, involved impacting drops on repellent surfaces where there is no contact with the surface, hence eliminating shear dissipation. Such surfaces may include superhydrophobic surfaces, [16][17][18][19] hot plates above the Leidenfrost temperature, 20,21 and, more recently, cold plates covered with liquid nitrogen to exploit the cold Leidenfrost effect. [22][23][24][25] We have demonstrated elsewhere 24 that, in the so-called viscous regime, biaxial extensional dissipation dominates the sheet dynamics produced using the cold Leidenfrost effect.…”