Significance Comment

The paper has good overall significance due to the novelty of the low recorded contact time, the demonstration that the phenomenon is general to a range of surfaces (Fig. 4), and the fact that drop splashes are inherently of high aesthetic and practical interest.

Two issues relating to significance deserve some comment.

Firstly, the previous studies (and arguably the “theoretical limit”) used for comparison here are something of a straw man, because they do not use macrotextures. SHSs come in a wide variety of structures [3,4], including complex biological hierarchical systems. However, structure on scales greater than ~10 micron is not required for drop bouncing (e.g. this paper includes an SHS formed by 10 x 10 micron square pillars). If structure can be introduced on any length scale, one might trivially consider the contact time on a sloping surface (e.g. curvature of a leaf). On the other hand, the authors can be credited for the novelty and clear sinusoidal definition of their macrotextured SHSs. Also, the macrotexture length scale may be of particular interest because it is comparable to the size of a drop.

Secondly, application potential can be considered. Application examples given here include stay-dry, self-cleaning, and ice-resistant surfaces. Considering ice prevention, reduced contact time should reduce heat transfer on the first bounce. However, successive impacts are also important, and macrotextures (unlike microstructures) can significantly affect the nature of a secondary impact. In this paper, reduced contact time is achieved by splitting the drop, and perhaps imparting some horizontal momentum. We can consider the comparison between the second impacts of (i) a split drop on a macrotextured SHS and (ii) an intact drop falling vertically on a macroscopically flat surface. The smaller, non-spherical split drops have greater surface area to volume ratio, and therefore increased potential for heat transfer on the second bounce. Surface interactions could also be promoted by horizontal momentum, although the contact time could also reduce with drop size (proportional to R^1.5 for a vertical impact). Considering briefly the other example applications, contact time is not an obvious measure of quality for self-cleaning surfaces; while mobility of quasi-static drops on the surface (i.e. “roll-off”) is arguably of more practical interest for all three applications.

[3] D. Quéré and M. Reyssat, “Non-adhesive lotus and other hydrophobic materials”, Phil. Trans. R. Soc. A 366, 1539-1556 (2008).

[4] N. J. Shirtcliffe, G. McHale and M. I. Newton, “The superhydrophobicity of polymer surfaces: Recent developments”, J. Polym. Sci. Pol. Phys. 49, 1203-1217 (2011).

Quality Comment

Bird et al.’s paper describes experiments in which high speed photography is used to film small drops (a few mm in diameter) as they drop vertically on to horizontal surfaces. Superhydrophobic surfaces (SHSs) are used so that the drops bounce. The clear novelty of this paper is, as claimed, to demonstrate approaches which “allow us to reduce the overall contact time between a bouncing drop and a surface below what was previously though possible”. This is achieved using surface structures that have similar length scale to the impacting drops, labelled “macrotextures”.

Overall this is a really nice paper. Drop splashes have been widely studied using high-speed photography [1,2], and previous bouncing contact time work is usefully summarized in Extended Data Table 1. The experiments employ an array of well-defined synthetic SHSs (see Methods Section), and biological SHSs. The laser-ablated silicon macrotextured SHSs which achieve particularly low contact times (Fig. 3) do not ‘pin’ to the structure of the fabricated surfaces – an indication of SHS quality. The spatial arrangement of the sinusoidal macrotextures has even been optimised so that the drop impact point is not highly significant. The data analysis and theory introduced are adequate to demonstrate the main principles of the observations, and appropriate to the scope of the paper.

[1] A. L. Yarin, “Drop impact dynamics”, Annu. Rev. Fluid Mech. 38, 159-192 (2006).

[2] S. T. Thoroddsen, T. G. Etoh and K. Takehara, “High-speed imaging of drops and bubbles”, Annu. Rev. Fluid Mech. 40, 257-285 (2008).

Summary

In summary, the study is interesting and novel, with high quality experiments. The observation of unprecedented low contact times is specific to initial, vertical impacts of drops on to a horizontal surface with structure specific to the size of the drop. There is clear potential for related experimentation to broaden the scope of this work.