Understanding droplet mobility on surfaces is crucial for various biological and technological applications [1–3]. Traditionally, wetting dynamics is modelled on regular surfaces like micropillar arrays [4–8], yet many real-world superhydrophobic surfaces exhibit stochastic roughness with irregular characteristics [9–11]. Our study reveals that the wetting dynamics on these stochastic surfaces fundamentally differ from those on regular surfaces [12]. We found that droplet friction in the quasi-static regime (𝑣d ~ μm/s) is significantly higher than in dynamic conditions (𝑣d ~ cm/s to m/s), contrasting with the behavior observed on regular surfaces. Using state-of-the-art imaging and modelling, we demonstrate that at lower velocities, the liquid-air interface has extended adaptation times to surface irregularities (τ ~ s). This adaptation leads to an increased liquid-solid contact fraction. Conversely, at higher velocities, the interface has reduced adaptation time (τ ~ μs). Thus, we introduce the concepts of static and dynamic liquid-solid contact fractions to explain these differences while highlighting the stochasticity levels required to repel droplets of varying sizes effectively. These findings provide important insights that will contribute to advancements in fabrication methods and lead to novel superhydrophobic materials with potential applications in various fields, such as microfluidics, coatings, and biomimetic surfaces.

References

[1] D. Quéré, Annu. Rev. Mater. Res. 2008, 38, 71.

[2] T. M. Schutzius, S. Jung, T. Maitra, G. Graeber, M. Köhme, D. Poulikakos, Nature 2015, 527, 82.

[3] J. C. Bird, R. Dhiman, H.-M. Kwon, K. K. Varanasi, Nature 2013, 503, 385.

[4] M. Reyssat, D. Quéré, J. Phys. Chem. B 2009, 113, 3906.

[5] D. W. Pilat, P. Papadopoulos, D. Schäffel, D. Vollmer, R. Berger, H.-J. Butt, Langmuir 2012, 28, 16812.

[6] C. W. Extrand, Langmuir 2002, 18, 7991.

[7] C. Dorrer, J. Rühe, Langmuir 2007, 23, 3179.

[8] H.-J. Butt, N. Gao, P. Papadopoulos, W. Steffen, M. Kappl, R. Berger, Langmuir 2017, 33, 107.

[9] H. J. Ensikat, P. Ditsche-Kuru, C. Neinhuis, W. Barthlott, Beilstein J. Nanotechnol. 2011, 2, 152.

[10] W. Barthlott, T. Schimmel, S. Wiersch, K. Koch, M. Brede, M. Barczewski, S. Walheim, A. Weis, A. Kaltenmaier, A. Leder, H. F. Bohn, Adv. Mater. 2010, 22, 2325.

[11] L. Wang, Soft Matter 2023, 19, 1058.

[12] Vuckovac, M. et al., in preparation.