Description
Superhydrophobicity is a remarkable surface property observed in nature and widely exploited in engineering applications such as anti-wetting, anti-fogging, and anti-fouling coatings [1]. Achieving robust superhydrophobicity typically relies on micro-/nanostructure, among which doubly re-entrant architectures have emerged as particularly effective due to their re-entrant curvature that stabilizes liquid–air interfaces even for intrinsically wetting liquids [2]. While the static and dynamic wetting properties of doubly re-entrant surfaces have been widely reported, a quantitative understanding of how their geometric parameters govern droplet friction and mobility remains largely unexplored [3,4]. Here, we systematically investigate the frictional behaviour of water droplets moving on superhydrophobic doubly re-entrant surfaces using a cantilever-based force sensing approach coupled to a motorized translation stage. This approach enables direct measurement of friction and adhesion forces in the micro-Newton (µN) range. Water droplets were translated across the surfaces at controlled velocities spanning two orders of magnitude (0.1–10 mm s⁻¹). A series of doubly re-entrant surfaces were fabricated with independently varied structural parameters, including pillar spacing and spacing-to-diameter ratio. Despite exhibiting similar apparent static water contact angles in the range of approximately 130°–148°, the surfaces displayed pronounced differences in frictional behaviour. This indicates that contact angle alone is insufficient to predict droplet mobility on microstructured surfaces. Systematic friction measurements reveal that pillar geometry plays a dominant role in controlling droplet friction. In particular, reducing pillar diameter while increasing inter-pillar spacing significantly lowers friction forces by minimizing solid–liquid contact and suppressing contact-line pinning. An optimized design with a pillar diameter of 50 µm and a spacing-to-diameter ratio of 4:1 exhibited ultra-low friction forces as low as 3.47–4.12 ± 2.86 µN, representing one of the lowest friction regimes observed in this study. However, excessively large spacing was found to compromise the ability of the structure to mechanically support droplets, indicating an important trade-off between friction reduction and liquid retention. Velocity-dependent measurements further reveal non-linear friction at higher translation speeds, suggesting changes in contact-line dynamics as velocity increases. Complementary evaporation studies demonstrate that surface geometry influences droplet pinning and evaporation modes, with implications for deposit formation and coffee-ring effects on superhydrophobic microstructures. Overall, this work provides new mechanistic insight into how doubly re-entrant microstructural design governs droplet friction and mobility, beyond what can be inferred from static wetting measurements. The findings establish design principles for engineering low-friction, high-mobility superhydrophobic surfaces, with direct relevance to microfluidic droplet transport, and self-cleaning materials.
[1] Vu, H. H., Nguyen, N. T., & Kashaninejad, N. (2023). Re‐entrant microstructures for robust liquid repellent surfaces. Advanced Materials Technologies, 8(5), 2201836.
[2] Liu, T. L., & Kim, C. J. C. (2014). Turning a surface superrepellent even to completely wetting liquids. Science, 346(6213), 1096-1100.
[3] Backholm, M., Molpeceres, D., Vuckovac, M., Nurmi, H., Hokkanen, M. J., Jokinen, V., ... & Ras, R. H. (2020). Water droplet friction and rolling dynamics on superhydrophobic surfaces. Communications Materials, 1(1), 64.
[4] Singh, N. S., Jitniyom, T., Navarro-Cía, M., & Gao, N. (2024). Droplet impact on doubly re-entrant structures. Scientific Reports, 14(1), 2700.