Curvature-Enhanced Superomniphobic Property for Minimizing Contact Time of Low-Surface-Tension Liquid

Abstract

In nature, the springtail represents an ideal superomniphobic system, exhibiting remarkable resistance to organic liquids in both static and dynamic states. This behavior is attributed to the hierarchical structure of their skin, consists of micro- and nanostructures. While numerous artificial superomniphobic surfaces have been developed to mimic its geometry and properties, previous designs are limited to flat surfaces and failed to incorporate the curvature of the springtail's cuticle. Here, a curved superomniphobic surface is first developed that mimics both the curved shape and hierarchical structure of springtail skin. This system developed on the flexible substrate reveals the significant role that curvature plays in reducing the contact time of low-surface-tension liquid. While the static repellency on curved and flat surfaces is comparable, droplet rebound dynamics are distinctive on curved surfaces, showing asymmetric bouncing that conforms to the curvature. This effect intensifies with increased curvature, leading to a reduction in contact time by up to 54%, a record for organic liquid. This study uncovers the crucial role of surface curvature in springtail superomniphobicity and offers valuable insights for designing advanced omniphobic systems.

Keywords: asymmetric bouncing; contact times; hierarchical structures; superomniphobic surfaces; surface curvatures.

Figure 1

Design of the curved superomniphobic surface inspired by the springtail. a) Schematic of the springtail with curvature and hierarchical structures. b) Photograph images of the springtail resisting raindrops. c) Photograph of the curved superomniphobic surface and SEM image of fabricated hierarchical structures. Scale bar, 10 μm. d) Photograph of curved superomniphobic surface shielding organic liquids. e) Schematic of fabricating the curved superomniphobic surface. f) Photograph of curvature‐controlled surfaces. g) SEM images of the superomniphobic surface with hierarchical structure incorporating microwrinkle and serif‐T‐shaped nanostructures in top‐view and cross‐sectional view (inset). Scale bars: 10, 1 μm, and 200 nm.

Figure 2

Static repellency of various organic liquids on flat and curved superomniphobic surfaces. a) Apparent contact angles of various organic liquids (ethanol, acetonitrile, olive oil, and ethylene glycol) on flat surfaces with four different wrinkle dimensions (λ₅, λ₈, λ₁₀, and λ₁₇). b) Dynamic contact angles and CAH of tested organic liquids on flat λ₁₀. c) Photograph image of dyed organic liquids on flat λ₁₀. Scale bar, 2 mm. d) CA of ethanol on surface curvature engineered surfaces (D/D ₀ ≈ 1, 3, 5, and 9). e) Ethanol droplet rolling off from D/D ₀ ≈ 1 surface. f) Bending stability of superomniphobic property under 1000 cycles of bending (D/D ₀ ≈ 1). Error bars denote the standard deviation over three experiments. Data are presented as the mean ± standard deviation (n = 3).

Figure 3

Asymmetric bouncing of acetonitrile droplet on the curved surface (D/D ₀ ≈ 5) which induces rapid droplet rebound. a,b) Sequential snapshot images (top and side view) of (a) asymmetric bouncing at We ≈79 on the curved surface (D/D ₀ ≈ 5) and (b) droplet rebound at We ≈79 on the flat surface. c,d) Temporal evolution of dimensionless contact line distance (D ^* ) in the axial and azimuthal directions at We ≈79 on the (c) curved surface and (d) flat surface. e,f) Sequential snapshot images (top and side view) of asymmetric bouncing (e) at We ≈225 on the curved surface (D/D ₀ ≈ 5) and droplet rebound (f) at We ≈225 on flat surface. g,h) Temporal evolution of dimensionless contact line distance (D ^* ) in the axial and azimuthal directions at We ≈225 (g) on the curved surface and (h) on the flat surface. i) The spreading asymmetry ratio (k) under different We. j) Contact time on the curved surface compared to the flat surface under different We. Regime I, asymmetric bouncing; Regime II, asymmetric bouncing with splash. k) Spreading and retracting time constituting contact time under different We.

Figure 4

Evaluation of asymmetric bouncing on curvature‐engineered surfaces (D/D ₀ ≈ 1, D/D ₀ ≈ 3, D/D ₀ ≈ 5, D/D ₀ ≈ 9). a,b) Sequential snapshot images of acetonitrile asymmetric bouncing behaviors on curvature‐engineered surfaces (a) at We ≈45 and (b) at We ≈337.5. c) The asymmetric ratio (k) as a function of We on each curved surface. d) Contact time reduction as a function of We on each curved surface compared to the flat surface. e) Comparison of contact time reduction (%) between our study with previous literature.