Salvinia-like slippery surface with stable and mobile water/air contact line

Abstract

Superhydrophobic surfaces are widely used in many industrial settings, and mainly consist of rough solid protrusions that entrap air to minimize the liquid/solid area. The stability of the superhydrophobic state favors relatively small spacing between protrusions. However, this in turn increases the lateral adhesion force that retards the mobility of drops. Here we propose a novel approach that optimizes both properties simultaneously. Inspired by the hydrophobic leaves of Salvinia molesta and the slippery Nepenthes pitcher plants, we designed a Salvinia-like slippery surface (SSS) consisting of protrusions with slippery heads. We demonstrate that compared to a control surface, the SSS exhibits increased stability against pressure and impact, and enhanced lateral mobility of water drops as well as reduced hydrodynamic drag. We also systematically investigate the wetting dynamics on the SSS. With its easy fabrication and enhanced performance, we envision that SSS will be useful in a variety of fields in industry.

Keywords: Salvinia molesta; drag reduction; low adhesion; slippery Cassie state; stability.

Figure 1.

Design of the SSS. (a) Salvinia molesta floating leaf on which water drop displays stable Cassie state. Although the rational design of elastic eggbeater-shaped microstructure with surface energy gradient in vertical direction could stabilize the contact line to prevent impalement, such structure has strong adhesion because of the hydrophilic patches. (b) A pitcher plant-inspired slippery surface with molecularly smooth lubricant fixed on the top of the microstructure, which enables fast drop or liquid transportation. (c) The combination constitutes the SSS on which water drop shows slippery stable Cassie state. The black, purple and red solid arrows represent the directions of pressure (P), gravity (g) and the velocity of drop transport (V), respectively.

Figure 2.

Fabrication process and topological structure of the SSS. (a) Combination of hard and soft lithography. Micropillars are first made of the SU-8 photoresist and then coated with PDMS hemispheres. (b) ESEM images taken at an angle of 45° showing high regularity and structural details of the SSS (d = 36 , D = 25 μm, b = 120 μm, h = 33 μm, H = 45 μm). (c) Three-dimensional confocal microscopy image of a drop on the SSS and its cross-section (d = 15 , D = 10 , b = 60 μm, h = 16 μm, H = 20 μm). Fluorescent emissions from water, oil-infused PDMS and SU-8 pillars are shown in red, green and blue, respectively. Reflection is shown in purple.

Figure 3.

Lateral pinning and adhesion forces on the SSS. (a) Schematic of the experimental setup. The micro-needle moved at 0.01 mm/s and its deflection was used to measure force. The confocal microscope was used for imaging. (b) Confocal vertical cross-section of the receding side of a water drop moving to the right on the SSS. Colors as in Fig. 2c. (c) Apparent during needle movement. When the receding angle value was reached (dashed line), the contact line jumped. (d) Measured lateral adhesion force of water drop on the control surface and the SSS with time. The dimension is: d = 15 μm, D = 10 μm, b = 60 μm, h = 16 μm, H = 20 μm.

Figure 4.

Stability of the Cassie state on the SSS during evaporation. (a) Change of the contact base diameter of water drop L_CL with time on the control surface and the SSS (the insets are the selected snapshots, obtained with a high-speed camera, of water drops evaporating on the control surface and the SSS). (b) Apparent contact angle and (c) Laplace pressure of a water drop on the control surface and the SSS. (d) Confocal image of the reflection from the water/air cushion interface of a drop deposited on the surface and its X-Z cross-section along the white dotted line. We extract the microscopic contact angle θ_CL and the minimum height of the air cushion . Micropillars have been reconstructed with dimensions known from SEM measurements. (e) Thickness of the water/air cushion on the control surface and the SSS with time (the inset shows profiles of the air cushion through the center of the water drop, taken every 6 s). (f) Microscopic contact angle θ_CL on the control surface and the SSS with time (the intrinsic angles of the surface showed in Table S2). The dimension is: d = 36 μm, D = 25 μm, b = 120 μm, h = 33 μm, H = 45 μm.

Figure 5.

Contact line barrier at the contact line between a slippery surface and a hydrophobic surface (HPOS). (a) Schematic of a water drop spreading on a chemically homogeneous hydrophobic surface and a heterogeneous hydrophobic surface with a circular slippery surface at the center. A drop with a volume of 13 μL was deposited on the surface (circle 1) and expanded until the critical advancing contact angle was reached for homogeneous hydrophobic surface (circle 2). By further injecting water, the drop on homogeneous hydrophobic surface started to spread while the contact line of drop on the heterogeneous surface kept still until a higher critical advancing contact angle was reached (circle 3). (b) The change of apparent contact angle and the diameter of water drop as a function of time during spreading on the homogeneous surface and the heterogeneous surface. Numbers correspond to (a). (c) Contact line moving from a macro perspective to a micro perspective. (d) Critical force exerted on the water drop when impalement happened on the control surface and the SSS (dashed rectangles is the position when the Cassie-to-Wenzel transition happened). The dimension is: d = 15 μm, D = 10 μm, b = 30 μm, h = 16 μm, H = 20 μm. (e) Theoretical model of a drop (cyan) surrounded by an oil ridge (yellow) on a flat surface (grey). θ₁ and θ₂ are the contact angle values of the oil on the two sides of the ridge, θ_A, θ_B and θ₃ are the angles of the water/oil/air Neumann triangle, and R₁, R₂ are the radii of curvature of the water/oil and oil/air interfaces. Angles α, β define the position of the water/oil/air contact line.

Figure 6.

Application of SSS in drop impact and drag reduction. (a) Time-resolved variation of spread factor Lc/D (Lc is droplet contact length and D is the droplet diameter) on the control surface and SSS. (b) The restitution coefficient e as a function of Weber number We. (c) The velocity of tracers at the interface as a function of flow rate. The sizes of the structures used in the drop impact experiment are d = 15 μm, D = 10 μm, b = 30 μm, h = 22 μm, H = 26 μm. (d) Water/air interface impalement process at critical flow rate. The yellow color area in the images corresponds to the water/air interface on top or the reflection artifacts (yellow-black circle), and the black color indicates absence of reflection and thus corresponds to the water/solid interface. The sizes of the structures used in the drag reduction experiment are d = 15 μm, D = 10 μm, b = 30 μm, h = 16 μm, H = 20 μm.