Water Wetting on Sub-Micron Scale
Leaf Surfaces Studied with In Situ Electron Microscopy
- Fig. 1: Leaf of Nelumbo nucerifa (A taken from ) and Aquilegia vulgaris (B) and corresponding secondary electron images (C and D) taken at an accelerating voltage of 2 kV and 100 Pa water vapor in low vacuum mode after 10° sample tilting.
- Fig. 2: (A-R) Water micro-droplet evaporation on the leaf surface of Aquilegia vulgaris at T = 276 K and p = 810 - 790 Pa (image size 70 µm x 70 µm). (S) Side view of water droplet (blue) lying on a wax (red) covered surface. (T) p-T phase diagram of water.
- Fig. 3: (A) ESEM image of Aquilegia vulgaris at p = 810 and p = 790 Pa (B). (C) Contact angle determination for the asperities and the wrinkles (D). The red line represents a linear fit of the results of 10 water micro-droplets. (E) ESEM image of water patches left on the surface of Aquilegia vulgaris. Noise-filtered secondary electron image of a wax covered asperity (F) and wrinkle (G). The perimeter of the red colored part divided by the projected area represents the contact line density.
The wetting behavior of natural or technical surfaces is usually studied by contact angle goniometry. By use of an Environmental Scanning Electron Microscope (ESEM), it is possible to measure the nucleation, growing and contact angle of aqueous micro-droplets in-situ. Taking gravitation and capillary forces into account, the contact angle of water on the leaves of Aquilegia vulgaris is determined by the contact line density of the nanoscopic wax crystals that cover the entire leaf surface.
Many leaves show superhydrophobic wetting behavior reflected by contact angles of Θ > 150° and roll-off angles typically lower than 10° . This wetting behavior is mainly caused by the hydrophobic nature of the epicuticular waxes found on the leaf surfaces . As an example, the sacred lotus (Nelumbo nucerifa) exhibits a contact angle of about 160° and a roll-off angle below 5° measured by contact angle goniometry . Its extraordinary self-cleaning properties were referred to as “lotus effect” and resulted into several technical applications . It has been claimed that the combination of surface chemistry and surface topography at the micron and sub-micron scale are the prerequisite to achieve superhydrophobicity .
Environmental Scanning Electron Microscopy, a technique developed in the 1970s , is capable of resolving surface features under dynamic water wetting conditions during imaging . Although wetting experiments of the lotus plant were performed both by light and scanning electron microscopy , a microstructural water contact angle analysis of superhydrophobic plant surfaces under dynamic wetting conditions using in situ electron microscopy has not been performed.
For a detailed analysis of the wetting phenomena on a superhydrophobic plant leaf, we chose Aquilegia vulgaris, a plant that shows epicuticular wax crystals at the top face of the leaf  and a surface topography similar to the lotus plant (see fig. 1A-D). The wetting experiments were performed using an environmental scanning electron microscope FEI Quanta 400 FEG with a built-in cooling stage operating at a temperature of T = 276 K and a water vapor pressure range of p = 700-1000 Pa.
According to the pressure-temperature phase diagram of water (see fig. 2T), the equilibrium water vapor pressure at the border to the liquid state is p = 760 Pa. Increasing the water vapor pressure inside the SEM chamber leads to condensation of water onto the sample; decreasing the water vapor pressure leads to evaporation of liquid water. For our study we initiated water droplet formation on the leaf surface by increasing the water vapor pressure up to 1000 Pa and controlled the droplet size by changing the water vapor pressure inside the SEM chamber. To visualize the droplet size and shape in situ, secondary electron images were captured at an accelerating voltage of 10 kV.
Water droplet formation started both on the wax covered asperities (papillae) and on the wax covered wrinkles of the leaf surface and grew as described by Rykaczewski . By reducing the pressure inside the chamber, the water droplets shrunk and water patches were left on the surface (fig. 2A-R). It is assumed that the diameter of these patches is equivalent to the contact area between the droplet and the underlying surface due to pinning of the contact line at the surface features. The maximum (2R) and minimum (D) droplet diameters were analyzed using the model of Extrand  and the contact angles Θ for the asperities and wrinkles were calculated by (fig. 2S): sinΘ = D/2R. (1)
Results and Discussion
Several water micro-droplets (ten droplets that arose at the wax covered asperities and another ten droplets that arose at the wax covered wrinkles after condensation at T = 276 K) were measured to determine their maximum diameter 2R at p = 810 Pa and minimum diameter D at p = 790 Pa (fig. 3A and 3B). In this experiment, the equilibrium water vapor pressure is slightly higher due to heat loss from the cooling stage to the top of the wax covered surface. A linear fit of all data points leads to a water contact angle of 159.4° ± 0.9° for the asperities and 160.3 ± 2.1° for the wrinkles (fig. 3C and 3D). These values are similar to that found for the lotus plant and other superhydrophobic leaf surfaces . Looking at the remained patches at the surface of Aquilegia vulgaris that left upon evaporation, we could observe that the water micro-droplets had only contact to the surface via the wax crystals both at the asperities and the wrinkles of the leaf surface (fig. 3E).
As discussed by Extrand , a drop will be suspended by a rough (or micro-structured) surface due to the interplay between body and surface forces. As a result the contact line density L = l/A (l: contact line between water droplet and the underlying surface, A: apparent contact area of the drop) determines whether a water droplet is suspended or will collapse. In the case of wax crystals on top of a leaf surface, the contact line is represented by the perimeter of wax crystals that are in contact to the water micro-droplet. For Aquilegia vulgaris we find a contact line density of L = 8.3*107 m-1 for the wax crystals on the asperities and of L = 8.6*107 m-1 for the wax crystals on the wrinkles (see fig. 3F and 3G). Extrand  calculated a critical contact line density of Lc = 3*104 m-1 for small droplets on a hydrophobic surface. If the contact line density L > Lc, a small water droplet will suspend, otherwise it will collapse. The measured values for the contact line density L on the wax covered asperities and wrinkles of Aquilegia vulgaris are much higher than the critical contact line density Lc for small droplets. As a result, the water micro-droplets are suspended by the underlying wax covered surface.
Water contact angles on Aquilegia vulgaris were determined by in situ electron microscopy for the asperity and the wrinkle part of the leaf surface. The measured contact angles are similar to the values found for other super-hydrophobic leaf surfaces measured by contact angle goniometry. Contact line densities of the wax covered asperities and wrinkles could be determined and are much higher than the critical contact line density given by Extrand. Overall, we conclude that the wax crystal density on top of a leaf surface is responsible for the high contact angle of water droplets on the leaf surface of Aquilegia vulgaris.
I would like to thank Dr. René Hensel for fruitful discussions and Prof. Eduard Arzt for ongoing support.
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