Plants have a unique method of influencing the wetting properties of their surfaces by controlling the composition and morphology of exterior wax layers excreted from the plant surfaces. In this way nature has been able to use one material to achieve both the hydrophobic nature and the high degree of roughness needed to achieve a superhydrophobic wetting state. Superhydrophobicity is of significant interest as it has many applications which range from self-cleaning surfaces, and anti-icing surfaces, to drag reduction and antifouling.
Figure 1 – (a) Schematic illustration of the hierarchical surface created via thermal deposition of paraffin wax. Numbers 1-3 indicated on the illustration match the corresponding features in (b). (b) HR-SEM image of said fluorinated wax structures. (c) Schematic illustration of a liquid drop on top of the hierarchical wax structures.
Our lab has studied these biological systems to get inspiration for fabrication methods and engineering applications of single-component superhydrophobic surfaces. Using thermally evaporated paraffin waxes deposited on various substrates we showed that an increase of the hydrophobicity occurs over time until a superhydrophobic state is achieved. This is was shown to occur via the self-assembly of the paraffin waxes at room temperature into rough hierarchical surfaces (1, 2). Fluorinated paraffin waxes were also used in a similar method to create hierarchical surfaces exhibiting both superoleophobic and superhydrophobic properties in tandem with exceptional surface stability (3). This fabrication method can be easily utilized in a variety of applications where either a superhydrophobic, superoleophobic, or both are needed.
Furthermore the liquid-surface interface of a sessile drop in a superhydrophobic state was studied using direct imaging via confocal microscopy (4). This is the first time the shape of the water-air interface bellow a drop on a superhydrophobic surface was imaged directly and that the curvature of the water-air interface was shown to be constant and close to zero for liquids of varying density and surface tension.
Figure 2 – Imaging the water-air interface beneath a water drop on a superhydrophobic surface. (a) Macro view of the confocal immersion lens inside the water drop, (b) 3D image of the water-air interface, (c) water drop demonstrating a 170˚ contact angle indicating a superhydrophobic state, (d) 3D image of the microtextured surface (4).
1. S. Pechook and B. Pokroy, “Self‐Assembling, Bioinspired Wax Crystalline Surfaces with Time‐Dependent Wettability,” Advanced Functional Materials, vol. 22, pp. 745-750, 2012.
2. S. Pechook and B. Pokroy, “Bioinspired hierarchical superhydrophobic structures formed by n-paraffin waxes of varying chain lengths,” Soft Matter, vol. 9, pp. 5710-5715, 2013.
3. S. Pechook, et al. and B.Pokroy, “Bio-Inspired Superoleophobic Fluorinated Wax Crystalline Surfaces”, Advanced Functional Materials, vol. 23, pp. 4572-4576, 2013.
4. B. Haimov, et al. and B.Pokroy, “Shape of Water–Air Interface beneath a Drop on a Superhydrophobic Surface Revealed: Constant Curvature That Approaches Zero”. The Journal of Physical Chemistry C, vol. 117, pp. 6658-6663, 2013.