Mimicking natural superhydrophobic surfaces and grasping the wetting process: A review on recent progress in preparing superhydrophobic surfaces
Graphical abstract
(a) A glycerol drop on Euphorbia myrsinites, which is a robust specimen and well suited to show the surface's repellence against the liquid droplet. (b) The upper side surface of the lotus leaf without the shrinkage artifact. (c) The wax tubules from the upper side of the lotus leaf.
Highlights
► We review the most recent progress in preparing superhydrophobic surfaces. ► The fundamental theories of wetting phenomena are investigated. ► The natural models inspiring to the creation of superhydrophobic surfaces are inspected. ► The main discussion focuses on the formation of surface roughness and structure. ► We present the creation of superhydrophobic surfaces through a variety of techniques.
Introduction
The superhydrophobicity (hydrophobicity) of solid surfaces has been investigated with considerable attention over the past few years and remarkable progress has been achieved. It has been discovered that water droplets on hydrophobic surfaces can exhibit a contact angle higher than 90°, and some can even be approaching approximately up to 180° [1], [2], [3], [4], [5]. In particular, the contact angles related to superhydrophobic (or ultrahydrophobic) surfaces are greater than 150°. And those superhydrophobic surfaces are very likely to have phenomenal roughness with micro- or nanosized (or even smaller) protrusions coming out of the surface [6], [7]. Therefore, the liquid might contact only a few bits of the superhydrophobic surface without fully wetting it. Indeed, fluid interacting with superhydrophobic surfaces is one important discipline of research in the 21st century, and can essentially influence a lot of cutting-edge topics in engineering and biotech research which involve surface structures, fluid motivation, and their physical and chemical properties. Basically, the contact angle related wetting phenomena are of great interest and importance to current research progress. A considerable amount of work has been carried out to study the involved mechanisms and principles [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. Interestingly, many methods that are used to create manmade superhydrophobic surfaces are inspired by the “Lotus Effect” [32]. Recent research has indicated that there are even more superhydrophobic surfaces in nature [13], [33], [34], [35]; and this helps to promote the applications of biomimetic ideas into practical fields [36], [37], [38], [39].
Also, it is noticed that the underlying theories interpreting the phenomena are mainly focused on Young equation, Wenzel equation, and Cassie–Baxter equation [40], [41], while the wetting phenomena have been studied over the past decade. Based on these theories, people have understood that either surface energy or surface structure (roughness) can influence the contact angle of liquid droplets on solid surfaces. However, such equations are not sufficient to thoroughly explain the mechanisms of wetting phenomena, although they are still necessary. This review attempts to discuss concisely the most recent research progress into the superhydrophobicity of solid surfaces with regard to the wetting process, both theoretically and experimentally. From an overall point of view, the work discussed in this paper will include reviewing the fundamental theories and their complement, the superhydrophobic surfaces that are discovered in nature, and the way manmade superhydrophobic surfaces are prepared. To make this work clearly described, we refer to wetting as wetting phenomena of liquid droplets on solid surfaces within the context.
Section snippets
Theoretical models
As discussed in the Introduction, although some research work has been undertaken to study the wetting phenomena on superhydrophobic surfaces, there are still quite a few critical questions remaining unsolved. For example, it is not yet fully known how and when a stable wetting state can be achieved. An interesting observation that both homogeneous and heterogeneous wetting states could coexist on the same surface, as shown in Fig. 1, was reported by Callies and Quéré in 2005 [42]. The
Superhydrophobic surfaces
As explained by Wenzel and CB equations, surface roughness and its structure play a very important role in promoting extremely high contact angles. Also, thanks to the findings related to the surface of lotus leaves, people have started the bio-inspired research to understand and design those water repelling surfaces. The effort includes exploring natural surfaces which have complex roughness and significant hydrophobicity. This type of research work is extended in not only plants but also
Lithography
The lithographic process is a well-established technique and its sub-techniques used in making superhydrophobic surfaces include optical lithography (photolithography) [53], [59], [67], [101], [102], soft lithography [56], [103], [104], [105], [106], [107], nanoimprint lithography [94], [108], electron beam lithography [24], [109], X-ray lithography [92], and colloidal lithography [110]. On a general level, this sort of method prepares superhydrophobic surfaces by copying the information from a
Templating
Template-based methods are another imprint-related way to prepare superhydrophobic surfaces [117]. To develop the superhydrophobic surface pattern, templating can be involved with lithography [56], [107], [108], [110], [118], [119]. Thus it is plausible that the templates can be fabricated by using lithography. Also, templating is used to assist other methods preparing superhydrophobic surfaces [120]. The original prototypes of the templates can be filter paper [121], insect wings [122],
Plasma treatment
In quite a few cases, the surface pattern has been formed by the plasma treatment after lithography or templating methods [110], [126], [127]. However, the plasma treatment can also be used before lithography or templating [128], [129], and sometimes the plasma treatment and lithography can even alternate with each other during the surface processing [130]. Moreover, as mentioned earlier, the plasma treatment is somehow connected to etching techniques for preparing superhydrophobic surfaces.
CVD-based surface treatment
The plasma-enhanced chemical vapor deposition (PECVD) has recently received more attention and already been used to fabricate superhydrophobic surfaces. Phenomenally, superhydrophobic surfaces with supported Ag/TiO2 core-shell nanofibers were prepared at low temperature by PECVD [145]. The fibers were formed by an inner nanocrystalline silver thread covered by a TiO2 overlayer. Water contact angles depended on both the width of the fibers and their surface concentrations, and a maximum contact
Conclusions and outlook
Although superhydrophobicity is only a recently developed concept, it has already become important to a lot of research and will be potentially important to people's life. As stated in previous sections, a great amount of effort has been put into the research to understand the mechanisms that are related to superhydrophobicity on solid surfaces. This nature-inspired theory is an interdisciplinary subject which involves physics, chemistry, material science, and even biology. In the cutting edge
Acknowledgment
We thank H. J. Ensikat from University of Bonn, Germany, for his assistance to this work. We are grateful to UK Royal Society, China Scholarship Council and the Open Project of Key Laboratory of Bionic Engineering (Jilin University), Ministry of Education, China, 2010 (project no. K201008).
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