Historical perspectiveRecent advances in the study and design of parahydrophobic surfaces: From natural examples to synthetic approaches
Graphical abstract
Introduction
While nature has provided examples over centuries of interfaces that have unexpected and unique interactions with water, it has only been more recently that the number of studies concerning the design and fabrication of surfaces that mimic these properties has exploded in number and popularity. Amongst the naturally observed phenomena, two major wetting regimes have been identified: self-cleaning behavior most commonly observed on the lotus leaf and thus named the lotus leaf effect [1], [2] and the sticky behavior observed on rose petals and deemed the petal effect [3], [4]. Surfaces that follow these wetting phenomena are both characterized by high contact angles with water (θw) but they differ in their adhesive behaviors. Interfaces that mimic the lotus leaf effect have a very low sliding angle and contact angle hysteresis (typically CAH < 5°) and are described as superhydrophobic and self-cleaning. With this behavior, the lotus leaf remains dry even in rainy conditions, as any liquid droplets that encounter the surface readily roll off and pick up dust particulates as they are ejected off the plant, cleaning it in the process. Besides the lotus leaf, many other natural interfaces have this self-cleaning behavior, one example being the Rosa Hybrid Tea (cv. Showtime) [5]. Surfaces typical of the petal effect are strikingly different and have very strong water adhesion, which often manifests as water being pinned to a surface and staying adhered even if the interface is turned completely upside down. This regime is most notably associated with the behavior observed on various rose-petals [6], but is also observed on cacti [3] and beetles [7].
The source of inspiration for synthetic development and theoretical studies of surfaces with unique wetting properties has come from these natural observations on leaves, plants, animals and specimens that collect, repel or interact with water in a unique manner. While the thermodynamics that dictate the contact angle between a water droplet and a smooth surface has been understood for over 200 years [8], a general understanding of how macroscopically similar surfaces can have drastically different adhesive properties was developed much later. These findings were supported by the advent and development of modern microscopic and analytical techniques that permit nano-scale observation of natural surfaces. The use of these modern analytical tools has provided a wealth of information about the topographical structures and surface chemistry that lend to the wetting behaviors associated with both the lotus leaf and petal effects. After numerous observations of naturally occurring surfaces with unique wetting behaviors, it has become obvious that micro- and nano-scale surface topography are key contributors to the observed contact angle and adhesion with liquids on various interfaces. This understanding has encouraged and led to the development of more sophisticated theories to describe wetting behavior on non-smooth surfaces that combine both thermodynamic and physical contributions, such as the Cassie-Baxter and Wenzel theories which will be presented and described in this review [9], [10].
The number of research studies that design or study surfaces following the lotus leaf effect are especially numerous over the past 20 years, which is not surprising as various applications have been identified for interfaces with self-cleaning behavior including textiles [11], [12], anti-icing [13], [14], [15], anti-fog [16], [17] and anti-bioadhesive surfaces [18]. Similarly, sticky, adhesive surfaces that interact with liquids following the petal effect have been explored for a growing number of applications with significant global and industrial use. Included in this list are the fabrication of microarrays for small (nano or micron) scaled reactions [19] as well as membrane separations [20]. One of the most relevant and sought after applications for surfaces with strong adhesion to water or other liquids is towards the use of guided liquid transport or collection [1], [21], [22], [23], [24], [25], [26], [27], [28], [29]. Notably, strategies that are appropriate for large-scale and efficient collection of water especially in arid and dry climates are highly sought-after and researched. An understanding of adhesive surfaces and feasible strategies to develop them on a larger scale could provide a pathway to solve numerous global problems related to public health, economy, and water reserves.
Despite the tremendous need for fabrication and efficient design of both self-cleaning as well as adhesive types of surfaces, there exist significantly more comprehensive reviews concerning the study and development of interfaces following the lotus leaf effect. This is in part, a result of the more recent development and understanding of naturally adhesive surfaces. To bridge this gap and to address the recent developments and efforts towards both understanding adhesive behavior with liquids and trying to mimic this behavior synthetically, this paper presents a comprehensive review on the theoretical background and mechanism of the petal effect. Furthermore, it includes an overview of natural surfaces that display this behavior, as well as a discussion of currently employed synthetic strategies to replicate these interfaces.
Section snippets
Theories of wetting
On a smooth and homogeneous interface, the contact angle with a water is described by the Young equation, which combines the influences of the solid-vapor, solid-liquid, and liquid-vapor surface tensions into the following equation [30]:
Where θY is referred to as the Young's angle, and γSV is the interfacial tension between the solid (S) and vapor (V) phase, γSL the interfacial tension between the solid and liquid phase, and γLV that between the liquid and vapor phase. Usually, γ
Surfaces with strong adhesion found in nature
Parahydrophobic surfaces are commonly encountered in nature and as mentioned, it is from the study of these interfaces that the influence of surface topography and chemistry has become well understood in the context of contact angles and adhesion behavior. The surface topography of rose petals (Fig. 5), the most well known natural example of a parahydrophobic surface, is comprised of micropapillae evenly distributed across the petal surface. Since these papillae have a larger pitch value than
Synthetic methods to surfaces with strong adhesion
With numerous natural examples of parahydrophobic surfaces, much research has focused on identifying ways in which the surface nano- and micro-structures observed on plants and animals can be fabricated and mimicked synthetically. With this research, comes many design considerations including: the amount of time to fabricate the surface, overall cost, toxicity of materials used, efficiency of the process, ability to form surfaces on a large or industrial scale, etc. In general, the synthetic
Conclusion & outlook
Until recently, limited understanding and knowledge of the chemistry and physics behind surfaces that have both strong adhesion as well as high contact angles with water was available. This is somewhat surprising as this behavior, more commonly referred to as the petal effect has been observed on numerous natural surfaces and interfaces including: rose petals, gecko feet, scallion leaves, beetle shells, peach skin, etc. With the advent of microscopic techniques that permit the observation of
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2022, PolymerCitation Excerpt :Interestingly enough, SEM micrographs at high magnification did not show significant differences in these surfaces, as shown in Fig. 3d. Therefore, the distinct wetting behavior cannot be ascribed to the existence of a submicrometer structure in the surfaces exhibiting the lowest Δθw values. In consequence, it can be concluded that the roughness generated by the S/C treatment can be finely tuned by changing the coagulant solvent nature, varying from a rose-petal or parahydrophobic behavior with large Δθw values [34] for surfaces with low Sa values to a surface with superhydrophobic properties for the same chemical surface. Taking into account these results and for the sake of clarity, four S/C treatment conditions were chosen to comparatively study the topographies generated in all the blends and their relation to the final wetting properties (see Table 1).