Historical perspective
Recent advances in the study and design of parahydrophobic surfaces: From natural examples to synthetic approaches

https://doi.org/10.1016/j.cis.2017.01.002Get rights and content

Highlights

  • Parahydrophobic surfaces are of interest for numerous applications including water harvesting.

  • The physical and chemical factors that contribute to parahydrophobic properties are presented.

  • Natural surfaces that display adhesive, parahydrophobic behavior are presented.

  • Current methods for the fabrication of parahydrophobic surfaces are discussed in detail.

  • Promising, new, energy efficient, and in-situ fabrication techniques are described.

Abstract

Parahydrophobic surfaces are an interesting class of materials that combines both high contact angles and very strong adhesion with wetting fluids, most commonly water. This unique set of properties makes parahydrophobic surfaces attractive for a variety of applications, including water harvesting and collection, guided fluid transport, and membrane development, amongst many others. Taking inspiration from natural surfaces that display this same behavior such as rose petals and gecko feet, synthetic approaches aim to incorporate the nano- and micro-scale topography as well as the low surface energy chemistry found on these interfaces. Here, we discuss the chemical and physical factors that contribute to parahydrophobic behavior and provide a comprehensive overview on the current technologies and procedures used towards constructing surfaces that mimic this behavior already observed in nature. This includes etching processes, colloidal assemblies, deposition methods, and in situ growth of surface features. Furthermore, issues such as ease of scale-up, efficiency of technical procedures, and other current challenges associated with these methods will be discussed to provide insight as to the future directions for this growing area of research.

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]:cosθY=γSVγSLγLV

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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