Invited Feature Article pubs.acs.org/Langmuir
Bioinspired Structured Surfaces Bharat Bhushan* Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics, The Ohio State University, Columbus, Ohio 43210-1142, United States ABSTRACT: Nature has evolved objects with desired functionality using commonly found materials. Nature capitalizes on hierarchical structures to achieve functionality. The understanding of the functions provided by objects and processes found in nature can guide us to produce nanomaterials, nanodevices, and processes with desirable functionality. Various natural objects which provide functionality of commercial interest have been characterized to understand how a natural object provides functionality. We have modeled and fabricated structures in the lab using nature’s route and developed optimum structures. Once it is understood how nature does it, optimum structures have been fabricated using smart materials and fabrication techniques. This feature article provides an overview of four topics: Lotus effect, rose petal effect, gecko feet, and shark skin.
1. INTRODUCTION Nature has gone through evolution over the 3.8 billion years since life is estimated to have appeared on earth. Nature has evolved objects with desired functionality using commonly found materials. These function on the macroscale to the molecular scale. The understanding of the functions provided by objects and processes found in nature can guide us to produce nanomaterials, nanodevices, and processes with desirable functionality. Biologically inspired design or adaptation or derivation from nature is referred to as “biomimetics”. It means mimicking biology or nature. “Biomimetics” is derived from the Greek word biomimesis. Other words used include bionics, biomimicry, and biognosis. The field of biomimetics is highly interdisciplinary. It involves the understanding of biological functions, structures, and principles of various objects found in nature by biologists, physicists, chemists, and material scientists and the design and fabrication of various materials and devices of commercial interest from bioinspiration. Biological materials are highly organized from the molecular scale to the nanoscale, microscale, and macroscale, often in a hierarchical manner with intricate nanoarchitecture that ultimately makes up a myriad of different functional elements. Nature uses commonly found materials. The properties of the materials and surfaces result from a complex interplay between the surface structure and the morphology and physical and chemical properties. Many materials, surfaces, and devices provide multifunctionality. Molecular scale devices, superhydrophobicity, self-cleaning, drag reduction in fluid flow, antifouling, energy conversion and conservation, reversible adhesion, aerodynamic lift, materials and fibers with high mechanical strength, biological self-assembly, antireflection, structural coloration, thermal insulation, self-healing, and sensory aid mechanisms are some of the examples found in nature which are of commercial interest.1,2 Various features found in natural objects are on the nanoscale. The major emphasis on nanoscience and nanotechnology since the early 1990s has provided a significant impetus in mimicking © 2012 American Chemical Society
nature using nanofabrication techniques for commercial applications. Biomimetics has spurred interest across many disciplines. Various biomimetics-inspired materials and objects are being fabricated in laboratories around the world, and some have found industrial applications. In this feature article, an overview of four topics as shown in Figure 1 is provided: Lotus effect, rose petal effect, gecko feet, and shark skin. Each section starts with an introduction followed by some recent contributions to the field and a summary with future directions. The objective of our research is to select objects from nature which provide functionality of commercial interest. We characterized natural objects to understand how they provide functionality, modeled them, and fabricated structures in the lab using nature’s route and developed optimum structures using our models. Nature has a limited toolbox and uses rather basic materials and routine fabrication methods. Once we understand how nature does it, we then fabricate optimum structures using smart materials and fabrication techniques.
2. LOTUS EFFECT Superhydrophobic surfaces with a high static contact angle above 150° and contact angle hysteresis (the difference between the advancing and receding contact angles) of less than 10° exhibit extreme water repellence and self-cleaning properties.3 At a low value of contact angle hysteresis, water droplets may roll in addition to sliding, which facilitates the removal of contaminant particles. Surfaces with low contact angle hysteresis have a low water roll-off (tilt) angle, which denotes the angle to which a surface must be tilted for water droplets to roll off. Superhydrophobic and self-cleaning surfaces are of interest for various applications including self-cleaning windows, windshields, Received: August 30, 2011 Revised: December 20, 2011 Published: January 10, 2012 1698
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Invited Feature Article
droplet, and both the contact area and the adhesion to the surface are dramatically reduced.3 Based on the so-called Lotus effect, one of the ways to increase the hydrophobic property of the surface is to increase surface roughness, so roughness-induced hydrophobicity has become a subject of extensive investigations. Wenzel10 suggested a simple model predicting that the contact angle of a liquid with a rough surface is different from that with a smooth surface. Cassie and Baxter11 showed that a gaseous phase including water vapor, commonly referred to as “air” in the literature, may be trapped in the cavities of a rough surface, resulting in a composite solid− liquid−air interface, as opposed to the homogeneous solid− liquid interface. These two models describe two possible wetting regimes or states: the homogeneous (Wenzel) and the composite (Cassie−Baxter) regimes. 2.1. Modeling of Contact Angle for a Liquid in Contact with a Rough Surface. As stated earlier, superhydrophobic and self-cleaning surfaces should have both a high contact angle and a low contact angle hysteresis. Liquid may form either a homogeneous interface with a solid or a composite interface with air pockets trapped between the solid and liquid. In this section, mathematical models which provide the relationships between roughness and contact angle are discussed. 2.1.1. Homogeneous (Wenzel) Interface. Consider a rough solid surface with a typical size of roughness details smaller than the size of the droplet as shown in Figure 2 (left). For a droplet in contact with a rough surface without air pockets, referred to as a homogeneous interface with complete wetting, the contact angle is given as10
cos θ = R f cos θ0
(1)
where θ is the contact angle for the rough surface, θ0 is the contact angle for the smooth surface, and Rf is a roughness factor defined as the ratio of the solid−liquid area ASL to its projection on a flat plane, AF
Figure 1. Montage of four examples from nature.
exterior paints for buildings and navigation of ships, utensils, roof tiles, textiles, solar panels, and applications requiring a reduction of drag in fluid flow, e.g., in micro/nanochannels. They also exhibit antifouling which is of interest such as in membranes used for desalination and water purification. These surfaces can also be used for energy conversion and energy conservation. Condensation of water vapor from the environment and/or process liquid film can form menisci, leading to high adhesion in devices requiring relative motion.4 Superhydrophobic surfaces are needed to minimize adhesion between a surface and a liquid. A model surface for superhydrophobicity and self-cleaning is provided by the leaves of the Lotus plant (Nelumbo nucifera) (Figure 1, top left).5−9 The leaf surface is very rough due to socalled papillose epidermal cells, which form asperities or papillae. In addition to the microscale roughness, the surface of the papillae is also rough with submicrometer-sized asperities composed of 3-D epicuticular waxes. The waxes of the Lotus are tubules, but on other leaves waxes also exist in the form of platelets or other morphologies. Lotus leaves have hierarchical structures, which have been studied by Bhushan and Jung.8 The water droplets on these surfaces readily sit on the apex of the nanostructures, because air bubbles fill the valleys of the structure under the droplet. Therefore, these leaves exhibit considerable superhydrophobicity. The water droplets on the leaf surfaces remove any contaminant particles present when they roll off, leading to self-cleaning. It has been reported that nearly all superhydrophobic and self-cleaning leaves consist of an intrinsic hierarchical structure.6 Water on such a surface forms a spherical
Rf =
ASL AF
(2)
The model predicts that a hydrophobic surface (θ0 > 90°) becomes more hydrophobic with an increase in Rf, and a hydrophilic surface (θ0 < 90°) becomes more hydrophilic with an increase in Rf. 2.1.2. Composite (Cassie−Baxter) Interface. For a rough surface, a wetting liquid will be completely absorbed by the rough surface cavities while a nonwetting liquid may not penetrate into surface cavities, resulting in the formation of air pockets, leading to a composite solid−liquid−air interface as shown in Figure 2 (middle). Cassie and Baxter11 extended the Wenzel equation for the composite interface, which was originally developed for the homogeneous solid−liquid interface. For this case, there are two sets of interfaces: a solid− liquid interface with the ambient environment surrounding the droplet and a composite interface involving liquid−air and solid−air interfaces. In order to calculate the contact angle for the composite interface, Wenzel’s equation can be modified by combining the contribution of the fractional area of wet surfaces and the fractional area with air pockets (θ = 180°)
cos θ = R f cos θ0 − fLA (R f cos θ0 + 1) (3) where f LA is the fractional flat geometrical area of the liquid−air interface under the droplet. In this regime, a hydrophobic smooth surface can be changed to superhydrophobic with an 1699
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Figure 2. Schematic of a liquid droplet in contact with smooth and rough solid surfaces (left), schematic of the formation of a composite solid− liquid−air interface for a rough surface (middle), and schematic of a tilted surface (with tile angle α) with a moving liquid droplet (θadv and θrec are advancing and receding contact angles, respectively), showing the definition of contact angle hysteresis.
Nosonovsky and Bhushan3 derived a relationship for contact angle hysteresis as a function of roughness, given as
increase in Rf but a lower value as compared to that for the Wenzel regime. Even for a hydrophilic surface, the contact angle increases with an increase of f LA, and at a high value of f LA, a surface can become hydrophobic. However, the value of f LA required may be unachievable, or the formation of air pockets may become unstable. Using the Cassie−Baxter equation, the value of f LA at which a hydrophilic surface could turn into a hydrophobic one is given as12
R f cos θ0 fLA ≥ R f cos θ0 + 1
for θ0 < 90°
cos θadv0 − cos θrec0 − sin θ cos θrec0 − cos θadv0 1 − fLA R f 2(R f cos θ0 + 1)
θadv − θrec = (1 − fLA )R f =
(
)
(5)
For the homogeneous interface, f LA = 0, whereas for a composite interface f LA is a nonzero number. It is observed from eq 5 that, for a homogeneous interface, increasing roughness (high Rf) leads to increasing contact angle hysteresis (θadv − θrec), while for a composite interface, an approach to unity of f LA provides both a high contact angle and a small contact angle hysteresis. Therefore, the composite interface is most desirable for superhydrophobicity and self-cleaning. 2.1.4. Stability of a Composite Interface and Role of Hierarchical Structure. Stability of the composite interface is an important issue.14−17,3 Even though it may be geometrically possible for the system to become composite, it may be energetically possible for the liquid to penetrate into the valleys between asperities and form a homogeneous interface. The formation of a composite interface is a multiscale phenomenon which depends upon the relative sizes of the liquid droplet and roughness details. A composite interface is metastable, and its stability is an important issue. Even though it may be geometrically possible for the system to become composite, it may be energetically profitable for the liquid to penetrate into the valleys between asperities and form the homogeneous interface. The composite interface is fragile and can be irreversibly transformed into the homogeneous interface, thus damaging superhydrophobicity. Many authors have investigated the stability of artificial superhydrophobic surfaces and showed that whether the interface is homogeneous or composite may depend on the history of the system, in particular whether the liquid was applied from the top or condensed at the bottom. Nosonovsky and Bhushan3 have identified mechanisms which lead to the destabilization of the composite interface, namely, the capillary waves, condensation and accumulation of nanodroplets, and surface inhomogeneity. They also reported that a convex surface leads to a stable interface and high
(4)
High Rf can be achieved by both micropatterns and nanopatterns. For a high f LA, a nanopattern is desirable because generation of the liquid−air interface depends upon the ratio of the distance between two adjacent asperities and the droplet radius. Furthermore, nanoscale asperities can pin liquid droplets and thus prevent liquid from filling the valleys between asperities. Transition between Wenzel and Cassie-Baxter regimes is dependent upon the size of the droplet and the pitch values of the micro/nanopattern.13 2.1.3. Contact Angle Hysteresis. The contact angle hysteresis is another important characteristic of a solid−liquid interface. If a droplet sits over a tilted surface (as shown in Figure 2, right), the contact angle at the front and back of the droplet corresponds to the advancing contact angle and the receding contact angle, respectively. The advancing angle is greater than the receding angle, which results in contact angle hysteresis. Contact angle hysteresis occurs due to surface roughness and heterogeneity. Although for surfaces with the roughness carefully controlled on the molecular scale it is possible to achieve a contact angle hysteresis as low as
