Self-cleaning surfaces are a class of materials with the inherent ability to remove any debris or bacteria from their surfaces in a variety of ways. The self-cleaning functionality of these surfaces are commonly inspired by natural phenomena observed in lotus leaves, gecko feet, and water striders to name a few. The majority of self-cleaning surfaces can be placed into three categories: superhydrophobic superhydrophilic photocatalytic.
History The first instance of a self-cleaning surface was created in 1995. Paz et al. created a transparent titanium dioxide (TiO2) film that was used to coat glass and provide the ability for the glass to self-clean. The first commercial application of this self-cleaning surface, Pilkington Activ, was developed by Pilkington glass in 2001. This product implements a two-stage cleaning process. The first stage consists of photocatalysis of any fouling matter on the glass. This stage is followed by the glass becoming superhydrophilic and allowing water to wash away the catalyzed debris on the surface of the glass. Since the creation of self-cleaning glass, titanium dioxide has also been used to create self-cleaning nanoparticles that can be incorporated into other material surfaces to allow them to self-clean.
Surface characteristics The ability of a surface to self-clean commonly depends on the hydrophobicity or hydrophilicity of the surface. Whether cleaning aqueous or organic matter from a surface, water plays an important role in the self-cleaning process. Specifically, the contact angle of water on the surface is an important characteristic that helps determine the ability of a surface to self-clean. This angle is affected by the roughness of the surface and the following models have been developed to describe the "stickiness" or wettability of a self-cleaning surface.
Young's model
Young and colleagues proposed Young's model of wetting that relates the contact angle of a water droplet on a flat surface to the surface energies of the water, the surface, and the surrounding air. This model is typically an oversimplification of a water droplet on an ideally flat surface. This model has been expanded upon to consider surface roughness as a factor in predicting water contact angle on a surface. Young's model is described by the following equation:
where is the contact angle of water on the surface is the surface energy of the surface-air interface is the surface energy of surface–liquid interface is the surface energy of liquid–air interface
Wenzel's model When a water droplet is on a surface that is not flat and the surface topographical features lead to a surface area that is larger than that of a perfectly flat version of the same surface, the Wenzel model is a more accurate predictor of the wettability of this surface. Wenzel's model is described by the following equation:
where is the contact angle of water predicted by Wenzel's model is the ratio of surface area of rough surface to the surface area of a flat projection of the same surface
Cassie–Baxter model For more complex systems that are representative of water-surface interactions in nature, the Cassie–Baxter model is used. This model takes into consideration the fact that a water droplet may trap air between itself and the surface that it is on. The Cassie–Baxter model is described by the following equation:
where is the contact angle of water predicted by the Cassie–Baxter model is the liquid–air fraction, the fraction of the liquid droplet that is in contact with air
Mechanisms
Use of water
Control over surface wettability is a critical aspect of self-cleaning surfaces. Both superhydrophobic and superhydrophilic surfaces have been used as self-cleaning materials.
Superhydrophobic Superhydrophobic surfaces can be created in a number of different ways including plasma or ion etching, crystal growth on a material surface, and nanolithography to name a few. All of these processes create nano-topographical features that imbue a surface with superhydrophobicity. The ultimate goal in developing superhydrophobic surfaces is to recreate the self-cleaning properties of the Lotus Leaf that has the inherent ability to repel all water in nature. The basis for superhydrophobic self-cleaning is the ability of these surfaces to prevent water from spreading out when in contact with the surface. This is reflected in a water contact angle nearing 180°. Superhydrophobic self-cleaning surfaces also have low sliding angles, which allows for water that is collected on the surface to easily be removed, commonly by gravity. While superhydrophobic surfaces are great for removing any water-based debris, these surfaces likely will not be able to clean away other types of fouling matter such as oil.
Superhydrophilic
Superhydrophilicity allows for surfaces to clean away a wide variety of dirt or debris. This mechanism is very different than the aforementioned superhydrophobic surfaces. For superhydrophilic self-cleaning surfaces, cleaning occurs because water on the surface is able to spread out to a great degree (extremely low water contact angle) to get between any fouling debris and the surface to wash away the debris.
Photocatalysis One of the most commonly used self-cleaning products, titanium dioxide, utilizes a unique self-cleaning mechanism that combines an initial photocatalytic step and subsequent superhydrophilicity. A titanium dioxide coating, typically on glass windows, when exposed to UV light, will generate free electrons that will interact with oxygen and water in the air to create free radicals. These free radicals will in turn break down any fouling organic matter deposited on the surface of the glass. Titanium dioxide also changes the normally hydrophobic glass to a superhydrophilic surface. Thus, when rainfall occurs, instead of water beading up on the window surface and instantly falling down the glass, rain drops will rapidly spread out on the hydrophilic surface. The water will then move down the surface of the window, as a film rather than a droplet, essentially acting like a squeegee to remove surface debris.
Joule heating Heating of the surfaces via passing current through a conductive transparent film has been shown to repel and remove contamination. It has been used in inkjet printers to reduced ink contamination on sensor windows.
Electric curtain Cleaning surfaces in environments without water has been a challenge. Electric curtain devices were designed to remove particles by creating electric fields on the surface and carrying away particles due to their charged nature. It has been used in solar panels as well as 3D printers.Self-cleaning optical surfaces for the inkjet and 3D printing industry
In nature
Plants
Lotus leaf The lotus flower has been known as a symbol of purity in some Asian cultures. The lotus leaves (Nelumbo nucifera) are water-repellent and poorly adhesive, which keep them free from contamination or pollution even being immersed in dirty water. This ability, called self-cleaning, shields the plant from dirt and pathogens and plays a vital role in providing resistance towards invading microbes. Indeed, numerous spores and conidia of pathogenic life forms, mainly fungi, need water for germination and taint leaves within the first signs of water. It had been a curiosity how lotus flower could remain clean even in muddy water, until German botanists, Barthlott and Neinhuis, introduced the unique dual structure of leaves with the help of a scanning electron microscopy (SEM). Papillose epidermis cells carpet the exterior of a plant, particularly the leaf. These cells generate papillae or microasperties that make surface very rough. On top of microscale roughness, the papillae surface is superimposed with nanoscale asperities consisting of three-dimensional (3-D) hydrophobic hydrocarbons: epicuticular waxes. Basically, the plant cuticle is a composite material composed of a network of cutin and low surface energy waxes, designed at different hierarchical levels. The various leveled surface of lotus leaves is made out of convex cells (looks like bumps) and a much smaller layer of waxy tubules. The water beads on plant leaves rest on the apex of the nanofeatures since air is enclosed in the valley of convex cells, which minimizes the contact area of water droplet. Hence, the lotus leaves represent remarkable superhydrophobicity. Static contact angle and contact angle hysteresis of the lotus leaf are determined around 164° and 3°, respectively. With small tilting angles, water droplets on the leaf roll off and take any dirt or contaminant along, leading to self-cleaning. The ability of drops to form and roll off, depends not only on hydrophobicity, but also on contact angle hysteresis.
In plants world, the lotus leaf is not the only example of natural superhydrophobic surfaces. For instance, taro (Colocasia esculenta) leaves were found to exhibit self-cleaning behavior, too. They have a binary roughness built up by averagely 10 μm elliptical protrusions in diameter and nano-sized pins. India canna (Cannageneralis bailey) leaves and the rice leaves (whatever the kind of rice) also represent superhydrophobicity, arising from the hierarchical surface morphology.
Nepenthes pitcher plants The Nepenthes carnivorous pitcher, widespread in a lot of countries such as India, Indonesia, Malaysia and Australia, possesses a superhydrophilic surface, on which wetting angle approaches to zero to create uniform water film. Therefore, it increases the slipperiness of the surface and the prey slides off from its rims (peristome). Surface topography of Nepenthes rim demonstrates multiple scale radial ridges. The second order ridges are quite small in size and generated by straight rows of overlapping epidermidis cells. The surface of epidermidis cells are smooth and wax-free. The absence of wax crystals and microscopic roughness enhance the hydrophilicity and capillary forces, in doing so, water can swiftly wet the surface of rim.
Animals
Butterfly wings Butterfly wings possess not only ultra-hydrophobic trait but also directional adhesive characteristics. If the water bead is along the radial outward (RO) direction from the body's central axis, it rolls off and cleans the dirt away, leading to self-cleaning. On the other hand, if droplets stand against the opposite direction, they are pinned at the surface, leading adhesion and securing the flight stability of the butterfly by preventing deposit of dirt on the wings near the center of the body. SEM micrographs of wings exhibit hierarchy along the RO direction, arising from aligned microgrooves, covered by fine lamella-stacking nanostripes.
Water striders (Gerris remigis) Water striders (Gerris remigis), most commonly called Jesus bugs, have an extraordinary ability that lets them walk on the water. In a fashion similar to superhydrophobic plants, their legs are highly water repellent due to their hierarchical morphology. They are built up with hydrophobic waxy microhairs, microsetae, and each hair is covered with nanogrooves. As a result, air is entrapped between micro- and nanohairs, which repels water. Feng et al. measured how deep the leg can dip into water and the contact angle of the leg. They found the contact angle at least 168° and the maximum depth reported 4.38 ± 0.02 mm.
Gecko feet