Newnham Engineering Prize 2016 Question 1 Tatiana Lim

The science behind and the future ahead for self-cleaning technology

Introduction

Imagine dishes that never need to be cleaned. Imagine clothes that never need to be washed. Imagine buildings that never need to be re-painted.

Our rapidly advancing, fast-paced society is increasingly constrained by two factors: time and money. What if science could help us save both? Self-cleaning surfaces are a prime example of such a product, increasing time efficiency and reducing maintenance costs, which is why this industry has gained success over the past few decades. Every year, new types of self-cleaning surfaces come to market, and are used in a wide variety of applications ranging from skyscraper glass windows to hospital tiles, and even clothing textiles.

At present, the two main methods of self-cleaning stem from two contrasting concepts: superhydrophobic (meaning ‘very water-fearing’) materials and superhydrophilic (meaning ‘very water-loving’) materials. “The Lotus Effect”, a trademarked concept derived from the lotus leaf, inspires super-hydrophobic surfaces. This concept contradicts traditional beliefs on what is easy to clean. The rougher the surface (on a nanoscale), the better protected it is from organic matter and other pollutants. Conversely, superhydrophilic surfaces can be obtained by applying a TiO2 photocatalyst. Not only does TiO2 remove any organic matter on its surface, but it also increases the surface’s attraction for water making it an ideal self-cleaning material.

However, these products are far from perfect and more research into these manufacturing processes would optimise their potential. In particular, more research into their manufacturing processes could improve their current properties, adoption into applications in society and therefore economic feasibility- opening self-cleaning surfaces to a wider market.

Industry

The self-cleaning industry is a flourishing industry that has, and will increasingly, improve quality of life. Self-cleaning surfaces minimise the energy and cost spent on maintaining products ranging from clothing fabrics to building exteriors. Furthermore, these products only require natural and gentle conditions in order to function- there will no longer be wasted water or emission of harmful substances from chemical detergents, thus making them more kind to the environment. Eliminating window cleaning for the millions of m2 of building exterior surfaces in highly dense urban areas also not only saves costs normally spent on expensive cranes, outside lifts or scaffolding, but also limits the safety risks presented to window cleaners being many feet in the air.

For all these reasons, it is not a surprise that this industry is expected to grow to become one of the largest segments of the smart materials market, with predicted revenue of $3.3billion by 2020. In particular, this advancement is expected to be greatly

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Newnham Engineering Prize 2016 Question 1 Tatiana Lim

beneficial to the construction sector with an estimated $1.6billion revenue from this industry by 2020. Currently, 94% of revenue for the self-cleaning industry is from hydrophilic and hydrophobic technologies1.

Lotus EffectSurfaces using the Lotus Effect have many advantages over the traditional cleaning methods providing an alternative to chemical detergents that can be harmful to both humans and the environment. The Lotus Effect only relies on natural intervention. Annually, Germany spends €2-4billion on biocides2 which are used to remove fungi, algae and bacteria build-up on building exteriors. Unlike these products, surfaces using the Lotus effect do not require repeated application and are therefore more economically efficient. The reduced cost of energy that the Lotus effect brings have made the use of the Lotus Effect so attractive that in 2010; worldwide annual sale of products using the Lotus Effect was over $100,000,0002.

The first successful application of the Lotus Effect was by Ipso (now Sto Corp) when they created a paint called ‘Lotusan’3. Approximately 300,000 buildings4 and 40,000,000m² of façade surfaces5 have been coated with ‘StoColor Lotusan’ since 1999.

Nanotex has applied the Lotus Effect to clothing by attaching permanent, hydrophobic ‘whiskers’ to the ends of the fabric fibres7. These whiskers cause the water droplets to only pass the tips of these fibres, form beads and roll off the fabric, hence repelling stains and removes moisture. In 2011, it was reported that these Nanotex treatments were used in 80 textile mills worldwide and sold by more than 100 leading brands8.

Furthermore self-cleaning glass, produced by Ferro AG, is used throughout Germany in public high impact areas for optical sensors (e.g. toll bridge sensors on highways) since the Lotus Effect provides perpetual clarity5. The use of self-cleaning glass ensures that the surface will always allow a high transmittance of light and so is ideal for optical sensors.

Titanium DioxideThis photocatalytic coating is used for various applications (such as anti-fogging, water treatment, anti-bacterial and air cleaning) due to its photocatalytic, self-cleaning property. A titanium dioxide coating is easy to setup, reduces waste and does not require any post-processing, thus having a low running cost6.

Currently, TiO2 can be found in self-cleaning tiles used in hospitals, roofs and public restrooms due to its ability to break down biological organisms (e.g. bacteria, viruses, fungi, algae). It is also ideal for building surfaces under harsh conditions because it thrives under heavy rains and great intensities of sunlight. TiO2 can also be used to dramatically reduce air pollution because it can break down pollutants in the air such as nitrogen oxides levels7.

TOTO Ltd. has 270 patents for photocatalytic technology. From 1999 to 2013, TOTO Ltd. licensed this technology to over 100 companies worldwide8 showing the success of the use of photocatalysts in self-cleaning.

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Newnham Engineering Prize 2016 Question 1 Tatiana Lim

Pilkington Activ, considered one of the most successful photocatalytic products in the market, is a self-cleaning glass that is increasingly being used for various buildings and constructions worldwide. It has been used in zoos (ZSL London Zoo) for its ability to always provide a clear view through glass, hotels (Hilton Hotel in Finland) and car wing mirrors9.

Engineering Principles

A surface’s affinity for water can be described by its wettability. The main indication of the degree of the surface’s wettability is the contact angle between a water droplet and the surface (see figure 1). The more spherical its shape, the greater the contact angle, the more hydrophobic the surface is and vice versa. In other words, the angle of intersection between a liquid-solid interface and a liquid-vapour interface10 .

A water droplet has a spherical shape (when not in contact with a surface) because this shape has the minimum surface area:volume ratio10 and so is the most energetically favourable shape. In a water droplet, each molecule is pulled equally in all directions by neighbouring molecules. However, on the surface of the water droplet the molecules are exposed and are pulled inwards (contraction) by neighbouring molecules due to their strong intermolecular forces- surface tension. The adhesive force between the surface and the droplet also determines the shape of water droplet. A droplet will spread out and wet the surface more if the adhesive force from the surface is greater than the cohesive force holding the water together11. As shown in figure 1, a low contact angle between the water droplet and the surface would be any value less than or equal to 90° (hydrophilic) whereas any value greater than 90° would be considered a high contact angle (hydrophobic). A surface is considered ‘superhydrophobic’ when its contact angle exceeds 150° – the Lotus leaf’s surface with water has a contact angle of 170°12.

Two main methods can be used to create this much-needed self-cleaning property. For a successful self-cleaning surface, dirt particles that encounter the surface must either have a very strong affinity for the water droplets than for the surface or the surface must have a very weak affinity for the dirt particles. The use of TiO2 coating on glass uses the former and the ‘Lotus Effect’ follows the latter.

Lotus Effect

Figure 1- The contact angle between the water droplet and the surface is an indication of whether the surface is superhydrophilic, hydrophilic, hydrophobic or superhydrophobic 27

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Newnham Engineering Prize 2016 Question 1 Tatiana Lim

First described by Wilhelm Barthlott in 1976, research on the Lotus Effect began in 1989 and was patented in 1995 by Barthlott14. As a botanist, Barthlott was using the Scanning Electron Microscope (1965) to study the structures of plant surfaces when he realised that plants with rougher surfaces, most prominently the lotus leaf, never required cleaning in preparation for observation under the SEM.

When observed on a nanoscale level, the surface of the lotus leaf comprises many tiny bumps known as ‘papillae’, which are 1-5µm high15 and 10-15µm apart16.

The papillae are formed when the soft walls of the papillose epidermal cells are pushed outwards17. To further advance the surface’s hydrophobic nature, each of these papillae have micro- and nano-asperities made of epicuticular waxes (hydrophobic plant wax). This structure is also known as a ‘Hierarchical Structure’, as shown in figure 218.

Unlike the nano- and micro- structures, the hierarchical structure has a two-tiered roughness. This gives the surface its superhydrophobic nature and causes the liquid droplet to form a spherical droplet on its surface. Although the microstructure surface elevates the droplet to its tips, it is not as hydrophobic as the hierarchical structure so the water droplet is not as spherical. On the nano structure, the micro asperities also result in the elevation of the water droplet and hence, the surface’s hydrophobic nature. On the flat surface, the water droplet forms a dome- shape and is, therefore, not as hydrophobic.

Air gaps form in the ‘valleys’ of these papillae, causing the water droplets to sit on their peaks. This minimizes the contact area between the water droplet and surface, which decreases chances of adhesion. An experiment, using atomic force microscopy, concluded that contact area and adhesion have a directly proportional relationship22.

Organic matter has a greater affinity for the water droplet than for the superhydrophobic surface. As a result, water will form bead-like drops on the superhydrophobic surface and roll off the surface collecting any organic matter in its track (figure 3).

Figure 2- A water droplet resting on a four different surface structures21

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Newnham Engineering Prize 2016 Question 1 Tatiana Lim

Titanium DioxideTitanium Dioxide is a white pigment that is used in paints. After discovering its photocatalytic properties (meaning that it is a catalyst activated by light), these paints were coated with silica in order to prevent this process from happening. However, this exact property can be used to create a self-cleaning surface.

Although it is a white pigment, anatase (one of the 3 mineral forms of TiO2) can be thinly applied onto glass to form a transparent, self-cleaning coating. The anatase is activated when its electrons are excited and jump to a higher energy level by absorbing UV light. This ‘activated’ anatase reacts with water vapour in the air to form hydroxyl radicals, which not only break down organic molecules (ranging from dirt particles to sulfur oxides) but also increase the surface’s hydrophilic character. This is due to the fact that the hydroxyl groups can form hydrogen bonds with water molecules21.

Nitrogen oxides, which are common pollutants, are broken down into nitrates and organic compounds are broken down into carbon dioxide and water7. When activated, a TiO2 coated surface will have a contact angle with water of less than 1°. Therefore, water will form sheets instead of droplets on this surface and any excess material will flow off the surface with the water when the surface is tilted at an angle.

Improvements

Although the industry of self-cleaning materials is thriving, there are various aspects that can be improved to maximize their usage.

A key question arises when evaluating these two self-cleaning methods: What if the water droplets do not roll off the surface? In the case of the Lotus Effect, water might not come into contact with the whole surface that needs to be cleaned. This issue could be tackled by ensuring that the structure is tilted at a steep angle, creating a concave structure that allows all water droplets to flow downwards and inwards. In the case of the TiO2 coating, the sheets of water may evaporate before it can rinse the whole surface. Elegant Embellishments have approached this problem by changing the shape

particles 9

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Newnham Engineering Prize 2016 Question 1 Tatiana Lim

of the objects that use TiO2 coating (Figure 4). This is used to increase the area between TiO2 coating and air to maximize the rate of self-cleaning.

At present, the longevity of these self-cleaning products is limited. When a major road in Antwerp, using 10 km2 of photocatalytic paving block, was exposed to vehicular traffic, the photocatalytic efficiency of the paving blocks decreased by 20% after one year7. Over time, dirt will get lodged in the porous materials with TiO2, preventing interaction between UV light and the material. Following the same method used by Elegant Embellishments, a TiO2 coating could be applied onto a non-porous structure in front of any porous material. This would still allow for photocatalysis to occur and keep the advantages of the porous material in construction.

In addition, the products that either apply the Lotus Effect or use TiO2 have a high purchase cost. Although running costs are low, the methods to manufacture these novel technologies on a large-scale are limited. The price to use TX Active Aria (a cement with photocatalytic compounds) is $0.60/lb- six times the price of using normal cement10. Pilkington Activ, one of the more successful products that apply TiO2 coatings, use chemical vapour decomposition at high temperatures of 600°C, increasing the manufacturing cost26.

House paints that apply the Lotus Effect are currently being made by the following limited processes: changing the surface structures of hydrophobic polymers early in the manufacturing stage, moulding or etching surfaces in the post production stage or allowing previously manufactured materials, which meet the required surface structure requirements, to undergo hydrophobisation26. Moreover, as seen in Figure 5, the replication of the lotus leaf surface is not exact and therefore these surfaces do not reach their maximum potential. The fabricated surfaces’ papillae are not as tall and not in as high a density as the lotus leaf’s papillae. Further research and development could improve the manufacturing processes. By investing in more research to find more applications for these self-cleaning surfaces (e.g. food containers), the demand for this industry will rise. With an increase in demand, these surfaces will be manufactured in greater bulk and manufacturing

Figure 4- Elegant Embellishment’s smog- eating façade panel 31

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Newnham Engineering Prize 2016 Question 1 Tatiana Lim

technologies will be encouraged to develop at a greater rate so production costs will decrease. This would increase the material's accessibility thereby expanding the market for self-cleaning surfaces.

Conclusion

Currently, pollutants and chemical detergents are posing an increasingly alarming threat to the environment. Additionally, the use of energy for water pumps to clean buildings could be more efficiently utilized e.g. lighting for buildings) and so self-cleaning surfaces are becoming an increasingly more attractive, low-energy alternative.

To conclude, by addressing areas of improvement through further active research, these surfaces could reach their maximum potential and be used in every household whether for domestic use or for the coating of buildings. The self-cleaning industry is thriving with new applications for this innovative industry rapidly increasing. Their potential goes beyond their current offerings, with new applications such as the ability to aid water collecting or to, increase the ease of movement of hulls in ships or coat airplane wings to prevent de-icing. These remarkable surfaces gain no dirt but have undoubtedly gained great attention.

Figure 5- Diagrams a, b show the surface of a lotus leaf on at a magnification of x1000 (left) and x2000 (right). Diagrams c, d show the synthesized Lotus Effect surface at a magnification of x1000

(left) and x2000 (right).33

Figure 5- Diagrams a, b show the surface of a lotus leaf on at a magnification of x1000 (left) and x2000 (right). Diagrams c, d show the synthesized Lotus Effect surface at a magnification of x1000

(left) and x2000 (right).33

Figure 5- Diagrams a, b show the surface of a lotus leaf on at a magnification of x1000 (left) and x2000 (right). Diagrams c, d show the synthesized Lotus Effect surface at a magnification of x1000

(left) and x2000 (right).33

Figure 5- Diagrams a, b show the surface of a lotus leaf on at a magnification of x1000 (left) and x2000 (right). Diagrams c, d show the synthesized Lotus Effect surface at a magnification of x1000

(left) and x2000 (right).33

Figure 5- Diagrams a, b show the surface of a lotus leaf on at a magnification of x1000 (left) and x2000 (right). Diagrams c, d show the synthesized Lotus Effect surface at a magnification of x1000

(left) and x2000 (right).33

Figure 5- Diagrams a, b show the surface of a lotus leaf on at a magnification of x1000 (left) and x2000 (right). Diagrams c, d show the synthesized Lotus Effect surface at a magnification of x1000

(left) and x2000 (right).33

Figure 5- Diagrams a, b show the surface of a lotus leaf on at a magnification of x1000 (left) and x2000 (right). Diagrams c, d show the synthesized Lotus Effect surface at a magnification of x1000

(left) and x2000 (right).33

Figure 5- Diagrams a, b show the surface of a lotus leaf on at a magnification of x1000 (left) and x2000 (right). Diagrams c, d show the synthesized Lotus Effect surface at a magnification of x1000

(left) and x2000 (right).33

Figure 5- Diagrams a, b show the surface of a lotus leaf on at a magnification of x1000 (left) and x2000 (right). Diagrams c, d show the synthesized Lotus Effect surface at a magnification of x1000

(left) and x2000 (right).33

Figure 5- Diagrams a, b show the surface of a lotus leaf on at a magnification of x1000 (left) and x2000 (right). Diagrams c, d show the synthesized Lotus Effect surface at a magnification of x1000

(left) and x2000 (right).33

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Newnham Engineering Prize 2016 Question 1 Tatiana Lim

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