What efforts have scientists made to prevent your poop from sticking to the toilet?

Produced by: Science Popularization China

Author: Denovo Team

Producer: China Science Expo

When we use the toilet, we often encounter the embarrassment of "residue" on the toilet. When you press the flush button again and again, but the residual poop is still on the toilet wall, you may wonder: Why can't there be a toilet in this world that doesn't stick to poop?

In fact, this question has also aroused the interest of scientists.

Why do we need a toilet that doesn't stick to poop?

Why is poop so sticky?

SM Woolley and others from the University of KwaZulu-Natal in South Africa found that stool viscosity is closely related to water content, and the water content of stool is affected by the intake of dietary fiber and protein. However, the intake of protein-rich foods (such as meat and beans) and dietary fiber-rich foods (such as fruits, vegetables and whole grains) is essential for humans to maintain a healthy digestive system, and they have an indispensable position in the human diet.

It is not easy to change the viscosity of stool without making major adjustments to your diet.

Foods rich in protein and dietary fiber play an important role in the human diet.

(Image source: author)

In order to solve the embarrassment of "residue" on the toilet, people have developed a variety of toilets with unique principles and strong flushing force. However, the high water flow of these powerful flushing toilets has led to a significant increase in water consumption, bringing new problems.

Survey data shows that the world consumes more than 141 billion liters of fresh water every day to flush toilets (six times the total water consumption of the African population), and urban residents use 30%-35% of their total domestic water consumption for flushing toilets. At the same time, the 2023 United Nations World Water Development Report jointly released by UNESCO and UN-Water shows that there are still 2 billion to 3 billion people in the world facing water shortages. This contradictory situation forces people to find other solutions.

After studying the strange relationship between poop and toilets, scientists finally came up with a solution: design a toilet that doesn't stick to poop!

How to keep poop from sticking to the toilet?

Perhaps your first reaction is that to achieve non-stickness, all you need to do is add a layer of coating on the toilet, and scientists think so too.

Nepenthes, a well-known insectivorous plant in the biological world, has leaves that are specialized into bottle-shaped insect traps. The inner wall of the insect trap has a porous microstructure and is filled with lubricating fluid. Insects attracted by the nectar glands of the Nepenthes and falling into the insect traps cannot stand steadily on the inner wall and find it difficult to escape. This magical phenomenon has attracted the attention of scientists.

The inner wall of the pitcher plant cage is shaped like a toilet

(Image source: Wikipedia)

Tak-Sing Wong's team at Pennsylvania State University in the United States drew inspiration from pitcher plants and developed a "super-slippery coating" - LEES (liquid-entrenched smooth surface).

Just use polydimethylsiloxane to form a nano-scale "hair" base on the surface of the toilet, and then spray silicone oil lubricant. It only takes 5 minutes to form a LEES coating on the surface of an ordinary ceramic toilet, making it smoother.

Scientists have tested water, artificial mixed feces, and real feces on the inner walls of toilets made with this coating, and found that the LEES coating can reduce the adhesion of viscoelastic fluids such as feces by about 90%, basically achieving "non-stick".

The LESS coating also has an immediate effect on saving water resources. Depending on the water content of the "feces", the water consumption for flushing the toilet can be reduced by 50%-90%. The LEES layer also has a certain antibacterial effect, which can further reduce the generation of odor.

Comparison of the effectiveness of LEES-coated (left) and uncoated toilets (right) in removing synthetic feces

(Image source: Reference [3])

Based on LESS, scientists have continuously improved this coating technology in the past decade, developed coatings with better effects, and widely used them in anti-fouling, self-cleaning, anti-icing and other fields.

However, this coating technology also has its own problems. Take LESS as an example. After about 500 flushes, a new silicone oil coating needs to be sprayed on the toilet, and the polydimethylsiloxane base also needs to be repaired after long-term use, and each repair costs $15 (about RMB 109). In underdeveloped areas with water shortages, using this type of technology seems too cumbersome and costly.

Is there a permanent solution?

Professor Su Bin's team at Huazhong University of Science and Technology also drew inspiration from the microstructure of the inner wall of the pitcher plant's insect-catching cage and decided to start with the toilet itself. They used a mixture of plastic and special sand as raw materials and 3D printed a toilet model that is 1/10 the size of an ordinary toilet.

During the 3D printing process, the laser sintered the raw materials. Sintering is a chemical process that compacts and combines two types of solids, and importantly, it produces a porous structure - there are many tiny gaps between the solid particles. Therefore, the lubricant can be stored on the entire surface of the toilet and even penetrate into the interior. In this way, after the lubricant on the surface is lost, it can be replenished from deeper inside the toilet, achieving a once-and-for-all effect.

At the same time, this porous structure makes the toilet more wear-resistant. Even after being scrubbed with sandpaper more than 1,000 times, the mini toilet is still as smooth as new.

Of course, this kind of toilet still has a long way to go before it can become part of our lives, because we need to first print the toilet in actual size and then reduce manufacturing costs.

Synthetic feces can still slide out of the bottom hole of a mini toilet whose surface has been sanded 1,000 times.

(Video source: Reference [2])

Why doesn't poop stick to the toilet?

At the chemical level, the super-slippery properties of pitcher plants, LESS coatings, and mini toilets all stem from a material property called hydrophobicity. When a hydrophobic material surface comes into contact with water, water droplets form a bead shape that is difficult to wet.

This is because the interaction force between the molecules on the surface of the material is greater than the interaction force between the water molecules and the surface. Correspondingly, the surface where the interaction force between the molecules on the surface of the material is smaller than the interaction force between the water molecules and the surface is hydrophilic.

We usually determine the hydrophilic or hydrophobic effect of a solid surface by measuring the angle between the edge of the droplet and the solid surface (i.e., the contact angle). It is not difficult to see that when the water droplet is completely spread on the solid surface, the contact angle should be 0°, while if the water droplet remains completely spherical, the contact angle should be 180°.

Contact angles of water droplets on hydrophilic, hydrophobic and superhydrophobic surfaces

(Image source: Reference [5])

Therefore, the larger the contact angle, the stronger the hydrophobicity of the solid. When the contact angle exceeds 150°, scientists have given this property a new name: "superhydrophobicity" . In addition to the pitcher plants and coatings mentioned above, the leaves of plants such as lotus, taro, cabbage and the wings of Morpho butterflies in nature all have this property, which is determined by their rough or porous microstructures.

Lotus leaves (left) and Morpho butterfly wings (right) are both common hydrophobic surfaces.

(Image source: Wikipedia)

Conclusion

Scientists have made many efforts to prevent your poop from sticking to the toilet, but there are still many problems to be solved. However, we believe that with the advancement of materials science, one day, non-stick toilets will enter our daily lives.

References

[1] Woolley, SM, et al. "Shear rheological properties of fresh human faeces with different moisture content." Water sa 40.2 (2014): 273-276.

[2] Li, Yike, et al. "Abrasion‐Resistant and Enhanced Super‐Slippery Flush Toilets Fabricated by a Selective Laser Sintering 3D Printing Technology." Advanced Engineering Materials (2023): 2300703.

[3] Wang, Jing, et al. "Viscoelastic solid-repellent coatings for extreme water saving and global sanitation." Nature Sustainability 2.12 (2019): 1097-1105.

[4] Kreder, Michael J., et al. "Design of anti-icing surfaces: smooth, textured or slippery?." Nature Reviews Materials 1.1 (2016): 1-15.

[5] Zhang, P., and FY Lv. "A review of the recent advances in superhydrophobic surfaces and the emerging energy-related applications." Energy 82 (2015): 1068-1087.

[6] Chen, Huawei, et al. "Continuous directional water transport on the peristome surface of Nepenthes alata." Nature 532.7597 (2016): 85-89.

[7] Wang, Dagui, et al. "Liquid-like polymer lubricating surfaces: mechanism and applications." Nano Research (2023).