Origin of superhydrophobic materials and theory of surface wettability

【1】Origin of superhydrophobic materials

 

In 1063 AD, Mr. Zhou Dunyi came to the lotus pond. This great philosopher, who was later called the founder of Neo-Confucianism in the Song and Ming Dynasties, wrote the famous “Love of the Lotus” that was passed down to future generations. This extraordinary article of only 119 words pointed out the characteristic of lotus flowers that “can emerge from the mud without being stained”. Later scientific researchers called this property the self-cleaning ability of the surface. By observing the self-cleaning phenomenon in nature, people have concluded that superhydrophobicity on the surface is the prerequisite for self-cleaning. As shown in the picture, the water droplets on the surface of the lotus leaf are in the shape of balls that cannot be wetted, and can carry dust and roll off.

By the 1990s, with the development and improvement of electron microscopy technology, scientists observed the material basis supporting this property. By observing the surface morphology, scientists found that the surface of the lotus leaf is composed of micron-level papillae and nano-level wax crystals distributed on the papillae. Further surface chemical composition analysis showed that these micro-nano rough surfaces have low surface energy. The micron-scale papilla structure and rough surface on the lotus leaf surface are shown in the figure.

In addition to the surface of lotus leaves, people have successively discovered the superhydrophobic effect on the surfaces of water strider wings, leech wings, dragonfly wings, mosquito legs, taro leaves, cabbage leaves, poinsettia leaves, etc. After obtaining the knowledge of superhydrophobicity from the top university of Nature, the researchers embarked on the path of bionic manufacturing of superhydrophobic surfaces.

【2】Surface wetting theory

 

2.1 Contact angle and rolling angle

 

 

The contact angle is the angle between the tangent of the gas-liquid interface and the solid-liquid interface (including the liquid part). The contact angle (CA) is usually used to express the degree of infiltration of the liquid into the solid. In addition, water droplets may slide down on an inclined surface, or they may remain stationary, which is another manifestation of hydrophilicity and hydrophobicity, expressed by the rolling angle (SA). The rolling angle is the critical tilt angle at which a droplet starts rolling on a solid surface. The schematic diagram of contact angle and rolling angle is shown in the figure.

 

It can be intuitively found from the figure that the larger the contact angle and the smaller the rolling angle, the better the superhydrophobicity of the surface.

 

2.2 Basic wettability theoretical model

 

1) Young’ model

 

In 1805, Young constructed a model of liquid droplets on a flat solid surface, namely Young’s equation model. The contact angle of a droplet on a solid surface is the result of the balance of surface tension between the solid, liquid, and gas interfaces, and is the static contact angle of a droplet on a smooth surface.

 

 

The contact angle is related to the mutual surface tension between solid, liquid and gas phases. Reducing the free energy of the solid surface can increase the contact angle and improve the hydrophobicity of the surface. However, experiments found that the contact angle of this model can only reach about 120°, which is inconsistent with the actual superhydrophobic surface contact angle greater than 150°. Therefore, Young’s model is only suitable for smooth and uniform ideal models.

 

2) Wenzel model

 

Since there is no absolutely smooth and uniform surface in real life, the influence of surface structure needs to be considered. Under the premise that water droplets can completely cover the grooves on the rough surface, Wenzel modified Young’s model and proposed the Wenzel model. Generally speaking, due to the effect of rough structure, the actual area of the object surface will be greater than its projected area, so the surface roughness coefficient is usually greater than 1. When θ is less than 90°, the increase in surface roughness of the object will reduce the contact angle and increase the hydrophilicity of the surface. When θ is greater than 90°, the increase in surface roughness of the object will increase the contact angle and increase the hydrophobicity of the surface. The Wenzel model is shown in the figure. It can be seen that the Wenzel model is based on the liquid completely contacting the solid surface, that is, it is in a wetted state of the solid surface. This model has important implications for subsequent research, allowing researchers not only to focus on the material properties of objects, but also to change the hydrophilicity or hydrophobicity of the surface by changing the surface roughness.

 

 

3) Cassie-Baxter model

 

Further research found that hydrophilic surfaces can also be used to prepare superhydrophobic surfaces, indicating that the Wenzel model also has shortcomings. The Cassie model believes that the contact interface between the rough surface and the liquid is a composite interface of solid, liquid, and gas. For some rough surfaces, when the hydrophobic surface and the droplets are in contact with each other, the grooves on the rough surface cannot be completely filled by the droplets. This is because the air is stored in the grooves, part of the droplet contacts the air and part contacts the solid, and the internal grooves of the rough surface structure of the solid will not be wetted. In this case, the Wenzel model is difficult to apply. When θ>90°, for a rough solid surface, increasing the interface roughness can significantly increase the actual contact angle. When θ<90°, the actual contact angle will also become larger because the grooves on the rough surface are filled with gas as a barrier layer. When the contact area between solid and liquid is smaller, the more gas remains under the water droplet, the better the hydrophobicity. When it approaches 0 infinitely, the superhydrophobic performance reaches the ideal state.

 

【3】Summary and application

 

Through the above three models, we realize that the surface structure and low surface energy of materials are two important factors that affect the surface hydrophobicity. In addition, the surface in the Wenzel state is adhesive to droplets, making the surface in a wet state and unable to achieve the ideal self-cleaning effect. The superhydrophobic surface under the Cassie-Baxter model shows perfect self-cleaning performance.

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