Superhydrophobic coatings that "drip into the water" appear

Back to Nantian, the floor is watering, the wall is "sweat", making people crazy. In some industries, water is even more enemies: water can bring bacteria, cause corrosion and bring pollution. However, the water around us is everywhere, and it is impossible to prevent damage. Is there a way to keep it out of the door when it is not welcome? Superhydrophobic materials take on the heavy responsibility.

In a TED speech, the scientist poured a basin of water on a metal plate. The water drops rolled like steel balls, and the metal plate was still dry. A paddle was immersed in the water tank and was taken out without a drop of water. It’s like never put it in; a glass of water falls on a specially treated glass plate, and the water is close to the center “not half a step of the thunder pool”, even if it is stirred by hand or two, it will run back immediately...

Due to the special effects of waterproofing and anti-corrosion, it is urgent to be promoted from the laboratory to the actual promotion and application.

These phenomena that violate our "common sense" of the naked eye are the ghosts of "superhydrophobic materials". This new material, obtained by changing the surface free energy and surface roughness of the material, is inspired by the lotus leaf in nature. Due to its special effects of waterproofing, anti-corrosion and anti-bacterial, it has become an internationally popular research field, and it can be used in various fields that you can't imagine, such as environmental protection, industry, and medical care.

Micro-nano composite structure at microscopic scale

The free energy of the surface of the material determines whether the material is hydrophilic or hydrophobic. The lower the free energy, the stronger the hydrophobicity. The microscopic roughness of the surface determines the strength of the hydrophilic and hydrophobic. The rougher the surface, the stronger the hydrophobicity.

A drop of water drops on the surface of the material. If it spreads out quickly, it is a hydrophilic or super-hydrophilic surface. If the water droplets form a sphere, they can roll off and roll, which is a hydrophobic or even super-hydrophobic surface.

The surface of some plants in nature has super-hydrophobic properties and self-cleaning functions, the most typical of which is the surface of the lotus leaf, which forms a "self-cleaning effect of the lotus leaf" and "discharges the mud without dyeing".

How is the nature of superhydrophobicity formed? To figure this out, the superhydrophobic phenomenon in nature can be exploited by humans.

An expert in the study of superhydrophobic materials at the School of Chemistry and Chemical Engineering, South China University of Technology explained that according to the laws of thermodynamics, substances with high surface energy cannot be spread on the surface of materials with low surface energy. Water is a substance with a relatively high surface energy. Therefore, substances whose surface energy is lower than water, such as some substances containing silicon and fluorine, will exhibit hydrophobicity. Water on such a surface will try to shrink itself into a spherical shape.

The chemical composition of the low surface energy determines whether the material is hydrophobic, but only hydrophobic properties are not sufficient. In the 1930s and 1940s, scientists discovered the relationship between surface roughness microstructure and wettability. In the microscopic environment, the liquid drip on the solid surface and does not completely fill the concave surface on the rough solid surface, and there is still air between the droplet and the solid concave surface.

The macroscopically visible contact interface between solid and liquid is actually a mixed interface composed of a gas-liquid interface and a solid-liquid interface. The rougher the micro-surface, the more air is trapped, the less contact with water, and the more hydrophobic the solid.

In 1997, researchers such as German biologist Barthlott studied the surface of nearly 300 plant leaves, and believed that the self-cleaning properties of plant leaves were caused by micron-structured mastoids on rough surfaces and hydrophobic waxy surfaces. The materials are created together.

The smooth and smooth lotus leaf looks like another under the electron microscope: the surface is covered with granulated mastoids, which looks rough and uneven. These mastoids and mastoids are covered by numerous nanoscale wax crystals. Water-repellent wax and micron-sized mastoids give the lotus leaf surface super-hydrophobic properties.

According to the above experts, the free energy of the surface of the material determines whether the material is hydrophilic or hydrophobic. The lower the surface free energy, the stronger the hydrophobicity. The surface microscopic roughness determines the hydrophilic and hydrophobic strength. The rougher the surface, The more hydrophobic it is. Therefore, when the surface is hydrophobic, increasing the roughness of the solid surface increases the hydrophobicity of the surface.

In 2002, the team of China's famous nanomaterials expert Jiang Lei discovered that there are nanostructures on the micro-structured mastoids on the surface of the lotus leaf. The average diameter of the mastoids is 5-9 microns, and the diameter of each mastoid surface is distributed. (124 ± 3) nanometer fluff. Nanostructures also exist on the surface between the mastoids. In addition, nanostructures can be found on the lower surface of the lotus leaf, which effectively prevents the lower layer of the lotus leaf from being wetted.

It turns out that it is only a micron structure, and the hydrophobicity is not strong enough. The micro-nano multilayer structure is the ultimate mystery of the hydrophobic phenomenon in nature.

Researchers often use contact angles to express the extent to which a liquid infiltrates a solid, that is, the degree of hydrophilicity. The contact angle is the angle between the tangent of the gas-liquid interface through the liquid and the solid-liquid interface. If the water droplet is perfectly spherical on the surface of the material, it means that the plate is a completely hydrophobic material with a contact angle of 180°; if the water is completely tiled on the surface, the material is very hydrophilic and the contact angle is 0°.

The greater the contact angle, the lower the degree of infiltration. By definition, a superhydrophobic surface generally refers to a surface having a contact angle with water greater than 150°.

The plane in reality is often not horizontal, but more inclined. Water droplets may roll or stagnate on sloping surfaces, which is also a manifestation of hydrophilicity, which needs to be expressed by rolling angles. The roll angle refers to the critical surface tilt angle at which the droplet begins to roll on the solid surface. The smaller the tilt angle of the droplet starting to roll, the better the superhydrophobicity of this surface.

According to the above experts, the water droplets are rolled off, the decontamination ability is stronger than that of the sliding, and the inclined smooth surface water droplets are mostly in a sliding state, which explains the self-cleaning characteristics of the superhydrophobic surface.

2. Produce superhydrophobic materials for natural learning

People have been inspired by nature to create a variety of materials that are also superhydrophobic. The study of anisotropy can control which direction and extent of liquid infiltration occurs.

In addition to lotus leaves, there are many biological surfaces with superhydrophobic structures. According to the above experts, the surface of the flap is composed of a regularly arranged nano-columnar structure with a diameter of about 80 nm and a nano-column spacing of about 180 nm. The regularly arranged nanoprotrusions create a roughness that allows the flap surface to stably adsorb an air film, inducing superhydrophobic properties, thereby ensuring self-cleaning.

The gecko's toes also have a fascinating hierarchy. Microscopic observations can be seen that its toes are made up of thousands of silk-like "scales" and hundreds of shovel-like subtle structures contained in each piece of "silk." This structure makes the gecko's soles extremely rough and can crawl freely on the wall.

Although the water raft on the rivers and lakes is called "iron legs floating on the water", although it has a small weight, it can float on the water mainly by the super-hydrophobic structure of its legs. Jiang Lei's team conducted a profound and meticulous study on the scorpion legs and found that the surface of the otter legs is oriented with micron-sized acicular bristles and a spiral nano-scale groove structure on the bristles. The bristles can adsorb air bubbles in the grooving to form an air cushion, so that the leeches can freely shuttle on the water surface without wetting the legs.

Inspired by the leeches, many researchers have designed new super buoyancy materials. Dr. Pan Qinmin and other researchers from the Department of Applied Chemistry of Harbin Institute of Technology and other researchers used porous copper mesh as the substrate and made it into several stamp-sized miniature ships, and then soaked the solution with silver nitrate to make the surface of the ship super Hydrophobic.

This material also has a micro-nano-structured surface that forms an air cushion on the outer surface of the ship, changing the ship's contact with water, so that the surface of the hull is less resistant to water. This miniature boat can carry 50% more weight than its maximum displacement while floating freely on the surface.

Water droplets exhibit anisotropy as they roll on the leaf surface of certain plants and can be simply interpreted as different properties in different directions. Jiang Lei’s team observed that water droplets on the surface of rice leaves always roll in the direction of parallel veins. It turns out that the surface of rice leaves has a multi-stage structure combined with the micro-nano surface of the lotus leaf surface. However, on the surface of rice leaves, the mastoids are aligned in a direction parallel to the edge of the leaves, and the arrangement in the vertical direction is very arbitrary. ", so the water drops are more likely to roll down in the direction of the parallel veins.

In 2009, Jiang Lei’s team also found the anisotropy of water droplets rolling on the surface of the butterfly wings. The butterfly wings are covered by micron-sized scales, each of which is arranged with neatly arranged nano-strip structures, and each nano-strip is formed by sloping periodic sheets. This special microstructure causes the water droplets to be anisotropic as they roll over the surface of the butterfly's wings.

These findings provide important information for the preparation of invasively controllable solid surfaces. With this in mind, people can not only control whether solids and liquids infiltrate, but also control which direction and to what extent the liquid infiltrates in the solid.

3. Let super-hydrophobic materials get out of the lab

The application of superhydrophobic materials is quite extensive, covering aerospace military, construction, medical and other aspects. However, due to current technology and development costs, there are not many actual industrialization and commercialization.

What are the applications of superhydrophobic properties? Many researchers have raised their imaginations.

Think about it first and foremost with our lives. The super-hydrophobic surface with antibacterial self-cleaning effect is applied to daily necessities, which can reduce the trouble of cleaning; on the surface of the inner surface of refrigeration equipment such as refrigerators and freezers, there is no longer condensed water, frosting and icing; in the interior and exterior walls of buildings Waterproof, snow-proof and stain-resistant applications such as glass and metal frames can greatly reduce the cleaning and maintenance costs of buildings.

The idea is broad. The surface of the inner wall of natural gas and petroleum pipelines is coated with a super-hydrophobic molecular film, which can prevent pipeline corrosion and improve the transmission efficiency of oil and gas. Apply it to the bottom of an ocean-going ship to prevent contamination and corrosion.

Superhydrophobic materials also perform well in microfluidic control applications. Researchers have suggested that controlling the movement and flow of microdroplets and making microdroplets to control the needles allows for precise metering of the liquid droplets during the experiment or production process, and the addition of experimental reagents will be more convenient.

Some experts believe that if this kind of technology is applied to the field of electrostatic spraying, such as the use of super-hydrophobic materials to make spray nozzles such as spray paint, it will make the spray droplets more uniform and the atomization effect is better. Where the spraying effect has special requirements.

According to the above experts, there are several main preparation methods for superhydrophobic materials, including template method, plasma method, chemical vapor deposition method, electrospinning method, sol-gel method, etc., which are basically on low surface energy materials. Construct a rough surface.

These methods are either too expensive; either the equipment is demanding, the conditions are harsh, the cycle is long, and it can only be manufactured in small quantities in the laboratory; or the hydrophobic surface is not resistant to wear; or the hydrophobicity is not strong and is easily contaminated by oily substances... On the one hand, researchers are trying to create hydrophobic materials with different characteristics of different structures, such as some super-double-sparing materials that are both hydrophobic and oleophobic, and on the other hand, they are also racking their brains to put them into practical applications.

At present, the relevant team of the School of Chemistry and Chemical Engineering of South China University of Technology has made good progress in the preparation of superhydrophobic coatings. After preparing the micro-nano composite structure particles, they are combined with silicone to form a coating. Spraying the coating can prepare a super-hydrophobic coating film, which is one of the few technical methods with practical application value.

In view of the problem that the super-hydrophobic coating is easy to wear and the strength is not enough, the above team also proposed a new idea: first apply a layer of glue on the surface of the object, and then spray a hydrophobic coating, so that the hydrophobic coating can better adhere to the surface of the object, hydrophobic The strength is guaranteed.

In the latest issue of Science, Lu Yao, a Ph.D. student in the Department of Chemistry at University College London, also suggested that the method of spraying super-hydrophobic coatings on adhesives can effectively improve the wear and tear of super-hydrophobic coatings. Weak points are given to more mature glue technology to overcome."

[Follow the WeChat public number "Jiuzheng Paint Network"; pay attention to surprises, scan code to view "If your husband is selling paint..." Jiuzheng Coating Network Exchange Group]