Another winter has arrived. In the morning, you walk out of the house against the cold wind, ready to fly out to meet an important customer. When you don’t go far, you slide heavily to the ground – the sidewalk outside the building doesn’t know when a layer of ice has been formed. You struggle to stand up, glad you’re strong, and it’s okay to fall, but the clothes you washed clean last night were “reimbursed.”
Go home and get your clothes back on your way, and you walk carefully to the parking lot, ready to start the car, only to find that the windshield is covered with a thick layer of ice. By the time you finally cleared the ice from your car, it was more than half an hour away from the scheduled departure time.
Worried about missing the flight you hurried to the airport, found that the boarding has not yet begun, the heart of a big stone finally fell to the ground. But soon you’re not happy: you don’t know how to account to your boss if you can’t make an appointment on time because of the heavy ice on the airport runway and the wings of the plane, and a lot of delays in flights.
Such a “disaster is not alone” scene may be exaggerated, but the ice to all aspects of our lives bring a lot of inconvenience, and even cause serious property and casualties, it is an indisputable fact. Such as in 2008 in China’s southern provinces of the snow disaster, a large number of power lines, towers and other ice overburdened collapse, resulting in serious power supply and communication disruption.
It is precisely because of the serious threat that ice poses to human life that it is always a vital task to protect important solid surfaces from these “uninvited visitors” every winter.
The current methods of de-icing or anti-ice are as follows: crushing and breaking the ice with simple mechanical forces; heating solid surfaces to melt ice; spraying chemicals such as salt and alcohol to reduce the freezing point of water.
These techniques, known as “active ice prevention”, work, but the drawbacks are also obvious: mechanical de-icing is time-consuming and laborious, and operators can also face potential hazards, such as falling from a height or falling from a high place. In the 2008 snow disaster in the south, Zhou Jinghua, Luo Changming, Luo Haiwen three power workers in Hunan Province were killed in the sudden collapse of the tower during the de-icing of the power line tower, heating the solid surface requires a small energy input, and the use of chemical reagents de-icing may lead to surface runoff and groundwater pollution.
So in recent years, a whole new idea has been put forward: can a solid material be designed that will not attach to the ice at low temperatures, even without human intervention, to solve the problems of icing once and for all?
This is known as “passive ice prevention”.
This goal may sound like a nightmare at first, but if you look closely, it’s not entirely impossible. Of course, before we can begin to design this material, we first need to understand why the solid surface at the end suddenly attaches a thick layer of ice.
Super-hydrophobic surface: say no to freezing rain?
One of the causes of ice on solid surfaces is freezing rain. Freezing rain is a very painful natural disaster. In 2008 to the south of China caused serious loss of life and property snow disaster, there are many losses from the freezing rain brought about by the freezing. In the event of freezing rain, water droplets with temperatures below 0 oC, so-called over-cold water droplets, drop from the air and freeze quickly when they encounter solid surfaces.
So if after cold water droplets land on solid surfaces, before they can be solidified before they flow quickly, it is not possible to achieve the goal of never freezing?
To the lotus leaf “take the Bible” –
Strong combination of microstructure and wax cover
With this goal in mind, the researchers went to nature for inspiration, and they quickly noticed an object worth emulating: lotus leaves.
If we look closely, the droplets that fall on the lotus leaves not only always remain spherical, but when the breeze blows and the leaves tilt slightly, the droplets will soon fall off. On the contrary, the droplets that fall on the glass will not only spread out, but they will only flow down when we tilt the glass sharply.
So what’s the mystery of the lotus leaf surface? To answer this question, we need to understand a few basic concepts first.
When we place a drop of water on a solid surface, the gravity of the droplets drives the droplets to spread over the solid surface to form a thin film of liquid. But there are two other important forces that determine the fate of droplets: first, the molecular inter-force steam between water molecules and solid surface molecules, which is similar to gravity, will cause water to spread over the solid surface, and second, the molecular inter-molecular force between water molecules, which has the opposite effect, it will make the water droplets as far as possible to remain the original spherical.
When the volume of the water droplets is sufficient, the effect of gravity is negligible, and the interaction force between water molecules is fixed, so the water droplets on the solid surface “where to go” mainly depends on the size of the force between the solid molecules and the water molecules. If this force is strong enough, then water will spread on the solid surface, which we call water to immerse solids, which, correspondingly, are called hydrophilic surfaces;
To determine how hydrophilic or hydrophobic a solid surface is, we can determine the angle between the edge of the droplet and the solid surface, commonly referred to as the contact angle. It is not difficult to see that when the water droplets are fully spread on the solid surface, the contact angle should be 0o, and if the water droplets are completely spherical, the contact angle should be 180o. Therefore, the greater the contact angle, the stronger the solid hydrophobic.
A comparison of the contact angles of water on hydrophilic, hydrophobic, and super-hydrophobic surfaces.
Obviously, to improve the surface of the anti-ice effect, improve the hydrophobic surface is a natural choice.
To achieve this, we must first adjust the chemical structure of the solid surface, weakening the molecular inter-molecular force between the solid-liquid molecules. In common solid materials, plastics, rubber and other organic polymer materials are usually better than metals and ceramics, glass and other inorganic non-metallic materials, and high-molecular materials containing fluorine, silicon and other elements are more than extraordinary hydrophobic capacity. For example, the famous Teflon, or Teflon, which is often used in non-stick pot coatings, has a contact angle of about 130o on its surface.
However, the contact angle of water on the surface of the lotus leaf can easily exceed 150o. Obviously, the hydrophobic capacity of lotus leaves is still a lot higher than teflon. Where does this gap come from?
When scientists put the surface of the lotus leaf under an electron microscope, the mystery was solved: the surface of the lotus leaf was not smooth, but was filled with many small columns of diameter, height and spacing of only a dozen to tens of microns.
In fact, it is these rough microstructures that make the lotus leaves extremely hydrophobic. So what’s behind this?
We know that if gravity is ignored, water droplets should form a perfect sphere in the air. This means that if we think of air as a solid, then the contact angle of water on its surface should be 180o, which means that the air has a stronger hydrophobic condition than all other solids.
When a drop of water falls on the surface of the lotus leaf, due to microstructure constraints, the water droplets can not penetrate into the space, so a part of the surface of the water droplets will be in contact with the air this extremely hydrophobic “solid”. The lotus leaf surface is covered with a layer of wax, it itself has a good hydrophobic capacity. Therefore, the result of the two “strong combination” is that the surface of the lotus leaves show a strong hydrophobic ability. Not only that, but the increase in hydrophobicity has also brought about another “gift”, that is, the friction of water droplets as they flow. Surfaces like lotus leaves, not only the solids are slightly tilted droplets that will roll down. And when droplets falling from high places hit the surface, it is possible to bounce again.
Super-hydrophobic surface: advantages and limitations coexist
After learning the secrets of the lotus leaves, scientists, as a method, introduced microstructures into conventional hydrophobic surfaces, thus taking the hydrophobic effect of solids to the next level and getting what we usually call “super-hydrophobic surfaces”.
Looking forward to it, the scientists tested the ice-proofness of the super-hydrophobic surface, and indeed got some satisfactory results. In a 2010 study, for example, cold water droplets that fell on ordinary hydrophilic or even hydrophobic surfaces froze quickly, but fell on ultra-hydrophobic surfaces and bounced away quickly, keeping solid surfaces free from ice.
Over-cold water droplets fall on the tilted super-hydrophobic surface (Figure C) and bounce quickly, leaving the solid surface free of ice for a long time; The far right picture shows the microstructure of the super-hydrophobic surface seen under an electron microscope, with a scale of 10 microns. (Image Source: Ref.
But soon, the researchers were puzzled to find that in subsequent tests, the super-hydrophobic surface often “lost the wheat city”, its ice resistance is not much stronger than the ordinary solid surface. Why is this contradiction?
As we mentioned earlier, the key to ice-proofing on ultra-hydrophobic surfaces is that droplets that fall on the surface bounce quickly before they have a chance to freeze, but in many cases this is not so easy to do. For example, in the study, which had just been mentioned, scientists found that as the temperature dropped and the viscosity of water increased, it might not bounce on solid surfaces in time, but would freeze like a normal surface.
In addition, if raindrops hit the surface too fast, or if high ambient humidity causes water vapor to condense directly on the solid surface, it can lead to a worse situation, which is that the over-cold water droplets that hit the solid surface can enter the pores that were otherwise occupied by air between the microstructures. At this time, although the contact angle of the water droplets on the solid can still be close to 180o, but the friction of the water droplets flow is greatly increased, so when the solid tilt, the droplets are no longer rapid flow down, but “unyielding” to stay on the solid surface. It is not difficult to imagine that when freezing rain comes, such a surface is not only difficult to play an ice-proof effect, but also because the rough surface increases the adhesion between ice and solids, the surface of the ice is more difficult to remove.
Once the super-hydrophobic surface forms ice, we often have to use mechanical forces and other means to remove it, which is likely to lead to a worse result, that is, the microstructure of its surface in the de-icing process suffered partial damage, which can also make water droplets into the microstructure between the gap, resulting in a significant discount to its ice resistance. For example, studies have shown that the adhesion between the surface and the ice increases significantly after a 20-or-so icing cycle on the surface of the super-hydrophobic surface.
It is precisely because of the limitations of ultra-hydrophobic surfaces in anti-ice de-icing applications that in recent years researchers have begun to turn their attention to another type of surface structure, also inspired by nature, the famous insect-eating plant, the pig cage grass.