Shulou(Shulou.com)11/24 Report--

After a sound heavy rain

The lotus pond has regained its former calm.

Water droplets lie on the lotus leaves

Enjoying the afternoon sunshine.

A gust of wind blew

The lotus leaf shakes slightly

Water droplets roll happily on the leaves.

Behind the seemingly harmonious picture

It turned out to be hundreds of millions of years of resentment between molecules.

It is also an estrangement that will never be resolved!

All of this.

It all starts with a familiar noun.

surface tension.

Part 1: force, not simply pulling and pulling. Friends who are familiar with physics must be familiar with the forces around them: gravity, elasticity, support, Coulomb force. Every time you mention the name of a force, you can think of a diagram in which you push me and pull me between two objects that interact with each other.

▲ figure 1 Gravity and supporting force diagram

Although this kind of understanding is intuitive, it lacks some more in-depth perspective, and there is no way to really understand what surface tension is. What are we going to do? Let's look at the energy corresponding to the force (especially the potential energy). Take gravity as an example, you may have heard of gravitational potential energy, that is, the energy contained by an object near the earth because it is at a certain height. More intuitively, if the ground is taken as the zero point of potential energy, the energy released by the object falling to the ground under the action of gravity is the gravitational potential energy.

▲ figure 2 comparison of gravitational potential energy

As shown in figure 2, if the editor pushes down the same stone from the lower peak and the higher peak, which one releases more gravitational potential energy? It is obvious that the peak is bigger from the higher peak. So, how much bigger?

If you ignore the variation of gravity with height, that is, you think that the gravity of a stone is a constant G, then the answer is:

DL is the height difference between the two peaks. With a little deformation, we get

This shows that gravity is the change of gravitational potential energy per unit height! Similarly, the spring elasticity is the change of elastic potential energy caused by each elongation unit length, and the intermolecular force is the change of system potential energy caused by each change of intermolecular distance per unit length. It turns out that the force corresponding to any potential energy can be understood in this way.

At this point, the editor asked gracefully: so, what is the surface tension?

The reader replied in unison: the change of surface energy brought about by each increase in unit area of the surface!

After applauding, the editor asked: what is the surface energy? In what direction does the surface tension go?

The reader was silent and cast an expectant look at the editor.

Part 2: tension is just blowing a bubble. In fact, the problems mentioned above can be seen intuitively from the dimension. We have all come into contact with the concept of doing work, and the unit of work, like energy, is Joule J. Work is the product of a force on a body (in N) and the distance (in m) in the direction of the force, that is, 1 J = 1 N m. Then, in addition to N, the unit of force can also be written as J / m, that is, the change in energy caused by each movement of 1m.

However, there is a physical quantity whose dimension is J / m2, which represents the energy per unit area, which is the specific surface energy. Specific surface energy is a concept involving intermolecular interaction, which we need to understand from the microscopic point of view.

▲ figure 3. Diagram of the interaction potential energy between nitrogen and water molecules.

There are a variety of molecules in the micro world, such as nitrogen and water shown above. The potential energy of the intermolecular system is related to the distance between the molecules. This kind of relationship is like the relationship between two people, it is not good if it is far or near. If the distance is too close, there is no self-space between each other, the mood will be unstable, so it will have a mutually exclusive effect; if the distance is too far, everyone is too lonely, they will miss each other and attract each other. When the distance between them is just right, it corresponds to the lowest point of energy, which is more stable.

▲ figure 4 Water-air interface schematic

However, the stability of this relationship is related to the type of molecules on both sides. As shown in figure 4, for the water-air interface, there are a variety of molecules around the water molecules on the surface, including oxygen and nitrogen in the air in addition to other water molecules.

▲ figure 5 comparison of internal and surface water

As the saying goes, "birds of a feather flock together." more similar molecules tend to have a more harmonious relationship, while different kinds of molecules are relatively estranged. Physically, the lowest point of the potential energy curve between the same kind of molecules is lower. Therefore, for an internal water molecule, if it comes to the surface, it is obviously reluctant to disconnect part of the relationship with similar molecules and replace it with relatively "unfamiliar" air molecules. This is reflected in the increase in energy. So under what circumstances do water molecules come from the inside to the surface? It's time to blow bubbles.

▲ figure 6 schematic of bubble blowing process

Dip the ring in some soapy water, the volume of soapy water is basically constant. When we blow in air to make the surface area of the bubble larger, the thickness of the natural bubble becomes thinner. A further explanation is that there must be more water molecules coming from the inside to the surface, a process that internal molecules are reluctant to do, thus leading to an increase in energy. So, what is the energy increase per unit of expansion of the surface? This value is the specific surface energy.

▲ figure 7 the increase of the surface is accompanied by the thinning of soap bubbles.

We have mentioned that the unit of specific surface energy is J / m2, and understanding this physical quantity from the point of view of force is more intuitive for dealing with some problems, and we often call it surface tension. And J / m2 = N / m is not the unit N of the force, so the surface tension represents the force acting on the surface per unit length. In the experiment shown below, the soapy water film grows under the pull of force F, and the surface tension is

Among them, the 2 on the denominator comes from the surface on both sides.

▲ figure 8 schematic diagram of surface tension experiment | the picture is from [1]

So what is the direction of surface tension? We also compare gravity, elasticity and other forces. In a brief review, it is not difficult to find that if the surface tension is understood as an increase in energy caused by deformation per unit area, then the corresponding force should point in the direction that reduces the energy. For surface tension, it is naturally the direction in which the surface contracts along the tangent.

▲ figure 9 shows the direction of surface tension

Part 3: the struggle for spherical, tension and gravity begins with a question: what shape will the droplets look like in weightlessness?

With a little recollection, readers who have seen the space lecture should all think of the spherical water drop suspended in space.

▲ figure 10 screenshot of teaching in space

Yes, the droplets in weightlessness are spherical, because under a certain volume, the surface area of the sphere is the smallest, so the total surface energy of the spherical droplet is the lowest. So, what about gravity?

▲ figure 11 Surface molecules have a tendency to enter the interior.

On the surface of the droplet, each molecule is subjected to surface tension along the tangent, which is equivalent to the effect that each molecule (such as An and C) is pulling the surrounding molecules (such as B). The resultant force of the molecules on the surrounding surface of B points inward and tends to enter the interior. The effect of this tension keeps the droplets as nearly spherical as possible, that is, they have as small a surface area as possible. Gravity, however, plays a different role.

▲ figure 12 the effect of gravity

Gravity wants every water molecule to be as low as possible, and it is clear that a nearly spherical shape is not the lowest gravitational potential energy. Therefore, there is competition between surface tension and gravity.

▲ figure 13 Competition between gravity and tension

It is not difficult to imagine that if the tension is very strong, the droplet will maintain a nearly spherical shape without collapsing. If the tension is too weak to compete with gravity, the droplet will break. Going back to the problem of blowing bubbles, the reason why you usually have to dip in soapy water instead of pure water is that the surface tension of pure water is too small to compete with the gravity of the liquid film. However, in space, gravity no longer works, and the water film can exist stably!

▲ figure 14 Water film in space

Further, it involves a question: what does the surface tension of different substances have to do with? Qualitatively, we can introduce a concept called affinity, which reflects the affinity of materials on both sides of the interface. If the material on both sides is very friendly, then there is little difference between the interior and the surface. After all, everyone gets along, so the surface tension is relatively small. On the other hand, if the material relationship between the two sides is very distant, the surface tension will be very large. And this is the root cause of infiltration.

Part 4: infiltration, the phenomenon of love and hatred between molecules, this word may be strange, but the corresponding physical phenomenon is very common. For example, the waterproof clothes shown below and the water droplets on the bus windows are typical scenes of non-infiltration and infiltration.

▲ figure 15 infiltration and non-infiltration in life

Looking closely at the above scene, we find that there are not only two substances (precisely the phase, because the air has complex components), but three-- a solid as a base, a liquid and a gas in the environment. Therefore, the interface involved in the infiltration problem we face is not one, but three between two and two.

▲ figure 16 illustrating the relationship between matter

Naturally, the surface tension exists on any interface, and its magnitude is related to the relative affinity of the species on both sides, and the direction along the interface points in the direction that causes the interface to contract. And the final state needs to balance the components of these three forces.

▲ figure 17 tension competition at the junction of three interfaces

In the case of infiltration, the relationship between liquid and solid is very good, and it can be considered that the surface tension at the interface is negligible. In this way, the horizontal components of the red and blue forces need to be balanced, and the shape of the droplets tends to be flattened. Generally speaking, the angle between the tension at the gas-liquid interface and the liquid-solid interface is called the contact angle, and the contact angle of the hydrophilic surface with obvious infiltration is very small.

▲ figure 18 infiltration microscopic illustration

On the contrary, for the hydrophobic surface, the surface tension at the solid-liquid interface may be so large that the contact angle can be balanced only if the contact angle reaches an obtuse angle or even approaches 180 °, which is called non-infiltration or non-infiltration. This kind of surface is called super-hydrophobic surface because of its high hydrophobicity.

▲ figure 19 non-infiltrating microscopic illustration

At this point, some readers may have guessed that the surface of the lotus leaf is a natural super-hydrophobic substance, so the water is nearly spherical on the surface of the lotus leaf.

▲ figure 20 superhydrophobic phenomena in nature

Not only in nature, but also in the laboratory, scientists have prepared more effective super-hydrophobic or super-hydrophilic surfaces, such as extreme non-infiltration and infiltration in the image below.

▲ figure 21 artificial superhydrophobic / super hydrophilic surface | the picture is from [2]

The Night Lands,

A gust of evening wind blew

The lotus leaf shook violently

Several drops of water rolled down in the pond

There are ripples.

A puddle rowed across the water.

Just a few runs.

Disappeared into the whitish moonlight.

How does it know?

To lift one's own body.

Is the magical surface tension.

Reference:

Zhu Zhiang, Ruan Wenjuan. Physical chemistry. Edition 6 [M]. Science Press.

[2] Super hydrophobic / super hydrophilic surface modification-for all kinds of plastics

Editor: cloud opening and leaves falling

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Yun Kai Ye Luo

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