What is surface tension?
As we all know, we can explain the nature of the liquid according to the mutual attraction between molecules. The attraction of this molecule is called molecular cohesion or van der Waals force. The surface tension, interfacial tension and similar phenomena are used to explain the basic physical phenomena of molecular cohesion.
Specifically, the force of the molecules constituting the liquid on the surface is not the same as that within the body. The forces in the body are symmetrical and balanced. While the molecules on the surface are attracted by the molecules in the body without counterbalance. That is to say, it is pulled into the body of the force. In other words, trying to narrow the surface area, so that this imbalance in the state tends to balance the state. The theory of thermodynamics is to minimize the surface energy of the system, which is called "surface tension", also known as the free energy per unit area (J / m2), that is, the formation or expansion of the unit area of the interface The minimum energy required. Its value is consistent with the surface tension (N / m). Due to the habit, the commonly used surface tension represents the surface free energy, which plays a vital role in the physical and chemical phenomena on the liquid surface. In daily life, morning dew on the lotus leaves, cups in the curved surface, etc. are surface tension phenomenon.
Several methods commonly used to measure surface / interfacial tension
First, the platinum plate method
When the sensation of the platinum plate immersed in the measured liquid, the platinum plate will be affected by the role of surface tension, the liquid surface tension will pull down the platinum plate as much as possible. When the liquid surface tension and other related force and balance force to achieve a balance, the sensing platinum plate will stop into the liquid inside the immersion. At this time, the balance sensor of the instrument measures the depth of immersion and converts it into the surface tension of the liquid.
In the specific test process, the platinum plate method of the test steps are: (1) the platinum plate gradually immersed in the liquid; (2) immersed in the liquid surface state, by the sensor sense of balance; (3) will be the balance value Converted to surface tension values and displayed.
Second, the platinum ring method
Also known as du Nouy Ring method, du Nouy ring method, ring method surface tension meter, de-ring surface tension meter. The principle is: (1) the platinum ring gently immersed in the liquid; (2) the platinum ring slowly up, that is, the relative decline in liquid surface, making the platinum ring below the formation of a liquid column, and eventually with Separation of platinum rings. The platinum ring method is to sensate a maximum value, and this highest value is formed in the platinum ring with the liquid sample that will depart from the time. Because this method is used very early, so the original surface tension meter are basically using this method, a lot of existing data is measured in this way.
1. The platinum ring hangs over the liquid surface and has been zeroed.
2. Platinum ring has just come into contact with the liquid surface, because the platinum ring at this time contact with the liquid surface that layer of film; At this point the display began to show the value. (The number is small or negative)
3. Platinum ring began to immerse into the liquid inside, because the role of surface tension shows the value began to become larger.
4. Platinum ring immersed in the liquid to stop at 5mm, this time the value is displayed on the liquid two platinum rings connected to the role of the surface tension. (The size of the value depends on the wire diameter)
5. Start slowly lowering the sample stage.
6. The value on the display is getting larger.
7. As the sample stage slows down, the platinum ring forms a column of liquid film over the liquid surface. The value shown at this point has reached the maximum.
8. The final platinum ring and liquid from the liquid film rupture, showing a reduced value.
(The above two test methods are the most commonly used method)
Third, the largest bubble method
Four, spin drop method
This method is suitable for measuring ultra-low interfacial tension, the range is generally below 10mN / m.