Test method of surface tensiometer

Specifically, the force of the molecules that make up the liquid on the surface is different from that in the body. The forces on the molecules in the body are symmetrical and balanced. The molecules on the surface are attracted by the molecules in the body without a counterbalance force. That is to say, it receives a force that is pulled into the body. In other words, trying to reduce the surface area, so that this unbalanced state tends to equilibrium. The thermodynamic statement is: To minimize the surface energy of this system, this force is called "surface tension", which is also called the free energy per unit area (J / m2), that is, the formation or expansion of the interface per unit area The minimum energy required. Its value is consistent with the surface tension (N / m). Due to the habit, surface tension is commonly used to express surface free energy, which plays a vital role in the physical and chemical phenomena of the liquid surface. In daily life, dewdrops on lotus leaves in the morning and the curved water surface in cups are all surface tension phenomena.

1. Platinum plate method When the platinum plate is sensed to be immersed in the liquid to be measured, the surface of the platinum plate will be affected by surface tension. The surface tension of the liquid will pull the platinum plate down as much as possible. When the surface tension of the liquid and other related forces and balance force reach equilibrium, the sensing platinum plate will stop immersing into the liquid. At this time, the balance sensor of the instrument will measure the immersion depth and convert it into the surface tension value of the liquid.

In the specific test process, the test steps of the platinum plate method are: (1) the platinum plate is gradually immersed in the liquid; (2) in the state of being immersed in the surface of the liquid, the balance value is sensed by the sensor; (3) the balance value sensed Converted to surface tension value and displayed.
Second, platinum ring method Also known as du Nouy Ring method, du Nouy ring method, hanging ring method surface tension meter, off-loop method surface tension meter. The principle is as follows: (1) gently immerse the platinum ring in the liquid; (2) slowly lift the platinum ring upwards, that is, the liquid level drops relatively, so that a liquid column is formed under the platinum ring, and finally Platinum ring isolated. The platinum ring method is to sense a maximum value, and this maximum value is formed when the platinum ring and the liquid sample will be separated but not separated. Since this method was used very early, the original surface tensiometer basically adopted this method, and many existing data were also measured by this method.
1. The platinum ring hangs above the liquid surface and has been reset to zero.
2. The platinum ring has just started to contact the liquid surface, because the platinum ring is in contact with the thin film on the surface of the liquid; at this time, the display begins to show the value. (The display number is very small or negative)
3. The platinum ring began to immerse into the liquid, and the value began to increase due to the surface tension.
4. The platinum ring is immersed in the liquid at 5mm to stop. The value displayed at this time is the effect of the liquid on the surface tension of the two wires connecting the platinum ring. (The value depends on the wire diameter)
5. Start to slowly lower the sample stage.
6. The value on the display is gradually increasing.
7. As the sample stage descends slowly, the platinum ring forms a thin film of liquid above the liquid surface. The displayed value has reached the maximum.
8. Eventually, the platinum ring is detached from the liquid, and the liquid film is broken, indicating that the value decreases.
(The above two test methods are the most commonly used methods)
3. Maximum bubble method 4. Rotating drop method This method is suitable for measuring ultra-low interfacial tension, its range is generally less than 10mN / m.