Tensiometer (surface tension)
Tensiometers are instruments for the determination of surface tension.
For dynamic measurements in automation technology, particularly the bubble pressure method and the drop volume method is suitable.
The surface tension can also be measured by the capillary (see # surface tension measurement of surface tension ).
There are different measurement principles:
- 2.1 Contact angle measurement
- 2.2 pendant drop method
- 2.3 Spinning Drop Method
- 2.4 Expanding / Oscillating drop method (EDM / ODM)
- 3.1 Bubble Pressure Method
- 3.2 Drop Volume Method
- 3.3 Method of test inks
Force measurement
Ironing method
In the method of Lenard bracket (also pull-off ), a strap with a soldered- in extremely fine wire (usually platinum) suspended in the liquid so that it just immersed in the liquid and is wetted by it. With a precision spring balance the tension is then gradually increased to the bracket. The wire is then withdrawn from the liquid, and pulling a liquid film. At a certain point, this film breaks off.
Pulling the strap work is done against the surface tension. From the maximum pulling force on the strap, before the liquid film is torn off, the dimensions of the bow and the density of the liquid, the surface tension can be calculated.
Liquids such as ethanol, and wire length of 2-3 cm at a radius of 0.1 mm, the expected value for the composition in the two to three digit milligram range. So there are very precise scales necessary. At a measurement uncertainty of the scale of 5 mg and a measurement of the wire to 1 micron exactly the maximum error of the final result is already 8-12 %.
Ring method
The traditional method of Pierre Lecomte du Nouy to measure the interfacial tension and surface tension, critical even in difficult wet conditions. To measure the force of a raised ring of liquid lamella is similar to the bracket method.
Plate method
Universal method of Wilhelmy, especially suitable for surface tension measurements over a longer time range. Measures the force that results from the wetting of a vertically suspended plate.
Optical Measurement
Drops from the geometry, the surface tension is determined.
Contact angle measurement
Also reports on the wettability of a substance. About the Young's equation can be calculated from the cosine of the contact angle of the surface tension.
Pendant drop method
Suitable for surface and interfacial tension measurements. Measurement capability even under extreme pressures and temperatures. Optical measuring the drop shape. The size of the drops dripping from a capillary tube is proportional to the surface tension.
Spinning drop method
For the determination of interfacial tensions. Particularly suitable for low to extremely low measuring ranges. The diameter is measured of a rotating drop in the heavy phase.
Expanding / Oscillating Drop Method (EDM / ODM)
Method for detecting characteristics of liquids oberflächenrheologischen. Describes the dependence of surface tension on the degree and the speed of the surface area of a drop, which is either rapidly expanded and then is stopped (EDM ), or a sinusoidally oscillating oscillation subject (ODM). Using this measurement technique, the foam stability and emulsion stability can be described.
Other methods
Bubble pressure method
Is suitable for the metrological detection of the dynamic surface tension (measured as a function of the surface age ). For this, a gas stream is passed through an immersed in a liquid capillary. The thereby formed bubble surface bulges and continuously reduces the radius of the bubble. The maximum pressure in the bladder before leaving is used to determine the surface tension.
Drop volume method
The rate of the number of drops in the shares a given volume of liquid. Superior method for the dynamic measurement of interfacial tensions.
Test inks method
One in the industry (eg in the bonding of self-adhesive films on plastics ) used test. On the surface to be tested a colored liquid ( "ink" ) is applied with a defined surface tension with a brush. If the surface is wetted by the ink ( ie the brush stroke remains for more than 3 seconds exist without to contract ), the surface tension of the surface under test is equal to or greater than that of the test ink. On the other hand pulls the brush stroke within three seconds together, the surface tension of the tested surface is smaller than that of the test ink.