Tensiometer (surface tension)
Tensiometers are measuring devices for determining surface tension .
The bubble pressure method and the drop volume method are particularly suitable for dynamic measurements in automation technology .
The surface tension can also be measured by the capillary effect (see also surface tension # Measurement of surface tension ).
There are different measuring principles:
Force measurement
Ironing method
With the Lenard ironing method (also known as the tear-off method), a hanger with an extremely thin wire (mostly made of platinum ) soldered into it is hung in the liquid so that it is just immersed in the liquid and is wetted by it. The tensile force on the bracket is then gradually increased using a precision spring balance. The wire is then pulled out of the liquid and pulls a film of liquid with it. At a certain point this film breaks down.
By pulling on the bracket, work is done against the surface tension. The surface tension can then be calculated from the maximum possible tensile force on the bracket before the liquid film breaks, the dimensions of the bracket and the density of the liquid.
For liquids such as ethanol and wire lengths of 2–3 cm with a radius of 0.1 mm, the expected value for the mass is in the two to three-digit milligram range. So very precise scales are required. With a measurement uncertainty of the balance of 5 mg and a measurement of the wire with an accuracy of 1 µm, the greatest error in the final result is already 8–12%.
Ring method
Classic method by Pierre Lecomte du Noüy for measuring the interfacial tension and surface tension, not critical, even with difficult wetting conditions. The force of a liquid lamella pulled up by the ring is measured similarly to the ironing method.
Plate method
Universal method from Wilhelmy, especially suitable for surface tension measurements over a longer period of time. The force that results from wetting the vertically suspended plate is measured.
Optical measurement
The surface tension is determined from the drop geometry.
Contact angle measurement
Also provides information about the wettability of a substance. The surface tension can be calculated from the cosine of the contact angle using Young's equation.
Pendant drop method
Suitable for interface and surface tension measurements. Measurement options even at extreme pressures and temperatures. Optical detection of the drop geometry. The size of the droplets that drip from a capillary 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 of a rotating drop in the heavy phase is measured.
Expanding / Oscillating Drop Method (EDM / ODM)
Method for determining the surface rheological properties of liquids. The dependence of the surface tension on the degree and the speed of the areal expansion of a drop is described, which is either rapidly expanded and then stands still (EDM) or is subject to a sinoidally oscillating oscillation (ODM). With the help of this measurement technique, the foam stability and the emulsion stability can be described.
Other methods
Bladder pressure method
Is suitable for the metrological recording of the dynamic surface tension (measurement depending on the surface age). For this purpose, a gas flow is passed through a capillary immersed in a liquid. The bubble surface that forms in the process bulges and continuously reduces the bubble radius. The maximum pressure in the bladder before it ruptures is used to determine the surface tension.
Drop-volume method
The number of drops into which a given volume of liquid is divided is measured. Superior method for dynamic measurement of interfacial tension.
literature
- ^ Du Noüy, Pierre Lecomte: An Interfacial Tensiometer for Universal Use . In: The Journal of General Physiology . 7, No. 5, 1925, pp. 625-633. doi : 10.1085 / jgp.7.5.625 .