Surface tension

The plate method

In the plate method the liquid is raised until the contact between the surface or interface and the plate is registered. The maximum tension acts on the balance at this instant; this means that the sample does not need to be moved again during the measurement.

: Schematic diagram of the plate method

The tension is calculated using the following equation

$\sigma = \frac {F}{L \cdot \cos \theta}$

( $\sigma$ = surface or interfacial tension; $F$ = auf force acting on the balance; $L$ = wetted length; $\theta$ = contact angle)

The plate is made of roughened platinum and is optimally wetted so that the contact angle $\theta$ is virtually 0°. This means that the term cos $\theta$ has a value of approximately 1, so that only the measured force and the length of the plate need to be taken into consideration.

Correction calculations are not necessary with the plate method.

Ring and plate methods in comparison

With the KRÜSS tensiometers the ring and plate measurement are available as standard procedures for surface and interfacial tension measurements. This section provides you with a list of advantages of both methods. This should make the choice between the methods for the respective purpose more easy.

Advantages of the ring method

Many values in the literature have been obtained with the ring method. This means that in many cases the ring method should be preferred for comparison purposes.
The wetted length of the ring exceeds that of the plate by the factor 3. This leads to a higher force on the balance and accordingly to a better accuracy. This effect doesn’t influence the results of surface tension measurements, but small interfacial tensions can be carried out more accurately with the ring method.
Some substances, e.g. cationic surfactants, show poor wetting properties on platinum. In such cases the surface line between a ring and the liquid is more even than that of a plate.

Advantages of the plate method

Unlike the ring method, no correction is required for measurement values obtained by the plate method.
With the plate method, the densities of the liquids don’t have to be known as they have to be with the ring method.
In an interfacial tension measurement the surface is only touched and not pressed into/pulled out of the other phase. This avoids the phases becoming mixed.
With the ring method the surface or interface is renewed permanently due to the movement of the ring. If the ring is moving with high velocity, but also if solutions of large molecules or with high viscosities are to be measured, the maximum force is obtained when the diffusion equilibrium at the surface or interface is still not reached. The measurement failure caused by this effect does not occur with the plate method. The plate method is a static measurement, i.e. the plate does not move after the surface or interface has been detected.