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Measuring principles
As the film is stretched a maximum force is experienced; this is recorded in the
measurement. At the maximum the force vector is exactly parallel to the direction of
motion; at this moment the force correlates to the surface or interfacial tension. The
contact angle θ is 0°for a material with optimum wetting properties as Pt/Ir.
The following illustration shows the change in force as the distance of the ring from the
surface increases.
In practice the distance is first increased until the area of maximum force has been
passed through. The sample vessel containing the liquid is then moved back so that
the maximum point is passed through a second time. The maximum force is only
determined exactly on this return movement and used to calculate the tension. The
lamella disruption shown in the above figure is avoided in the EasyDyne ring method.
The calculation is made according to the following equation:
F
σ
=
max
θ
⋅
L
cos
(σ = surface or interfacial tension; F
= maximum force; L = wetted length,
max
θ = contact angle).
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