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Measuring principles page 99
15 Measuring principles
This chapter contains a brief introduction into the principles of the measuring methods
integrated in the K11.
15.1 Surface and interfacial tension
Due to the attractive forces between molecules (cohesion) a certain work is necessary
to change the size of a liquid interface or surface. The interfacial or surface tension is
the force to be spent referring to the circumference of the surface. The term surface
tension is used when the liquid phase borders to a gaseous phase, the term interfacial
tension refers to an interface between two liquids.
The K11 measures the surface or interfacial tension with a measuring probe
suspended from a force sensor. This probe is a ring or a plate consisting of a material
with optimum wetting properties material (platinum respectively platinum-iridium). A
force acts on the balance as soon as the probe touches the surface. If the wetted
length of the probe is known the force measured can be used to calculate the
interfacial or surface tension.
15.1.1 The ring method according to Du Noüy
Historically the ring method was the first to be developed; this is why many of the
values for interfacial and surface tension given in the literature are the results of the
ring method.
In the ring method the liquid is raised until contact with the surface is registered. The
sample is then lowered again so that the liquid film produced beneath the ring is
stretched.