Abstract:
The ability to precisely analyse the composition of liquid mixtures by non-contact
techniques in both static and flow situations is extremely desirable for a diversity of
industrial, analytical and quality control procedures. Microwave resonators allow very
accurate and sensitive characterisation of the dielectric properties of polar liquids due to
the strong interaction of the latter with microwave electric fields. They have the useful
dual role of both exact characterisation of the complex permittivity of a dielectric sample
when it is inserted within a region of high electric field of the resonator, and effective
volumetric heating of the same sample if its dielectric loss is large enough to permit
heating. They offer tremendous potential for investigation of very small amounts of polar
solvents in non-polar hosts. In this regard they are superior to other traditional
composition analysis techniques such as liquid chromatography, gas chromatography and
mass spectrometry in the speed of analysis (≈ 1 s), non destructive nature and scope for
miniaturisation of the resonator size to suit the system under test.
For minute sample volumes, the resonator perturbation technique is extensively used for
dielectric measurements on polar liquids. In this project, it has been employed for highly
sensitive compositional analysis of two-component dielectric mixtures contained in
microcapillary segments. The first evaluation system used here was mixtures of
acetonitrile and toluene, chosen because of the large difference in their molecular electric
dipole moments. The results obtained from this first system provided the inspiration to
assess mixtures made of acetonitrile and water, which are much more closely matched in
terms of their electric dipole moments. Three different types of resonators namely hairpin
resonator, split ring resonator and sapphire dielectric resonator were used to analyse both
the aforementioned solution systems. The results show very sensitive characterisation and
are in close agreement with the theoretical predictions governing perturbation of
resonators by dielectric samples. In the last phase of this research, a miniaturised sapphire
dielectric resonator was designed and fabricated that provided added enhancement in
measured sensitivity of both evaluation mixtures.
