Microwave spectra of mixtures of non-absorbing solid particles and absorbing liquids

Microwave spectroscopy of condensed phases is sensitive to sample compositional factors such as water or fat content, solids content and salt concentration which leads to applications for in-line process monitoring. In this paper we show that microwave spectra can also be sensitive to sample structure at fixed composition which has implications for analytical applications involving samples which are multi-phase mixtures. We have measured microwave spectra for samples containing a constant volume fraction of non-absorbing glass spheres immersed within an absorbing liquid phase; the only difference between samples is the mean diameter of the glass spheres. The large differences in the spectra for different diameters is successfully modelled in terms of the relative volume fractions of glass and absorbing liquid within a measurement penetration depth adjacent to the microwave measuring probe.