Application of near-field microwave microscopy in in-situ detection of microfluids under dielectric cover*

Based on the capability of penetrating through low permittivity materials and the sensitivity to impedance of microwave, we have applied the near-field scanning microwave microscopy (NSMM) to the in-situ detection of microfluids packed with dielectric covers. By means of a NSMM system, we obtained two dimensional maps showing the frequency shift and magnitude of the reflection coefficient S$_{11}$, which correspond well to the spatial distribution and electrical conductance of various microfluidic structures underneath 15-200 $\mu$m thick dielectric covers. The spatial resolution and sensitivity are found closely related to the thickness of the cover layer. The underlying physics is discussed in detail. The time-resolvable detection of ionic concentration in microfluids is also demonstrated in different conditions for study of transport of particles in microfluids. This technique offers a real-time, in-situ and non-invasive approach for monitoring local chemical reactions, motion of fluids, distribution and concentration of ions in lab-on-a-chip systems, and has a potential to be developed for detection of cells and tissues.