THz spectroscopy techniques for water gradient quantification

Understanding depth dependent hydration, or gradients, is important for a wide range of applications from paper manufacturing to biological tissue imaging. However, in situ characterization, especially in a non-destructive, reflective measurement is difficult in practice. Moreover, uncontrolled sample drying, water migration, and surface warping create significant challenges towards experimental testing. In this work we show a simple scenario with a standard cleanroom wipe of around $350 \mu \mathrm{m}$ thickness being exposed to water and left for complete drying, while its reflectivity together with changing mass data were recorded. The reflectivity was measured using VNA-based quasioptical system. High dynamic range, frequency domain measurements show a non-trivial frequency dependence on reflectivity that cannot be explained by aqueous dispersion alone. Further, the measurements reveal complicated drying dynamics where broadband reflectionless behavior was observed even when the sample was still $\sim 50$% water by mass. These investigations are an important step towards model model-free based analysis of spectra that should provide direct extraction of water at depth.