Difficulties of Subsurface Liquid Front Tracking in Porous Media with Terahertz Pulsed Imaging

Terahertz pulsed imaging (TPI) coupled with a customised flow cell has previously been used successfully to study liquid transport in porous media, such as powder compacts. In some samples, the tracking of the liquid front remains challenging when the refractive indices of liquid and sample are similar and hence little contrast is observed. In this work, we found that even when the relative difference between the refractive indices of liquid and sample is high, it can still be challenging to track the reflection peak of water in some samples while it works fine in others. It is hypothesised that this is due to the formation of a water gradient in these samples due to their specific microstructure and we explore methods to overcome this limitation.