Estimation of the diffusion coefficient of hygroscopic materials using a contactless transient THz water vapour content imaging and excitation device

Abstract

The investigation of humidity movement during drying or imbibition processes represents a serious challenge for several industries, such as the wood-based material industry and paper industry. Recently, several studies have been conducted on the efficiency contactless Terahertz technique (THz) to map the water content. In this study, this technique based on coupling infrared camera and developed Tera-Thermo-Converter, is used for the estimation of mass diffusion coefficient within hygroscopic materials initially dry. A water vapour water content excitation device is designed to excite the samples in a THz semitransparent enclosure during the monitoring. The recorded infrared films showed a pronounced mitigation of the signal over time, indicating penetration of vapour through the sample. Processing image techniques such as the four-image, algorithm and Singular Value Decomposition (SVD) and averaging are used to obtain the space-time water content map. A simplified one-dimensional model of mass transfer is proposed to estimate the diffusion coefficient using two inverse methods: a) a statistical method based on Bayesian approach, and b) a minimization method based on Levenberg‒Marquardt algorithm (LM). The results showed that THz imaging technique coupled with the appropriate processing and the adequate modelling, is able not only to map the water content field but also to monitor the transient transfer and estimate the diffusion coefficient without contact.