Non-Destructive Measurement of Water Diffusion in Natural Cork Enclosures Using Terahertz Spectroscopy and Imaging

Abstract

Terahertz (THz) imaging enables non-destructive evaluation of many materials' internal structures which could not be probed by visual analysis alone. In this paper, we apply THz imaging to non-destructively probe and image the diffusion of water through natural cork samples. The temporal and spatial distribution of water is analyzed as it diffuses throughout the cork structure. An average diffusion coefficient comparable to previous work in the field is extracted from this analysis. Data is also presented to show that the diffusion coefficient is not uniform throughout the cork but changes dramatically due to the local structure and composition of the cork.

Review of Moisture and Liquid Detection and Mapping using Terahertz Imaging

“…For example, typical real indices of refraction and absorption coefficients at 1 THz for wood [10] (n~1.3, α~15 cm -1 ), leaves (solid material) [11] (n~1.7, α~40 cm -1 ), polyamide plastic [12] (n~1.75, α~10 cm -1 ), and cork cell walls [13] (n~1.2, α1 0 cm -1 ), are smaller than that of water (n~2, α~230 cm -1 ). Note that the absorption coefficients for liquid water are typically an order of magnitude or more larger than most other materials.…”

“…[11][12][13]16] As an example, [17] used a simple mixing model which incorporates the presence of water via a linear model for the effective absorption coefficient and the volume fraction of water :…”

“…There are many applications of THz imaging for non-destructive evaluation [13,[22][23][24][25] for which the contrast mechanism results from enhanced scattering by cracks, voids, and defects in an otherwise isotropic material. The diffusion of water in these materials, such as natural cork (Section 5.3) and concrete (Section 8.2) preferentially tend to follow the channels formed by the cracks, voids, and defects.…”

“…As another example of liquid diffusion measurements in a THz transmission geometry, [13] measured the time dependant diffusion through a thin disk of natural cork. The diffusion process can be modeled using the solution to the diffusion equation Eq.…”

“…[13,25] Natural cork, a product from the Cork Oak (Quercus suber), is utilized in a variety of products including cork stoppers for wine and other beverages. As an enclosure for liquids, it has the desirable properties of being largely impermeable to liquids and gases, as well as compressible.…”