Photon Walk in Transparent Wood: Scattering and Absorption in Hierarchically Structured Materials

TL;DR

This study investigates how light interacts with transparent wood, revealing that scattering and absorption are influenced by hierarchical structures, and provides methods to control optical properties for improved material design.

Contribution

It introduces a combined experimental and modeling approach to quantify scattering and absorption in transparent wood, enabling targeted optical property tuning.

Findings

Forward scattering mainly causes haze in TW

Refractive index mismatch is a key factor in scattering

Transmittance and haze can be experimentally controlled

Abstract

The optical response of hierarchical materials is convoluted, which hinders their direct study and property control. Transparent wood (TW) is an emerging biocomposite in this category, which adds optical function to the structural properties of wood. Nano- and microscale inhomogeneities in composition, structure and at interfaces strongly affect light transmission and haze. While interface manipulation can tailor TW properties, the realization of optically clear wood requires detailed understanding of light-TW interaction mechanisms. Here we show how material scattering and absorption coefficients can be extracted from a combination of experimental spectroscopic measurements and a photon diffusion model. Contributions from different length scales can thus be deciphered and quantified. It is shown that forward scattering dominates haze in TW, primarily caused by refractive index mismatch between the wood substrate and the polymer phase. Rayleigh scattering from the wood cell wall and absorption from residual lignin have minor effects on transmittance, but the former affects haze. Results provide guidance for material design of transparent hierarchical composites towards desired optical functionality; we demonstrate experimentally how transmittance and haze of TW can be controlled over a broad range.