Observation of anisotropic diffusion of light in compacted granular porous materials

TL;DR

This study demonstrates that compacted granular materials exhibit anisotropic light diffusion, with diffusion properties varying with compression force, providing new insights for material characterization and implications for spectroscopic analysis.

Contribution

It reveals the anisotropic diffusion of light in compacted granular materials and highlights the inadequacy of isotropic models, advancing diagnostic tools for material microstructure analysis.

Findings

Optical anisotropy increases with compaction force.

Radial diffusion constants can be up to 1.7 times the longitudinal.

Anisotropic diffusion impacts spectroscopic analysis of powder compacts.

Abstract

It is known that compaction of granular matter can lead to anisotropic mechanical properties. Recent work has confirmed the link to pore space anisotropy, but the relation between compression, mechanical properties and material microstructure remains poorly understood and new diagnostic tools are needed. By studying the temporal and spatial characteristics of short optical pulses diffusively transmitted through compacted granular materials, we show that powder compaction can also give rise to strongly anisotropic diffusion of light. Investigating technologically important materials such as microcrystalline cellulose, lactose and calcium phosphate, we report increasing optical anisotropy with compaction force and radial diffusion constants being up to 1.7 times the longitudinal. This open new and attractive routes to material characterization and investigation of compression-induced structural anisotropy. In addition, by revealing inadequacy of isotropic diffusion models, our observations also have important implications for quantitative spectroscopy of powder compacts (e.g., pharmaceutical tablets).