Observation of anisotropic diffusion of light in compacted granular porous materials

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).