Confocal Microscopy and Digital Volume Correlation Methods for Intergranular Force Transmission Experiments

In this work we develop an experimental method to study interparticle force transmission in 3D. An initial 2D study of volumetric data for commercially available polyethylene fluorescent microspheres shows that three-dimensional effects play a significant role in deformation of real granular assemblies. This highlights the need for more experimental work to validate existing numerical models and motivate further development of theory and models describing real 3D granular media. A full 3D analysis method is described where volumetric images are captured via confocal microscopy and the displacement fields for each particle are determined using digital volume correlation (DVC). This data is then used to determine the average strain in each particle as well as the assembly's fabric (geometric descriptors of particles and contacts) which are in turn used for the Granular Element Method (GEM) to determine interparticle forces. Additionally, we perform a DVC analysis for an in-house produced polyacrylamide copolymer grain with a volumetric fluorescent speckle pattern. This demonstrates that with sufficiently small particles, it is possible to use our imaging and analysis methodology to determine intergranular force transmission in 3D experiments.