Three-Dimensional Printing of Macropore Networks of an Undisturbed Soil Sample

Macropore systems predominantly determine rapid water flow and solute transport in undisturbed soils. Repeated experiments are needed to investigate the relationship between the nature of the macropore network and the resulting water and solute transport under different hydraulic initial and boundary conditions. However, the large heterogeneity in soil macropore network structures renders each soil sample unique and multiple identical samples impossible. In addition, the fragile nature of soil strongly limits the possible number of repeated experiments on one individual sample. Micromodels that mimic the precise shape and location of the macropores in undisturbed soil are therefore necessary to allow repeated experiments. In this study we investigated whether such micromodels can be obtained using contemporary three-dimensional (3-D) printing techniques and materials. We used X-ray computed tomography to digitize the 3-D macropore structure of an undisturbed soil sample. We printed a subsection of this macropore system in five different materials. Four out of the five investigated materials had essential parts of their macropore system clogged with residual printing or printing-aid material. Only one reprint, namely the prime-gray sample that was printed using stereo lithography, exhibited no pore clogging and had the largest hydraulic conductivity of all investigated reprints. Prime gray showed subcritical water repellency with a medium contact angle of approximately 65°, which is similar to contact angles found in natural soil. We conclude that the 3-D printing of undisturbed soil macropore systems is in principle possible with contemporary 3-D printing systems.