Capillary-Driven Flow Through Biological Porous Media: X-ray Microtomography and Computational Fluid Dynamics

Abstract This study investigates the use of X-ray microtomography (XMT) to reveal the structure of complex porous biological tissues and the fluid flow through them during wetting. It also evaluates fluid dynamical simulations based on XMT data to reproduce and analyse these flows, with a final aim of revealing fluid transport and void formation in such tissues. To fulfil the objectives, the wetting flow of a polymer liquid through an initially dry conditioned Norway spruce wood sample is visualised using XMT at the MAX IV synchrotron. The liquid flow front progression captured after 24 s and 48 s reveals uneven filling of longitudinal tracheids and flow between them via the tiny pits which connect tracheids. Most tracheids fill between 24 and 48 s, possibly due to removal of air inclusions. Large density gradients near cell walls suggest that the fluid followed and deposited along wall structures. Computational fluid dynamics simulations (CFD) of saturated flow through the tomography-based geometry indicate velocity profiles that resemble pipe flow in longitudinal tracheids and flow rate differences among them. The latter indicates that the geometry itself may cause the experimentally observed uneven flow. Streamlines show intra-tracheid flow development and clear flow direction change at the pits. Additionally, wetting simulations, using a constant contact angle, capture initial uneven filling between the tracheids on shorter time scales than could be capture by the experiments. These simulations furthermore show air entrapment during filling, consistent with experimental observations. Combining XMT with CFD enables detailed studies of flow in biological porous media. Faster X-ray scanning, incorporating dynamic contact angles and accounting for diffusion in simulations could further refine insights into fluid progression during capillary-driven flow into complex structures of porous biological tissues.