Multiscale structural control of airflow, diffusion and heat transfer in non-fungus farming termite mounds

Termite mounds exhibit highly heterogeneous porous architectures that support efficient ventilation and thermoregulation under variable environmental conditions. Despite considerable interest, particularly in the context of bio-inspired design, the structural properties governing these processes remain only partially understood. In this study, we combine X-ray tomography and numerical simulations to investigate the relationship between structure and climatic properties in non-fungus farming termite mounds built by Trinervitermes geminatus, Cubitermes sp., Apicotermes sp., and Thoracotermes sp. Our results reveal substantial variation in airflow, diffusive transport, and thermal behaviour within mound interiors, driven by differences in internal architecture. At the scale of the whole mound, transport is strongly constrained by the outer walls, which act as the primary resistance to flow and diffusion, thereby largely determining overall permeability and CO2 diffusivity. At the microscale, wall microporosity plays a key role in regulating both CO2 diffusion and thermal conductivity, with increased microporosity enhancing gas exchange while reducing heat transfer. Within the mound interior, however, structural properties, such as macroporosity, connectivity, and tortuosity, govern transport. Together, these findings demonstrate that transport in termite mounds is inherently multiscale, with microscale wall properties exerting a strong influence on macroscale behaviour. This work advances understanding of the mechanisms underlying passive ventilation and thermoregulation in termite mounds and offers potential guidance for the design of energy efficient, bio-inspired systems.