Bio-Architectural Blueprint - Part 3: Internal Architecture Revealed by Tomography

The Challenge of Opaque Systems

The success of biomimicry in sustainable architecture hinges on a complete understanding of the natural blueprint. For decades, studying the functional principles of termite mounds, particularly the precise flow of air and gas, was hampered by the mound’s opaque and complex structure. Traditional visualization methods, such as casting the tunnels with plaster or physically sectioning the mound, are inherently destructive, offering only partial and non-reusable information. Furthermore, the complexity of internal structures, featuring a network of tunnels, chimneys, and pores, makes direct in situ flow measurement difficult and localized.

Gaining a “complete picture of the behavior of mound temperatures through the year” and correlating structural elements with physiological functions requires advanced, non-invasive techniques. Researchers now recognize that understanding the physics of airflow requires knowing the exact geometry of the entire internal network, from millimeter-scale channels to micro-scale pores. This pursuit is driven by fundamental questions: What precise roles do chambers and galleries play in ventilation? How does connectivity support gas regulation? And what structural properties truly optimize CO2 transport?

Translating Structure to Science

The central claim is that X-ray tomography, particularly when combined with numerical simulations, has emerged as the definitive non-destructive method for visualizing and quantifying the intricate internal architecture of termite mounds across multiple scales. This integrated approach allows structural properties—such as macroporosity, channel thickness, and network connectivity—to be mapped directly onto fluid dynamics models. This process translates the biological ingenuity of the termite from a mere object of observation into a set of engineering parameters, providing the necessary depth to identify universal principles of bio-inspired climate control.

Quantifying Nature’s Labyrinth

X-ray tomography, a non-invasive tool widely used in engineering and material science, enables scientists to create highly detailed, three-dimensional digital models of the mound’s interior. By applying sophisticated image processing and segmentation techniques, the solid parts of the mound can be distinguished from the void spaces (air channels and pores). This allows for the quantification of previously inaccessible structural characteristics that dictate thermal and flow behavior.

Foundation & Mechanism: Porosity and Channel Networks

The internal architecture of the mound is a hierarchy of interconnected air spaces. The larger spaces, including chambers, galleries, and conduits, are collectively quantified as macroporosity, providing the pathways for bulk airflow. Values vary significantly by species and environment, ranging from 13.9% in some Macrotermes michaelseni mounds to 30% to 55% in Coptotermes lacteus. Crucially, the solid walls separating these macro-channels are not impermeable; they contain micropores (microscale voids).

In Macrotermes michaelseni mounds, porosity is measured between 37% and 47% air by volume, with small average pore diameters around 5 $\mu$m. These micropores are vital, aiding in insulation, CO2 diffusion, drainage, and structural stability. While the macroporous channels allow for rapid internal bulk transport of air, the microstructure dictates the second phase of ventilation: the slow, necessary exchange of gas across the mound wall to the exterior environment.

The Crucible of Context: Connectivity and Directional Flow

The effectiveness of ventilation depends not just on the volume of air space (porosity) but on how well those spaces are linked. X-ray tomography is used to map the network of chambers and galleries to calculate connectivity, a metric essential for communication, defense, and most importantly, maintaining continuous airflow. Studies show high connectivity in many species, such as Trinervitermes geminatus, which exhibits connectivity ranging from 92% to 98%.

Connectivity analysis can reveal anisotropy, or preferential directional flow, within the mound. For instance, certain mound subsets might show connectivity in the x-direction but none in the z-direction, suggesting a natural tendency for fluid flow along a specific axis. This high level of organization is particularly important for solar-powered ventilation, where even a strong thermal gradient would fail if the internal network lacked the connectivity needed to sustain a closed-loop convective current. The complex geometry of these tunnels, however, can introduce variability in local velocities, making interpretation challenging.

Cascade of Effects: From Measurement to Modeling

The digitization of the mound structure via tomography provides the geometric input necessary for computational fluid dynamics (CFD) modeling. Before this technique became common, air velocity measurements relied on custom-made, three-bead thermistor sensors, which could quantify flow speed and direction in specific conduits. These direct measurements confirmed the low, centimeter-per-second air speeds typical in termite mounds, suitable for Darcy’s flow models (Reynolds numbers less than 1).

Now, structural data quantified by tomography, such as wall thickness and pore size distribution, can be fed into numerical models like the Darcy–Brinkman–Stokes (DBS) equation. This allows researchers to simulate heat flow, CO2 transport, and pressure fields across the entire structure. For example, simulations based on tomography data can demonstrate how larger and better-connected channels (pores) facilitate superior airflow, while smaller or isolated channels restrict ventilation, effectively linking microscopic structure to macroscopic function.

The Unseen Dimensions of Self-Regulation

X-ray tomography, whether millimetre-scale (macro-features) or microscale (micropores), allows scientists to move beyond mere observation to precise quantification. This capability is crucial because termite mounds are not static; they are continuously remodeled in response to environmental cues like temperature and rainfall. For instance, during the rainy season, when walls become wet and less permeable, complex egress tunnels may be opened to maintain necessary ventilation.

By monitoring these structural changes over time using tomography, scientists can finally link internal remodeling actions directly to specific seasonal conditions and climatic shifts. The ability to translate the termite’s earth-and-saliva architecture into quantifiable engineering parameters—like porosity (37–47%), channel thickness (3 to 25 mm), and high connectivity (up to 98%)—underpins the next generation of bio-inspired design, moving past simple imitation toward true functional replication.