The architectural design of smart ventilation and drainage systems in termite nests

Ventilation and drainage in termite nests are controlled by microscale morphological features of the outer walls.


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Air Percolation Analysis Fig. S1. XRD analysis of the nest material.  Table S1. Mineral compositions of the Senegal and Guinea nest material obtained from the XRD analysis.
Reference (46) We computed a percolating path (see the Materials and Methods section) of maximum inscribed spheres ( fig. S7A and fig. S7B) extending across the porous sample ( fig. S7C). From all the inscribed spheres in the pore space ( fig. S7C), we estimate the radius of the largest sphere that fits into the pore space and allows the connectivity (percolation) of spheres across the sample. This radius, called the threshold radius, is used to estimate the threshold capillary entry pressure required for air invasion into the water-saturated sample using the Young-Laplace equation, = 2 , where P c is the capillary pressure, is the surface tension between water and air = 72.75 mN/m at 20°C (46). The mean curvature of the water-air interface was calculated from the threshold radius of the inscribed sphere of the percolating cluster ( ) using = cos / , where is the contact angle at the three-phase contact points of grains, water and air. A contact angle of 20° is used in the analysis (36), which represents strongly water-wet (hydrophilic) conditions. For the particular case of the wall of the Senegal nest, we found a threshold radius of 60 µm for percolation of the maximum inscribed spheres across the sample. This threshold radius covers all the larger pores (compare the pore size distributions of percolating pores in fig. S7D and the pore size distribution of the nest in Fig. 3G). On the other hand, the Guinea nest samples show smaller threshold radii 25-32.5 µm. These values are consistent with the pore size distribution shown in Fig. 3G.
The smaller pores in the subsets of the outer wall of the Senegal nest have a significantly lower threshold in the range 7.5-10 µm. The pore size distribution of the percolating cluster of the smaller pores, as expected, is towards the left side of the distributions of both nest samples (fig .  S7D). The Senegal random pack shows a threshold value of 15 µm, which is close to that for the smaller pores of the Senegal nest. This is consistent with the overall pore size distribution of the smaller pores and the pores of the Senegal random pack (red and yellow curves in Fig. 3G).
The calculated capillary entry pressure required for air to percolate through the samples of Senegal and Guinea nest wall is in the range of 2.28 to 5.47 kPa corresponding to a water column height of 0.23 to 0.56 m respectively. This is significantly lower than that of the smaller pores of the subsets of the wall of the Senegal nest sample, which is in the range of 13.67 to 18.23 kPa, corresponding to a water-column height of 1.39 to 1.86 m.
These findings illustrate that if the rain water fills all the pores of the nest wall, the water in the larger pores can still percolate into the ground soil along with the infiltrating water-air front (once the rain stops), driven by a combination of gravity and the capillary forces. The higher permeability of the larger pores allow water to drain at a higher rate. The drainage of water will help to re-establish ventilation and CO 2 exchange through the larger pores of the outer wall of the nest.   Here, quartz, clay, pores and metallic elements are represented by yellow, green, blue and red respectively. The segmented image was used for thermal conductivity simulations in which these phases were assigned their respective thermal conductivity values. C. Three-dimensional visualization of the segmented image. The three-dimensional raw gray-scale image is shown in Fig. 3E. The local thickness map of the pore space representing the radius of the largest spheres that fit into the pore space. C. Pore space obtained from thickness map with a threshold radius of 17 µm. The image shows voxels inside spheres with a radius of more than 17 µm. Different colors in the image show disconnected clusters of the pore space. D. Isolated connected cluster of the pore space. E. The connected cluster obtained from (D) was applied as a mask on image (B) to obtain the thickness map of the connected cluster. F. Pore space obtained from thickness map with a threshold radius of 17.5 µm. Different colors in the image show disconnected clusters of the pore space. The dotted circle indicates the location where the connected pore space becomes disconnected and does not span across the image.