Cactus plants are great at surviving in hot, arid climates.
What are their secrets and what can we learn from them?
One of cacti survival techniques is using their spines as fog collectors to ensure they get water even in arid conditions. What allows them to do this is structural features on micro- and nano-scale on the plants surface. Scientist’s Jie Ju, Hao Bai, Yongmei Zheng, Tianyi Zhao, Ruochen Fang and Lei Jiang studied these effects which can be read about in A multi-structural and multi-functional integrated fog collection system in cactus published in Nature communications (2012).
Microscopic image figur 2 (Jie Ju et al. 2012 p.3)
The picture show that the driving forces which make the water drops travel inwards from the tip of the spine are so strong that it works even when the spine is vertical with the tip down. So the way the spines grow from the stem is not very important for the movement of water.
”where R is the local radius of the spine (R1 and R2 are the local radii of the spine at the two opposite sides of the drop), g is the surface tension of water, R0 is the drop radius, a is the half-apex angle of the conical spine, and dz is the incremental radius of the spine (Fig. 4b). The Laplace pressure on the region near the
spine’s tip (small radius R1) is larger than that near the base (largeradius R2). This difference (DPcurvature) within the water dropinitiates a driving force that makes the drop move from the tip to the base side along the cactus spine.”(Jie Ju et al. 2012 p.5)
The other driving force is the gradient of the surface-free energy. This force is possible because the microgrooves on the spines have a difference in width. They are rougher and more hydrophobic near the tip of the spine than at the base. Because of this the tip of the spine has a lower surface-free energy compared to the base. The difference, again, creates a driving force, driving the water towards the base.
It can be described as follows:
”where 0A and 0R are the advancing and receding contact angles of water drops on the middle of the spine, respectively, and dl is the integral variable along the length of the middle of the spine from the region near the tip (ltip) to the region near the base (lbase).” (Jie Ju et al. 2012 p.5)
The third thing which is increasing the movement of the water has to do with the alignment of the grooves and barbs. The alignment creates an anisotropic contact angle hysteresis which generates movement along the grooves. Basically the drop is more inclined to go in the direction parallell to the grooves or barbs. Since the barbs are only on the tip of the pine and oriented towards the base the water is directed towards the base where the trichomes will absorb it.(Jie Ju et al. 2012)
Microscopic image figur 1 (Jie Ju et al. 2012 p.2)
Cactuses from the Saguara species can be over 15 meters high. Their stems can contain up to 95% water and they have little hard wood tissue which makes their structural strength low. This in combination with a root system of only around 30 cm in hight makes the cactus ability to withstand high wind interesting. Pradeep Babu and Krishnan Mahesh researched aerodynamic performance. Their research article Aerodynamic loads on cactus-shaped cylinders at low Reynolds numbers is published in Physics of Fluids (2008).
“Vector plot showing flow within a cactus groove at a Reynolds of a 20 and b 100. In b, note the lift of the primary recirculation zone closer to the free-stram and the formation of a smaller secondary recirculation zone closer to the surface in between the grooves.”Figur 11 (Babu, Mahesh 2008 p.8)
MECHANISMS FOR COOLING
Ju,J. Bai,H. Zheng,Y. Zhao,T. Fang,R. & Jiang,L. (2012) A multi-structural and multi-functional integrated fog collection system in cactus. Nature Communications (3:1247)[Microscopic image figur 1]http://www.nature.com/ncomms/journal/v3/n12/pdf/ncomms2253.pdf
Ju,J. Bai,H. Zheng,Y. Zhao,T. Fang,R. & Jiang,L. (2012) A multi-structural and multi-functional integrated fog collection system in cactus. Nature Communications (3:1247 [Microscopic image figur 2] http://www.nature.com/ncomms/journal/v3/n12/pdf/ncomms2253.pdf