Plants in desert climate

 

mindre first page

Cactus plants are great at surviving in hot, arid climates.

What are their secrets and what can we learn from them?

IMG_2822

ATTRACTING WATER

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).

There are three different structures of the cactus which are important in fog collecting. The conelike structure of the spine, the oriented barbs at the top and the difference in the grooves in the stem. Then there is the trichomes at the base of the stem which quickly absorbs the water because of the capillary force formed by the trichomes and the spines.

mikroskopbild kaktusdroppar.jpg 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.

One of the drivers of the water is the conical shape. Because of the Laplace pressure gradient a drop on a conical-shaped object is usually driven to the side with the larger radius.   In the cactus spine there is a pressure difference between the two sides of the water drop.
This can be shown as follows:

räkna på lapace.jpg”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:

andra ekvationen.jpg”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)

mikroskop taggar.jpgMicroscopic image figur 1 (Jie Ju et al. 2012 p.2)


 

IMG_2811.JPG

AERODYNAMIC PERFORMANCE

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).

The shape of the cactus is excellent for handling wind loads. The plants have longitudinal cavities along the stem and spines at the tip, making it cross-section almost star-shaped. This assist in reduction of the fluctuating aerodynamic loading which are performing on the cactus at high wind velocities. The cavities of the Saguara cactus changes as it grows. The number of cavities vary between 10 to 30 and the depth of them adapt so that the average cavity depth is always maintained.
Experiments have been made studying the difference in aerodynamics between a smooth cylider compared to a cactus-shaped cylinder. Tests show that the pressure coefficients is largest in the valleys and lower at the tips. Meanwhile the spanwise vorticity is lowest in the valleys and largest at the tips.  The viscous drag is reduced by 69% but the pressure drag is increased by more than 1.3% on a cactus-shaped cylinder compared to a smooth cylinder. Since the effects are competing the total drag is still decreased by about 22%.(Babu, Mahesh 2008)
 graf stor.jpg
“Comparison results of a pressure coefficient Cp and b spanwise vorticity z at Re= 20 for smooth cylinder and cactus-shaped cylinder. ” Figur 4 (Babu, Mahesh 2008 p.3)

luftrörelserna.jpg

“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)


 

IMG_2814.JPG

MECHANISMS FOR COOLING

The cactus ability to self-shade itself is one of the topics in Helmut Tributsch book How life learned to live (1984).
Many cactuses have ridges going along their stem. According to Tributsch, the structure has multiple positive effects on the cactus ability to survive in the desert. First of all the shape of the plant, with an almost star-shaped cross-section, allows for some much needed shade from the sun. This also has the effect that the cactus has alternating parts of it that are warmer and parts that are cooler. This produces rising and falling air-currents  which improves heat radiation. This is especially good since the cactus has a reduced surface because of the need evaporate as little as possible. Generally this would lower its ability to reflect heat. Because of the smart geometry of the plant it is able to both cut down on surface area but still be cooled downed. (Tributsch 1984)
akvarell.jpg
Diagram showing the air-currents

IMG_2823.JPGIMG_2829.JPGIMG_2863.JPG
It is evident that the cactus has some genious ways of surviving in the desert heat. Maybe some ideas could be translated and used in architecture. Perhaps the fog collection of the needels could be converted as a way to retain water from an evaporative cooling system. The cactus shape could be used as a reference when designing for high wind loads. Its ability to self-shade and create cooling windmovements might be used in smart facade-systems.
rendering 2 sv bak.jpg
There are so many different shapes and varieties of cactus. In order to be able to examine cactus inspired forms further  I made different kinds in Grasshopper. The next step  is to study them using environmental plugins and simulations.
sunpath printscreen.jpg
work in progress…

 REFERENCES
Babu,P. & Mahesh, K. (2008) Aerodynamic loads on cactus-shaped cylinders at low Reynolds numbers. Physics of Fluids  (20)
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)
Tributsch, H. (1984). How life learned to live. Cambridge, MA: The MIT Press
Images from articles
Babu,P. & Mahesh, K. (2008) Aerodynamic loads on cactus-shaped cylinders at low Reynolds numbers. Physics of Fluids  (20)
[diagram figur 4]http://www.aem.umn.edu/~kmahesh/publpdf/journal/pradeep_pfl_cactus.pdf
Babu,P. & Mahesh, K. (2008) Aerodynamic loads on cactus-shaped cylinders at low Reynolds numbers. Physics of Fluids  (20)
[diagram figur 11]http://www.aem.umn.edu/~kmahesh/publpdf/journal/pradeep_pfl_cactus.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 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

Photos, renderings, watercolor: My Brandt
 
 
 

 

 

 

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: