Biomimetics, the abstraction of good design from nature[i], is again providing a useful solution, one of hundreds that have been developed as the result of a need and the insight of a variety of interdisciplinary scientists to solve those needs. The Deepwater Horizon oil spill in the Gulf of Mexico in 2010 created massively damaging ecological events, with oil spreading for many thousands of miles. The majority of the clean-up operation typically focused on collecting the oil near the surface of the water, and very little could be done at the time to find an effective way of capturing submerged oil droplets, which are normally missed in the cleanup operations.
The cactus species, Opuntia microdasys is one of several hundred varieties of “prickly pear” cacti.
The type we have has a longer thorn and just a few of the white hairs. We let the little pads grow and cut them when about 6 inches long scrape off the thorns and make “nopales” which are good at breakfast with huevos. The small pads are great roasted over a mesquite fire also.
Then we let them bud out into the “pears and when they turn red pick them to make jelly.
However, this is not what we are discussing here. The Opuntia microdasys has a very unique property, which we will discuss so that you may understand how scientists are using its natural protection from the harsh arid surroundings. It was discovered that the cactus efficiently collects water droplets from fog using a “unique system composed of well-distributed clusters of conical spines and trichomes on the cactus stem”.[ii]
This occurs because the cactus spines have an interesting effect on the water droplets. When micron-sized spherically shaped water droplets in the air land on them, the spine’s conical shape distorts them, forcing them into a clam-like shape instead. However, water droplets are inclined to be spherical, and exert a strong inward pressure to try to remain so.[iii] (Be sure to read the footnote so you will understand what follows). The battle between these two forces pushes the droplets from the tip of the spine[iv] toward the cactus plant at the base of the spine, where the spine’s surface is less curved and the radius is larger.[v] The cactus’s trichomes[vi] at the base of the spine then immediately absorb the droplets of water.
Cactus spines have many different functions, such as offering protection, shade, and slowing down air currents around the epidermis to lessen water loss. The discovery of this new function, added to the CAM physiology, waxy skin, succulent tissue, and specialized root system, are a wonderful example of design that had to be present and functional to allow the cactus to live in such harsh ecosystems as the Chihuahua Desert. Some people will claim that it took billions and billions and billions of years to have slowly changed and morphed and “evolved.” And during this time, the ecosystem wasn’t as harsh as it is now so that these things could develop- but why would they need to develop if it wasn’t so harsh?
Inspired by the research on the newly discovered function of the cactus spines, the design was mimicked and applied to collecting oil droplets in water. While oil normally floats on water, oil spills also produce some denser micron-sized droplets as the oil breaks down, which do not float on the surface. The current range of cleanup technologies used on oil spills such as mechanical skimmers or membrane filters mainly remove oil from the surface but miss those denser droplets that have sunken further down.
To tackle the problem, a team of scientists in Beijing (it figures, does not it not an American scientist but a foreign one) identified that “micron-sized oil droplets in water and micron-sized water droplets in air are similar, so we can move the cactus-inspired system underwater”.[vii] This was done by creating arrays of conical copper and silicone polymer-based[viii] needles to replicate the cactus spines. “The ability to deposit oil is based on the intrinsic oleophilicity (meaning that the materials have a strong affinity for oils rather than water) of these materials under water, and the canonical structure induces the directional motion of the collected oil droplets.”[ix]
The arrays were then submerged into a mixture of silicone oil and water that was blasted with ultrasonic sound waves to generate the micron-sized oil droplets. The team then observed that underwater oil droplets collect on and flow along the needles in a similar way that the water collects on and moves along the cactus spines. Testing different array set-ups, the team found that an array with hexagonal grids of the silicone polymer needles proved to be the most effective, due to a higher density of needles, separating up to 99% of the oil from the water.
Very few cactus fossil specimens have ever been found.[x] After billions and billions and billions of years of development, there is not hardly any fossilized cactus, not spiny pads, no backbones or skeletons, no nothing to suggest that they have been around. You would think that some would have been dislodged during a flash flood and that they would have been stuck in the sand and found later by archeologists. For the evolutionist, “the timing of cactus origins and diversification has remained enigmatic”.[xi] From a biblical perspective we know both the origin of the cactus spine and that of the fossils. Cactus spines are a reminder of the entrance of sin into the perfect world that God had created for man, and the subsequent Curse on creation detailed in Genesis 3. “Because you listened to your wife and ate from the tree about which I commanded you, ‘You must not eat of it’, cursed is the ground because of you; through painful toil you will eat of it all the days of your life. It will produce thorns and thistles for you, and you will eat the plants of the field. By the sweat of your brow you will eat your food until you return to the ground, since from it you were taken; for dust you are and to dust you will return” (Genesis 3:17–19).
[i] Biomimetics or biomimicry is the imitation of the models, systems, and elements of nature for the purpose of solving complex human problems. Living organisms have well-adapted structures and materials. Biomimetics has given rise to new technologies inspired by biological solutions at macro and nanoscales. Humans have looked at nature for answers to problems throughout our existence.
[ii] Ju, J. et al., A multi-structural and multi-functional integrated fog collection system in cactus, Nat. Commun. 3:1247, 2012 | doi: 10.1038/ncomms2253.
[iii] The cohesive forces among liquid molecules are responsible for the phenomenon of surface tension. In the bulk of the liquid, each molecule is pulled equally in every direction by neighboring liquid molecules, resulting in a net force of zero. The molecules at the surface do not have the same molecules on all sides of them and therefore are pulled inwards. This creates some internal pressure and forces liquid surfaces to contract to the minimal area. Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the imbalance in cohesive forces of the surface layer. In the absence of other forces, including gravity, drops of virtually all liquids would be approximately spherical.
[iv] Or the barb that it lands on
[v] The gradient of surface-free energy and gradient of Laplace pressure are believed to be the primary driving forces behind these phenomena. The water droplet always moves towards the base of the spine regardless of the angle of the spine.
[vi] Trichomes are fine outgrowths or appendages on plants, algae, lichens, and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae. A covering of any kind of hair on a plant is an indumentum, and the surface bearing them is said to be pubescent
[vii] Li, K. et al., Structured cone arrays for continuous and effective collection of micron-sized oil droplets from water, Nat. Commun. 4:2276, 2013 | doi: 10.1038/ncomms3276
[viii] The silicon-based organic polymer was Polydimethylsiloxane (PDMS)
[ix] Li, et al. ref. vii, p. 5.
[x] Chaney, R., A fossil cactus from the Eocene of Utah, American J. Botany 31(8):507–528, 1944.
[xi] Arakaki, M. et al., Contemporaneous and recent radiations of the world’s major succulent plant lineages, PNAS 108(20):8379–8384, 2011.