Caution—slippery surface

by

Published: October 13, 2016
lotus
Figure 1: The surface texture of the leaves of the lotus plant, showing water beading (magnified).
Credit: Wikipedia.org

We have probably all seen the sign, “Caution—slippery surface,” and likely experienced the slipperiness of a wet floor firsthand. Not surprisingly, surfaces which repel fluids efficiently are correspondingly slippery, and such surfaces can have many useful applications in industry.

Vast amounts of money are spent each year in developing materials that are more and more repellent. Researchers have developed materials that are very good at repelling water on its own. 

Scientists and design engineers have increasingly been copying designs from nature, in a rapidly growing field known as biomimetics—and surfaces are part of this trend.

 Current state-of-the-art research in highly water repellent (superhydrophobic) surfaces has been inspired by an item from the Creator’s repertoire, the lotus flower (Nelumbo). Its leaves resist water because of their surface texture—a phenomenon called the lotus effect.1

Each leaf is covered with tiny bumps called papillae, so that air is trapped between the water droplets and the papillae, making the surface repel the water. The repelled water also carries away dust, so the surface cleans itself.

Needed: a multi-repellent surface

But it is much more difficult to manufacture materials that repel oils as well as water.

Despite more than a decade of intense research into slippery surfaces, a breakthrough had been lacking in such multipurpose oil- and water-repellent materials. Advancements were made with the lotus plant derivatives, but even with their legendary slipperiness, the surface materials developed have distinct limitations.2

Now imagine a man-made surface so slippery that it will repel not just water, but blood and even crude oil. Imagine also that it can do this in harsh environments, and can even rapidly heal itself if damaged, and clean itself if dirty. Sounds incredible, but it really exists—meet the pitcher-plant-inspired slippery surface technology!

Aquaplaning ants

wikipedia.org pitcher

The plants in the pitcher plant genus Nepenthes prey on creatures (mostly insects) that slip and fall into it after wandering on to the plant’s peristome, the area surrounding the ‘mouth’ (Figure 2).3 These plants have long been studied and researchers have only recently become aware of perhaps their most important capture mechanism—the amazing slipperiness of this peristome. Its microstructure traps nectar from the plant, or water from rainfall, which then forms a thin slippery film on the surface. In contrast to the lotus effect where the microtexture in combination with air produces the slipperiness, the slipperiness of these pitcher plant peristomes is produced by the combination of the lubricant and the surface texture.

The plant’s prey ‘aquaplanes’ down the peristome to the juices in the pitcher where it drowns. It is then slowly digested to provide the nutrients that the plant needs to survive in its otherwise nutrient-poor habitat. “The larger ones frequently take frogs, lizards and mice, and the biggest ones have been found with rats in them” and, unusually, even birds.4

Biomimetics expert Joanna Aizenberg, Professor of Materials Science at Harvard, said that, “Inspired by the pitcher plant, we developed a new coating that outperforms its natural and synthetic counterparts and provides a simple and versatile solution for liquid and solid repellency.”5

Incredible SLIPS

The man-made counterpart to this ingenious design is called “slippery liquid-infused porous surface(s)”, or SLIPS for short. Inspired by the pitcher plant, scientists generated these surfaces “… through liquid imbibition [saturation] into the porous materials, resulting in a homogenous and nearly molecularly smooth surface with a roughness of about 1 nm [one billionth of a metre].”6 Talk about a smooth surface!

Whereas lotus-inspired surfaces are only water repellent, surfaces created using the SLIPS technique can be made so that they repel oils as well as water (omniphobic). Depending on the lubricant used with the porous microstructure, SLIPS-type surfaces can repel almost anything. The authors reported that SLIPS can repel things such as water, ice/frost, oils, blood, and brine. They have exceptional performance against biofouling, which refers to the accumulation of living organisms on a surface—generally microorganisms but also invertebrates, such as barnacles on the hull of a ship. They also act as a pest barrier, mimicking the pitcher plants. In addition, the authors reported that “SLIPS are capable of repelling water and liquid hydrocarbons both at and while transitioning to a pressure of 676 atm (i.e. 676 times the pressure of our atmosphere at sea level, the highest available pressure in [their] setup).”7,8,9

The surface can also self-clean, and even when damaged repeatedly, can self-heal within 0.1–1 second because of the lubricating film.5 “Even after we damage a sample by scraping it with a knife or blade, the surface repairs itself almost instantaneously and the repellent qualities remain, making SLIPS self-healing.”10 These incredible surfaces can also be made transparent.5

Failure to credit the Creator

Like countless other plants and animals, these fascinating carnivorous pitcher plants (see box) display amazing design that showcases the Master Designer’s ability to create an elegant solution to a variety of problems. Because of the design’s superiority, SLIPS can be used in a huge range of applications ranging from architecture to aviation, and from consumer products to medical systems.

It has taken scientists decades of study and research to copy the design already present in a humble plant. So it displays an amazing lack of logic (or at the least a lack of the will to use it) to fail to recognize the Master Designer behind these plants.11 No wonder that Romans 1:20 says that people are “without excuse” for not seeing God in His created world. Just as the SLIPS surface repels fluids, sin, it seems, our rebellion against Him, is a potent truth repellant.

We need to give credit where credit is due; the array of ingenious designs continually found in the natural world is indeed powerful evidence for nature’s Intelligent Designer, the God of the Bible!12

Carnivorous plants in a pre-Fall world?

The pre-Fall world was ‘very good’ (Gen 1:31) with no death or suffering. Perhaps the features of pitcher plants now used carnivorously were originally used for non-carnivorous purposes. (Consider the predominantly vegetarian Bagheera kiplingi spiders, which use their webs to capture pollen to eat.1) Or perhaps God, foreknowing the Fall, programmed them with latent carnivorous features to enable adaptation to a harsher fallen world where plants might eventually struggle to obtain sufficient nutrients from poorer soils.2 Note too that insects, the main prey of pitcher plants, do not have life (nephesh chayyah) in the biblical sense, and that even today, soil is still the primary source of nutrition for pitcher plants.

References and notes

  1. Catchpoole, D., Vegetarian spider, Creation 31(4):46, September 2009; creation.com/vegetarian-spider.
  2. For more on the advent of carnivory, see chapter 6 “How did ‘bad things’ come about?” of The Creation Answers Book, creation.com/cab6.

References and notes

  1. Barthlott, W., Neinhuis, C., Purity of the sacred lotus, or escape from contamination in biological surfaces, Planta 202(1):1– 8, 1997. Return to text.
  2. The authors list problems such as “limited repellency to oils”, “fail under pressure”, “fail … under physical stress”, “cannot self-heal”, “are expensive to produce”, Ref. 7. Return to text.
  3. Bohn, H. F. and Federle, W., Insect aquaplaning: Nepenthes pitcher plants capture prey with the peristome, a fully wettable water-lubricated anisotropic surface, Proc. Natl. Acad. Sci. U.S.A. 101(39):14138–14143, 2004; ncbi.nlm.nih.gov. Return to text.
  4. Killer plant ‘eats’ great tit at Somerset nursery, bbc.co.uk, 5 August 2011. Return to text.
  5. Slippery slope: researchers take advice from carnivorous plant, seas.harvard.edu, accessed 28 August 2012. Return to text.
  6. Wong, T.-S. et al., Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity, Nature 477(7365):443–447, 22 September 2011. Return to text.
  7. Kim, P. et al., Liquid-Infused Nanostructured Surfaces with Extreme Anti-Ice and Anti-Frost Performance, ACSNANO | doi:dx.doi.org:10.1021/nn302310q, published online 10 June 2012. Return to text.
  8. SLIPS: Slippery Liquid-Infused Porous Surfaces, wyss.harvard.edu, accessed 21 August 2012. Return to text.
  9. Epstein, A. K. et al., Liquid-infused structured surfaces with exceptional anti-biofouling performance, Proc. Natl. Acad. Sci. | doi: dx.doi.org/10.1073/pnas.1201973109, published online 30 July 2012. Return to text.
  10. Bio-inspired coating resists liquids; phys.org, 21 September 2011. Return to text.
  11. Aizenberg says concerning biomimetics, the area of her research, “In the course of evolution, Nature has developed strategies that endow biological processes with exquisite selectivity and specificity, and produce superior materials and structures” seas.harvard.edu/directory/jaiz, accessed 4 April 2013. But another biomimetics expert does give proper credit—see Bell, P., Expert engineer eschews “evolutionary design”, Creation 32(1):35–37, 2010; creation.com/burgess. Return to text.
  12. Sarfati, J., By Design: Evidence for nature's Intelligent Designer—the God of the Bible, Creation Book Publishers, Powder Springs, GA, 2008. Return to text.

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Readers’ comments
Bryan R., Australia, 13 October 2016
This is a really fascinating article! Thank you so much for writing it.