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Snakeskin Texturing

September 28, 2015

I was educated in the time when "wood shop" was still a part of an elementary school education, at least for the boys. In those days of full gender stereotyping, the girls were shuffled off to "home economics." I've done quite a lot of woodworking, having learned may tricks from my father, who was a carpenter. He learned from his father, who was a carpenter and cabinet-maker.

Saws, chisels, and files are good for rough-forming of wood, but the better pieces are finished with sandpaper. Sandpaper for wood working is typically made by gluing particles of garnet or aluminum oxide onto thick paper. The form of aluminum oxide is typically corundum, which is aluminum oxide with traces of some transition metals.

Sandpaper is graded by grit; that is, a number specific to the average size of the abrasive particles. Extra coarse sandpaper, for removal of paint and varnish from surfaces, has particles more than a half millimeter in size, while the particles in super fine sandpaper, used for a final polish, are more than an order of magnitude smaller. The following chart lists the particles sizes for various sandpaper grades.

  
Grade
 
Grit
Particle Size
(micrometer)
 Extra Coarse30632
 Medium60265
 Fine 100140
 Very Fine15092
 Extra Fine 32036
 Super Fine 40023

Sandpaper is a good example that the properties of materials depend on their structure as well as their composition. The atomically-smooth surfaces of polished wafers of garnet or sapphire, which are the same materials as sandpaper grit, won't abrade anything. They're the limiting case of sandpaper in which the particle size goes to zero, although it can be argued, philosophically, that the particle size becomes the wafer diameter.

Particle size of sandpaper as a function of grit numberParticle size of sandpaper as a function of grit number

It's close to an exponential function, but not exactly.

(Graphed using Gnumeric.)

Scientists at the Institute for Applied Materials of the Karlsruhe Institute of Technology (Karlsruhe, Germany) have used the idea that surface structure modifies material properties in a biomimetic approach to friction reduction for steel surfaces.[1-3] They laser-scribed textures inspired by snake scales onto the alloy surface, and they found that friction was reduced in the dry state by more than 40%.[1] Says Christian Greiner, principal author of the study, "If we'd managed just a 1% reduction in friction, our engineering colleagues would have been delighted; 40% really is a leap forward and everyone is very excited."[3]

Interestingly, the scale patterns caused an increase in friction when the surfaces were lubricated with mineral oil, sometimes by a factor of three.[1,3] Says Greiner, "This wasn't a huge surprise, since we were looking to nature for inspiration, and the species we mimicked – the royal python (Phyton regius) and a lizard called a sandfish skink (Scincus scincus) – live in very dry environments and don't secrete oils or other liquids onto their skin."[3]

Biomimetics has been applied successfully to several engineering problems, but bio-inspired surface texturing is a relatively new research field.[1] An efficient way to texture surfaces is by laser, and laser processing to create circular dimples produced an 80% friction reduction in unidirectional steel-on-steel sliding in the presence of a lubricant.[1]

The German researchers decided to try patterns that mimic the scales of snakes and certain lizards.[1] As shown in the following figures, they used two patterns, one with completely overlapping scales, and the other with scales arranged in individual rows.[1] The biomimicry was inspired by the ventral scales of the Phyton regius, and the Scincus scincus sandfish lizard. The skin of the sandfish is known for low friction and its resistance to wear against sand, and a replica of its surface has already been shown to have a high resistance to erosion by particles.[1]

Horizontally overlapping scale structureHorizontally overlapping scale structure.

(Karlsruhe Institute of Technology interferometer image by Christian Greiner.)

Vertically overlapping scale structureVertically overlapping scale structure.

(Karlsruhe Institute of Technology interferometer image by Christian Greiner.)

The experiments were done using an alloy steel of iron containing 1.5% chromium and 1.0% carbon). A 7.5 mm diameter area was polished to flatness using a colloidal silica suspension.[1] The surface texturing was done with a Q-switched ytterbium fiber laser. The pattern dimensions were determined by what could successfully be laser textured, resulting in a feature size of about 50 micrometers and a raised texture of about 5 micrometers.[1] Says Greiner,
"The distance between the rows in our experiment was the smallest possible distance we could produce with the laser. The structure, hence, does not entirely correspond to that of the sandfish skink."[2]

While the dimensions of the laser-generated pattern were of the order of 50 μm, snake scales have a feature size of about 300 - 600 nanometers, while the scales of a sandfish are 2 millimeter x 3 millimeter. The counter material for lubricated friction tests was an an alloy of the same composition. For the dry friction tests, however, this contact material experienced severe galling. For those tests, high hardness sapphire disks were used as the counter material.[1]

Snake skin shows an anisotropy in its friction depending on the direction of travel, with the sliding friction being smallest in the travel direction. The artificially produced textures demonstrated reduced friction in at least two direction.[3] With use of a mineral oil lubricant, large dimension patterns increased friction by a factor of 1.6, while small patterns increases friction by a factor of 3.[2] In the dry state, the large dimension patterns reduced friction by more than 40 percent, while small patterns reduced friction by 22 percent.[2] The friction difference between dry and lubricated surfaces was unexpected.[2]

A 3-D image of the scale-like structure.A 3-D image of the scale-like structure.

(Karlsruhe Institute of Technology image by Christian Greiner.)

These results may have application where contacting surfaces can't be lubricated, such as in vacuum systems.[1] The authors of the study suggest application in such devices as sensors in automobile anti-lock braking systems, and actuators in mechanical hard disk drives for computers.[3] There may even be an application in snake-shaped robots.[3] There are plans for a more detailed study of how the feature size affects the tribological performance.[1-2] This research was funded by the Deutsche Forschungsgemeinschaft.[1]

References:

  1. Christian Greiner and Michael Schäfer, "Bio-inspired scale-like surface textures and their tribological properties," Bioinspiration & Biomimetics, vol. 10, no. 4 (August, 2015), doi:10.1088/1748-3190/10/4/044001 This is an open access publication with a PDF file available here.
  2. Snake Scales Protect Steel against Friction, Karlsruhe Institute of Technology Press Release No. 089/2015, August 6, 2015.
  3. Friction reduction breakthrough is no snake oil, Institute of Physics Press Release, July 1, 2015.

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