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Whiter Whites
September 5, 2014
Some people prefer to see the world as
black and
white, but the
reality is that nothing is that certain.
Scientifically, pure black and pure white are
technologically useful, but they are difficult to obtain. I wrote about various aspects of white and black in some previous articles (
Thin and Black, August 2, 2013,
Paint it Black, February 13, 2013,
Very White and Very Black, November 23, 2011, and
White Roofs, March 19, 2012).
One measure of the whiteness of an object is its
diffuse reflectivity, known also as
albedo (from the Latin,
alba, "white"). Albedo is the
ratio of
reflected to
incident light, but it's defined at each
wavelength of light. What this means is that something that's black at
visible wavelengths might be white at other wavelengths. The purest white has an albedo of one, and the purest black has an albedo of zero.
Uncooked, polished, white jasmine rice from Thailand.
The characteristic aroma of white jasmine rice comes from 2-Acetyl-1-pyrroline (C6H9NO).
(Photograph by Takeaway, via Wikimedia Commons.)
The
natural world has a wide range of albedo. The expression, "
pure as the driven snow," is a
misnomer, since fresh
snow has an albedo of only 0.85. Fresh
asphalt, however, is fairly close to pure black, having an albedo of just 0.02. The albedo of the
Moon is about 0.135.
Two technologically useful white materials are
barium sulfate, which has an albedo of better than 97% at visible wavelengths, and
Teflon (PTFE), with an albedo better than 98%.
Coatings of these materials are used as diffuse reflectors on the inside surface of
integrating spheres for
photometric and
radiometric measurements.
Obtaining a good black poses a problem, since the surface of any
material with a
refractive index n different from
air (n=1) will necessarily reflect light according to the
reflectance equation,
where
n1 and
n2 are the refractive indices of air and your material (in any order).
Glass, for example, gives a reflectance
R of about 4% at
normal incidence, so a black glass is still just 96% black.
The solution to the reflectance problem is to have a textured
surface in which incident light is multiply scattered from low reflectance surfaces before it's returned. One black material that incorporates texture to produce a better black is
black gold, which is formed when
gold is
evaporated under an
argon or
nitrogen pressure of about 750
millitorr. Under these conditions, the gold
atoms cool and cluster before attaching to a
substrate, forming a black layer.[1-2] The process works for other
metals, as well.[3]
Before the discovery of
graphene,
carbon nanotubes were the darlings of
materials science, and these offer a means of building highly textured black surfaces.
Scientists from
Rensselaer Polytechnic Institute grew arrays of vertically aligned carbon nanotubes to create a super-black material that reflected just 99.955% of incident light at visible wavelengths.[4-5]
Japanese scientists created similar arrays in 2009 to produce a broadband black absorber operable from 0.2 - 200
μm.[6] Similar
research has been pursued by
NASA.[7]
As you look at the world around you, there are a few things, such as some
flower petals, that are colored white. One unusual example of a
natural white is the coloring of the
Cyphochilus beetle, sometimes called the "ghost beetle." The high albedo of this
genus of beetle arises from the texture of its
scales. The properties of these scales has been elucidated by an international team of scientists from the
Università di Firenze (Firenze, Italy),
Harvard University (Cambridge, Massachusetts), the
Massachusetts Institute of Technology (Cambridge, Massachusetts),
Exeter University (Exeter, UK), and
Cambridge University (Cambridge, UK).[8-9]
Cyphochilus beetle. (left image and right image by Lorenzo Cortese and Silvia Vignolini, licensed under the Creative Commons Attribution 4.0 International License.)
A white surface will arise from
random scattering centers of high refractive index in a thick layer.[8] These scattering centers must reflect all wavelengths of light equally, at least in the visible light spectrum.[9] The Cyphochilus beetles appear white because of an
anisotropic network of
chitin filaments that comprise their scales.[9]
These chitin filaments are extremely thin, too thin to reflect light very well on their own. In the beetle scales, however, their tight
packing allows an efficient scattering of light, and the anisotropy of the packing produces the white color.[8-9] The multiple scattering has the lowest
transport mean free path of any low-refractive-index system discovered to date.[8]
Scanning electron micrograph of the cross-section of the scales of the Cyphochilus beetle.
(Fig. 1b of ref. 8, licensed under the Creative Commons Attribution 4.0 International License.[8]
Says research team leader,
Silvia Vignolini, of the
Cavendish Laboratory,
"Current technology is not able to produce a coating as white as these beetles can in such a thin layer... In order to survive, these beetles need to optimize their optical response but this comes with the strong constraint of using as little material as possible in order to save energy and to keep the scales light enough in order to fly. Curiously, these beetles succeed in this task using chitin, which has a relatively low refractive index."[9]
A random collection of scattering centers by themselves wouldn't give rise to such an intense white as with the chitin material. It's the particular arrangement of the filaments that allows this, and this high-brightness white comes from a material with a low
mass per unit
area.[8-9] The
artificial production of such structures would have many applications, such as making whiter
paper,
plastics and
paints.[8] Says Vignolini,
"The lessons we are learning from these beetles is two-fold... On one hand, we now know how to look to improve scattering strength of a given structure by varying its geometry. On the other hand the use of strongly scattering materials, such as the particles commonly used for white paint, is not mandatory to achieve an ultra-white coating."[8]
The
research was funded by the
European Research Council and the
Biotechnology and Biological Sciences Research Council.[8]
References:
- John Lehman, Evangelos Theocharous, George Eppeldauer and Chris Pannell, "Gold-black coatings for freestanding pyroelectric detectors," Measurement Science and Technology, vol. 14, no. 7 (July, 2003).
- W. Becker, R. Fettig and W. Ruppel, "Optical and electrical properties of black gold layers in the far infrared," Infrared Physics & Technology, vol. 40, no. 6 (December, 1999), pp. 431-445.
- Chih-Ming Wang, Ying-Chung Chen, Maw-Shung Lee and Kun-Jer Chen, "Microstructure and Absorption Property of Silver-Black Coatings," Jap. J. Appl. Phys. vol. 39, Part 1, no. 2A, (February 15, 2000) pp. 551-554.
- Zu-Po Yang, Lijie Ci, James A. Bur, Shawn-Yu Lin and Pulickel M. Ajayan, "Experimental Observation of an Extremely Dark Material Made By a Low-Density Nanotube Array," Nano Lett., vol. 8, no. 2 (February, 2008), pp 446-451.
- Researchers Develop Darkest Man-Made Material, Inside Rensselaer, vol. 2, no. 2, January 31, 2008.
- Kohei Mizuno, Juntaro Ishii, Hideo Kishida, Yuhei Hayamizu, Satoshi Yasuda, Don N. Futaba, Motoo Yumura and Kenji Hata, "A black body absorber from vertically aligned single-walled carbon nanotubes," Proc. Natl. Acad. Sci., vol. 106, no. 15 (April 14, 2009), pp. 6044-6047.
- Lori Keesey and Ed Campion, "NASA Develops Super-Black Material That Absorbs Light Across Multiple Wavelength Bands," NASA Goddard Press Release No. 11-070, November 8, 2011.
- Matteo Burresi, Lorenzo Cortese, Lorenzo Pattelli, Mathias Kolle, Peter Vukusic, Diederik S. Wiersma, Ullrich Steiner, and Silvia Vignolini, "Bright-White Beetle Scales Optimise Multiple Scattering of Light," Scientific Reports, vol. 4, article no. 6075 (August 15, 2014), doi:10.1038/srep06075. This is an open access article with a pdf file, here.
- The beetle's white album, University of Cambridge Press Release, August 15, 2014.
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