Why the soldier's black boot shines and the blackest synthetic material on earth

During the one year mandatory service to my nation, I was both fortunate and unfortunate to get an accommodation in a military barrack. I was lucky that the housing was rent-free, but "unfortunate" that I became subjected to a regimented lifestyle of the military. The order, rules, cleanliness, etc. were out of this world. I was particularly fascinated by the officer's boots. Here was the semi-arid part of the country where dust is on everything, and never on the officer's boots. It was black but always polished to the point that it shines and reflects light.

This phenomenon of shining boots may even make one wonder if black absorbs all colours, why do we experience reflection from a black surface? The index of refraction has a role to play here. It is constant, for different mediums, which expresses the speed of light waves as it moves from one substance to another. In other words, it merely checks the degree of refraction that light experiences as it moves or leaves a material (medium).

Index of refraction n = c/v
c= velocity of light in a vacuum
v= velocity of light in the medium

Some common index of refraction of some material is enumerated below:

Material	Refractive index (n)
Vacuum	1
Air	1.000277
Water	1.333
Flint Glass	1.57-1.75
Diamond	2.417

Looking at diamond's refractive index of 2.417, it means that light travels 2.417 times faster in a vacuum than it does in a diamond. The fast-medium have a smaller index of reflection than the slow medium. Hence light passing through a less dense medium to a denser medium bends towards the normal while the reverse occurs the other way round; if travelling from denser to less dense medium light turns away from the normal.

What has above got to do with black shiny reflective army boots? If a surface has a different refractive index from the air, some of the light that strikes the surface will reflect. The fraction of the light that will reflect is dependent on the angle the light is incident on the surface. The fraction of light reflected is a measure that the Fresnel equations can determine. The Fresnel equations or coefficients, proposed by Augustin-Jean Fresnel, describes the behaviour of electromagnetic wave's reflection and transmission at an interface.

The "shine" on a black object is merely a function of reflected light. The "transmitted" portion of the light merely is absorbed since we are dealing with a black object. A rough and smooth black surface respectively have a different reflective behaviour due to both possessing different refraction index.

The Blackest Man-Made Synthetic Object

In 2014, a small British tech company, Surrey Nanosystems Ltd, announced to the world that they had developed a world record holder in the category of the blackest material called vantablack. The name "vanta" is an acronym for Vertically Aligned NanoTube Arrays.

Though, Frederik De Wilde, a Belgian artist objects to Surrey Nanosystems claim of creating the blackest material with nanotechnology. A claim that Surrey Nanosystem refuted since their process is not the same and that theirs is darker than Frederik's material.

Moving on from their controversy, how did they achieve the world's blackest material? Vantablack is said to absorb 99.96% of any light that hits/strikes it perpendicularly at a wavelength of 663 nm. If it absorbs more 0.04% and all frequencies of the electromagnetic wave spectrum, it will become a blackbody, an ideal body that is capable of absorbing 100% of light incident on it. Vantablack holds a record light absorbing capacity.

How was that possible?

The answer lies in nanotechnology. Nano describes something tiny. To put it in perspective, a thing that is approximately 40,000 times smaller than the diameter of the human hair. The size is a minute one-billionth of a meter.

The Making

The vantablack is primarily not a colour, but rather a material. The making involves jam-packing "forests" of hollow carbon nanotubes. According to the website, a centimetre square surface contains approximately 1000 million nanotubes. These tiny microscopic tubes are "grown" using proprietary methods on top of a substrate in such a manner that any light incident on it gets absorbed with just a very tiny fraction escaping. The surface created is now devoid of any colour.

Therefore, unlike the black paints in the market, one cannot just walk into the neighbourhood hardware and home improvement store to buy a bucket or tin of vantablack.

The description of the making no doubt may give off the impression that a vantablack coated material will have a velvety feel to it, but that is not correct. Instead, we have a surface that is smooth.

To understand the scale of the nanotubes, we will use a real-world example of a wheat's height on a farm. The wheat in a farm usually stands about 3 or 4 feet high; if a carbon nanotube is a wheat in the farm, it will rise 1000 feet tall in nanoscale. This compaction and density of nanotubes make vantablack extremely good at absorbing most of the light but susceptible to damage. Hence application of it where there is no protection (outdoors exposed to the elements or where people physically touch it) is still a work in progress.

Applications

I guess you already know that the blackest material on earth created by different individuals and firms cannot be just for the fun of it. It must serve some purpose. Though initially created for the space industry to limit thermal cycling, exposure to high vacuum, mechanical vibrations in components in space, etc. Today, there are varied applications for the "darkest synthetic material on earth."

Since the material for making vantablack does not come cheap, high-end jewellers and automakers are looking at adorning the inside of a watch and making eye-popping super dark dashboard, interiors, etc. on luxury vehicles respectively. This added cost is something those with deep pockets can afford to pay for.

For the artist, due to the super light absorbing power, which can make a 3-D sculptor appear like a 2-D as seen in this incredible Youtube video while viewing the sculptor directly. It stands to reason that there are other mind-blowing "magic" that an artist can achieve with vantablack at the disposal. Presently, an artist, Anish Kapoor is working with vantablack.

Thank you for reading.

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^References

• ^{Fresnel Equations}

• ^Vantablack

• ^{Refractive index}

• ^Blackbody

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