Glass has been the key ingredient in scientific discoveries, medical breakthroughs and engineering feats. Glass is not just the stuff of windows – but windows into other worlds. It has helped us discover the miniature world of cells under the microscope as well as faraway galaxies viewed through telescopes.
My windows at home used to be clean but these days they provide a forensic record of my children’s exploits in and around the home via fingerprints and face smears (thank goodness for safety glass).
A bane of my life is cleaning the windows, but thankfully one particular food ingredient sometimes known as E171 is on my side.
E171, a food colorant that creates the pearlescent shimmer in jelly beans, is actually titanium dioxide – an incredibly versatile material that, when applied as a coating to glass (by a professional, I might add, and not by my children), has some remarkable properties.
A very thin layer of titanium dioxide is clear, and this coating can be used to self-clean glass in three great ways. Firstly, the titanium dioxide coating is hydrophilic, meaning that it attracts water and thus helps wash away any dirt that may have found its way onto the glass.
This is great for outdoor conditions, where the rain can wash the windows. Pilkington’s Activ glass is one commercial version that has a nanocrystalline titanium-dioxide coating.
Titanium dioxide also cleans glass via its strong oxidising power – oxidisers are often the active cleaning agents in laundry detergents and stain removers. Titanium dioxide coatings also have photo-catalysis effects – that is, they help breakdown organic ‘dirt’ using ultraviolet light (from the sun).
This last property of titanium dioxide is an advertiser’s dream come true, as this photo-catalytic activity has other key features: it is antimicrobial, antibacterial, antiviral, antifungal, and anti-fouling; it can deodorise and disinfect, and it self-cleans.
But wait, there’s more – it can be used as an anti-fogging agent on glass, as its hydrophilicity causes water vapour to form a continuous flat sheet of water rather than condense as droplets.
It’s not only clean but can be green, too, as another interesting application of titanium dioxide is its ability to perform hydrolysis – the splitting of water into hydrogen and oxygen. Imagine a future, where when it rains not only do your windows clean themselves, but you also get free fuel for your hydrogen-powered vehicle. Now that would be a very smart piece of glass.
The term ‘smart glass’, though, is more typically associated with a material that can change opacity at the flick of a switch. A thin layer of liquid crystal sandwiched between two transparent electrodes is the smart bit – when the power is ‘off’, the liquid crystal is randomly oriented and the glass goes opaque; apply a voltage across the electrodes, and the liquid crystal reorients and the glass becomes transparent.
The notion of getting light to travel along a curved path using refraction was first pioneered in the mid 19th century, when Daniel Colladon and Jacques Babinet demonstrated it in Paris in the early 1840s; later, John Tyndall demonstrated this phenomenon in public lectures in London.
Refraction simply refers to the change in direction that light (or any other form of wave energy for that matter) travels in when it passes from one medium to another (say, from air to glass to water). Light travels at different speeds through different densities of materials – that’s why when you place a straight stick into a pool of water the stick appears bent; the refractive index of water is higher than that of air.
In Colladon’s experiment, reference was made to a “light fountain” and a “light pipe”, whereby light travelled through a stream of liquid. Optical fibres, as we’ve come to know them, were first developed in the 1950s, though experimentation with light travelling along glass tubes had occurred much earlier.
John Logie Baird, the television visionary, for instance, had conducted experiments in the 1920s where images were transmitted along glass tubes.
In optical fibres, light bounces off the internal walls of the clear fibre in a phenomenon known as total internal reflection. The clear core of the fibre is coated in another material with a different refractive index, so light is reflected back into the core rather than scattering out through the coating, thus allowing light to propagate along the fibre.
Because the fibres can be so thin they can flex and light can travel around bends.
This important feature of fibre optics has revolutionised surgery and medical procedures. In 1956, Basil Isaac Hirschowitz patented the first flexible fibre-optic gastroscope, which allowed doctors to look inside a patient’s stomach relatively non-invasively.
Since then many fibre-optic based ‘scopes’ (the arthroscope and endoscope, for example) have been developed to investigate different parts of the body, and fibre-optic devices are now the mainstay of non-invasive surgery.
Last year surgeons in Paris performed ultra-keyhole surgery on a brain tumour using a water-cooled fibre-optic laser that was guided to the tumour site by the surgeon. The patient, who was awake during the procedure and felt nothing (a local anaesthetic was used), was encouraged to talk to the surgical team to ensure that brain functions were not being harmed by the surgery.
Industrial endoscopes (known as borescopes or fibrescopes) are also used to examine and maintain the ‘health’ of other things in hard-to-get-to places – from jet engines to drains, fibre-optics allow us to view things that have previously eluded us.
Fibre-optic devices can be used to monitor the health of structures, machines and systems, as they can act as sensors measuring changes in temperature, strain and pressure in environments that preclude conventional sensors because of extreme heat (for instance, measuring the temperature inside a jet engine or a furnace) or electromagnetic radiation.
Fibre-optics are not just for viewing and shedding light on a subject. They are of course widely used for telecommunication purposes, allowing large volumes of data to be transmitted great distances very quickly. This has made possible the evolution of the World Wide Web and has changed the world as we know it.
On a less world-changing scale, but nonetheless a fascinating application of optical fibres, we now have translucent concrete. Developed by Hungarian architect Áron Losonczi, Litracon is a structural concrete embedded with fibre-optic glass strands that has the same strength and weight as conventional concrete.
But, because light diffuses through the concrete, the physical mass of the material looks much less than it is, and the material looks a lot thinner than it actually is. At first glance the material looks more like a thin curtain or blind, with shadows cast through from behind.
The illusionary quality of light-transmitting concrete is nothing compared to that of the metamaterials. These materials have the scientific world aflutter. We’re talking about invisible cloaking materials.
Harry Potter and Lord of the Rings fans are of course familiar with the concept of a material that makes you invisible. The reality is not quite so seamless, but the structure of these materials can bend and manipulate light.
All materials have a positive refractive index, but metamaterials have a negative refractive index, meaning that they can bend light backwards. The woven-like structure of these materials negatively refracts light waves because the fibres of the material are smaller than the wavelength of the light (between 400 and 700 nanometres).
This ability to bend and manipulate the path of light means that objects can potentially appear to disappear when covered by a metamaterial, as the light from behind the object is transmitted through the metamaterial to the front.
The problem is getting the light to re-align on the other side – the image is likely to be blurred. Some interesting work is taking place at the University of California Berkeley on the use of metamaterials in superior lenses for microscopes and telescopes.
So we come full circle – and see that whilst there have been some incredible developments in glass, the motivation driving its development is simply the desire to see the world more clearly.