Scale matters more than matter

by Salmaan Craig, 12 January 2012

All the constituent elements of the universe exist in this rich chemical cloud. It looks like a biological cell of a diameter measured in microns, but its not. This image was taken in 2009 by NASA. It’s the remnant of a supernova explosion that occurred in our galaxy in 1572, as recorded by the Danish Astronomer Tycho Brahe. It’s 55 light years in diameter—that's 5¹7 meters. Credits: X-ray: NASA/CXC/Rutgers/ K.Eriksen et al.; Optical: DSS

All the constituent elements of the universe exist in this rich chemical cloud. It looks like a biological cell of a diameter measured in microns, but its not. This image was taken in 2009 by NASA. It’s the remnant of a supernova explosion that occurred in our galaxy in 1572, as recorded by the Danish Astronomer Tycho Brahe. It’s 55 light years in diameter—that's 5¹7 meters. Credits: X-ray: NASA/CXC/Rutgers/ K.Eriksen et al.; Optical: DSS

For millennia we have sought to gain mastery of new substances. Each new disclosure by nature revealed powerful new material properties— new behaviour not exhibited in the material palette of the day. Yet a recent breakthrough forecasts a change of emphasis. We know now that the behaviour of materials depends foremost not on substance, but on scale.

The dependence is most evident when the scale is that of nanometers. With the advance of tools capable of manipulating matter at the atomic level, it is now possible to build things the way nature does it: atom by atom, molecule by molecule. We can now practice architecture on ultra diminutive landscapes, repeating the activity of design as we move up the ladder towards scales which are more familiar. But a warning: To brave this ascent, our designerly intuition needs reprogramming. Down there, things do not behave as they ought to.

This article is my attempt at sketching the history of materials up to this subversive juncture. The thread owes much to the work of Michael Ashby, and to one article in particular*. I emphasise the drive for better material properties, the will to mastery over an expanding palette of chemical elements, and the role of high temperatures in accessing these new plains of performance. I end by explaining what it is that nature materials do so well—lessons that people like Julian Vincent, Peter Fratzl and George Jeronimidis do a better job at explaining.

Stardust

The story begins in the stars. One hundred billion degrees: The highest temperature found in the universe. Ten billion years in the making, it lasts for only fifteen seconds. When the inner core of a large dying star pumps against gravity for the last time, it succumbs to collapse in on itself with viscous speed, rebounding as a blast wave of colossal force. The blast wave turns into the most powerful explosion in the universe when it collides with the outer core of the star. The explosion is called a supernova, and it releases all the elements that the star has created during its life and death, creating a rich chemical cloud that spreads out into space.

In this chemical nebula exist all the building blocks of the universe; all the constituent elements recorded adroitly in the periodic table. The 26 common elements—the light and abundant stuff that makes up 99% of the earth: hydrogen, helium, carbon, oxygen, and so on—are made during the life and death of normal sized stars such as our sun. But only in large stars are the temperatures necessary for the genesis of the heavier elements found. In a galaxy of 100 billion stars, the conditions ripe for a supernova will exist on average for only a minute in every century. This explains why, throughout the universe and back on earth, the heavy elements such as gold, silver, platinum, copper, zinc, uranium, tin and lead, are scarce.

It is the mastery and exploitation of this elemental star dust to which much of civilisation’s development can be attributed. The story of civilisation is often recounted with the dominant materials of the day as chapter headings—the ages of stone, copper, bronze, iron, steel, then—fleetingly—polymers and composites, before silicon and the dawn of the information age. In this story, as in that of distant stellar factories, the accession of temperature is a recurrent theme.

Metallic millennia

When copper first came into use seven millennia ago, the complex shapes that it could be cold beaten into must have thrilled. However, like the other heavier elements, copper is scarce in its native form. Not before pottery kilns were capable of maintaining temperatures in excess of 400°C could we access the more abundant supplies of copper locked away in certain ores. And besides: flaked flint and quartz continued to offer the hardest, sharpest and most durable cutting edges. Until, that is, someone mixed tin with copper in the kiln, to create the first alloy—bronze.

Bronze was all but rendered obsolete when ways of reducing ferrous oxides to make iron were discovered. Iron revolutionized warfare and agriculture, edifying tools and weapons with greater stiffness, strength and hardness. Yet the temperatures sustained in the kilns of the day—close to 1100°C—were not enough to melt iron, only separate it from oxides. It came out of the kiln a spongy mass, mixed with slag, which had to be hammered out hot, so the remaining bounty—soft, hot, but still solid—could be forged into shape.

The high temperatures necessary to cast iron were not reached in kilns until the Renaissance. Blast furnaces got better and in the early nineteenth century fiery hearths of 1600°C were responsible for melting the iron that was poured into the pieces that were assembled into the great bridges and railway terminals of the day. The culmination was the Bessamier process, which expelled more impurities from iron and maintained its molten state for longer for it to enter a new material phase. The result was steel, which handed iron the dominant role in structural design that it still holds today.

It's worth emphasising the role of fossil fuels in developments hitherto. Without access to fossilised stores of solar energy, extracted in the form of coals and hydrocarbons (crude oils and natural gases), we would not have been able to access the temperatures necessary to gain mastery over the heavy elements. Time is in a sense more interesting than energy here. Fossil fuels were formed through slow heat and pressure transformations of accumulated biomass typically lasting 100 million to 1 billion years. In contrast, preindustrial societies derived their energy from sources that were almost immediate transformations of solar radiation (this includes flowing water and wind) or that took relatively short periods of time to become available in a convenient form. Times ranged from a few months of photosynthetic conversion to produce food and feed crops to a few decades to accumulate phytomass in mature trees to be harvested for fuel wood or charcoal.

Beyond Heat, Beat & Treat

Back to the unravelling of the twentieth century—to that bottleneck in time where steel took us high, but couldn’t help us fly. The demands of an expanding aircraft industry brought about a shift of emphasis, leading to the development of light and stiff alloys based on aluminium, magnesium and titanium. Jet engines demanded materials that could withstand temperatures in excess of 1200°C. Again, the alloying of iron provided a solution, this time with nickel based materials.

Meanwhile, polymers—cheap and easily moulded into complex shapes—ushered in a new era for consumerism. Today their combined tonnage rivals that of steel. Though not stiff enough for demanding applications, polymers do have properties that begged to be combined with other materials in composite form. Glass fibres were incorporated into thermosetting polyesters or epoxies, giving them the stiffness and strength of aluminium alloys. Exceptional structural properties came with the development of aramid and carbon fibres and their use as reinforcements, particularly in the products of aerospace and the sports equipment industry.

The era we live in now might have been called the polymer and composites age had silicon not fuelled the information revolution. Silicon was known as an element during the industrial revolution, but a use for it wasn’t found until, in the middle of the twentieth century, it was found that it could coax electricity to switch from an alternating to a single direct flow. This spawned the fields of electronics, mechatronics and modern computer science, revolutionising information storage, access and transmission, imaging, sensing and actuation, numerical modelling, and much more.

Biology does it better

Civilisation’s mastery of a large range of constituent elements has enabled the invention of many materials with special properties not exploited in nature. These materials include copper, bronze and iron in pre-industrial times, to steel and concrete during the industrial revolution, and silicon in the information age. All these materials require high temperatures for fabrication; temperatures which demanded the burning of fossil fuels.

In the hunt to expand material-property space, the focus is now shifting away from the exploitation of new constituent elements. During the 1990’s it was realised that material behaviour depended on scale. New tools allowed matter to be manipulated and resolved down at the atomic level to reveal remarkable behaviour back up top.

While we took our time in figuring this out, biological evolution continued to do what it had done for 3 billion years: spout exquisite examples of the mastery of scale. Nature has evolved flexible and robust materials which exhibit a remarkable range of properties. It does this using only relatively few constituent elements, and without access to temperatures higher than those already present in the immediate environment. The key is complex, hierarchical structuring: nested architecture grown from the bottom-up. Engineering materials can seem over-designed and over-specialised in comparison.

Hierarchical structuring allows the construction of complex organs based on much smaller and often very similar building blocks. Examples include bone, trees, seashells, spider-silk, optical microstructures, super-hydrophobic (self-cleaning) surfaces and the attachment systems of geckos. The ‘fabrication’ of these exquisite examples is an approximate outcome of a process of self-assembly. The process is guided by instructions stored in the genes and constrained by a range of external factors, such as temperature, mechanical loading and the supply of water, light and nutrition.

This subtle ‘bottom-up’ growth is in stark contrast to blunt ‘top-down’ fabrication in engineering. Engineers will design a part then select a suitable material that will perform for as long as the part is in service. But because natural materials grow they can adapt to the changing conditions that engineers must pre-empt.

Designers will have to wait a long time before they can do architecture on the nano-level. But we don’t have to wait to begin applying the lessons of nature. We just need to know that a wealth of functional variety can be achieved by manipulating the shape, structure and combinations of standard materials. Instead of developing new materials each time we need a new function, or changing the amount or type of material when faced with a problem, we should be adapting and combining the materials we already have. Let’s take functional advantage of space and shape on length scales we have easier access to, and focus on designing hybrids of two or more materials. To journey all the way to the bottom, let’s start with architecture we can still see with the naked eye: It’s the time for new lattices, sandwiches, foams, and composites, both particulate and fibrous.

Salmaan Craig studied Industrial Design before undertaking an Engineering Doctorate hosted by Buro Happold and Brunel University. During this time he designed and tested a biologically inspired material for protecting buildings from solar and ambient heat, while allowing them to cool through atmospheric re-radiation into space. He then became a Façade Engineer at Buro Happold, developing his interest in the interaction of materials with heat for projects such as the Abu Dhabi Louvre. Now a member of the Specialist Modelling Group at Foster+Partners, he tries his best to record, grow and put into practice the disparate ideas for thermal material structures he occasionally wakes up with.