What Is Soft Matter?

Philip Ball on Tom McLeish

Soft Matter: A Very Short Introduction. Tom McLeish. Oxford University Press, 2020. pp. 176 $11.95 (paperback)
The British biologist Peter Medawar called science the art of the soluble. It’s an apt characterization, recognizing that the questions scientists ask are directly related to the tools at their disposal. If you have only a spade, you aren’t likely to set yourself the task of dissecting a fly or raising a mountain. And then with the commitment bias to which we are all prone, you might find yourself declaring that digging a hole is obviously the most urgent thing, and that questions involving flies or mountains are irrelevant. Alternatively, you might insist that dissecting a fly or raising a mountain are to a first approximation not so different from cutting into the earth with a spade and flinging the soil onto a pile.

The term “soft matter” was coined in 1970, and has become common currency in science only in the past two or three decades. Yet the substances to which it refers – such as honey, glue, flesh, soap, leather, starch, bitumen, milk, pastes and gels – have been familiar components of our material world since antiquity. The phrase, incidentally, was coined by French scientist Madeleine Veyssié, a collaborator of physics Nobel laureate Pierre-Gilles de Gennes, one of the foremost pioneers of the field. The French term, matière molle, is something of a double entendre, which would doubtless have appealed to the suave and charismatic de Gennes, well known for his almost stereotypically Gallic romantic liaisons.

Modern theories that describe the molecular-scale nature of gases, simple liquids and solids were all taking shape by the end of the nineteenth century. But the sticky, viscous, rubbery or bendy fabrics of soft matter were not properly tackled until researchers such as de Gennes and Cambridge physicist Sam Edwards gave them serious attention in the 1970s and 80s; for many centuries they had been all but excluded from the conventional trio of the states of matter.

This was not, needless to say, because these substances were unimportant – they are, among other things, vital to a full understanding of living matter – but because until that time science lacked the conceptual and experimental tools to make much headway. That’s the first of the salutary lessons in Tom McLeish’s excellent slim volume, a new addition to Oxford University Press’s immensely popular Very Short Introductions series: Don’t assume that science studies problems in the order of significance, or that it has long cracked the familiar and is now working only on the esoteric.

The second lesson is that with difficult scientific problems – and there are few more challenging than soft matter, even in notoriously knotty subjects like high-energy physics or immunology – it is not enough to be diligent, nor even especially brilliant. You must also be bold and imaginative. That was certainly true of Albert Einstein, whose mathematical solution to the problem of a particle moving through space in a series of hops in random directions led to a proof of the reality of atoms (until the early twentieth century they were still hypothetical), as well as laying the foundation for some aspects of soft matter.

Sometimes the challenge is to find a new way of looking at the problem: as another Nobel laureate, biologist Albert Szent-Gyorgy put it, to see what everyone else has seen and think what nobody has thought. There is no one better placed than McLeish to convey this need for open-minded and inventive thinking. While immersed in soft-matter research himself, McLeish has also written extensively and with deep knowledge of how science intersects and interacts with the arts, especially poetry and music, and with religion, and how human creativity of all kinds arises from common sources.

The third lesson is, for many scientists, the most profound and indeed the most beautiful: there is a universality to scientific theory that transcends the immediate subject matter or even disciplines. Both de Gennes and Edwards were able to solve some problems in soft matter by drawing on their knowledge of other topics, such as quantum mechanics and superconductivity (the ability of some metals and other solids to conduct electricity without any resistance). Find the right analogy – it is in truth more than that – and you can import the conceptual framework from one area of science to another. The reasons for this universality are not fully understood, although they seem clearly to be saying that deep and generic principles such as symmetry and topology govern the way the universe works, more than do the fine details.

Why has it taken so long for soft matter to become established as a field of research? The answer here suggests a general lesson for science too. A theory of gases was made easier by the fact that one can consider it as a mass of randomly moving particles that never “feel” one another and therefore act independently: the very randomness and disorder leads to simplifications. A theory of solids comes at the other extreme. All the particles are jammed up against one another: tightly correlated, more or less immobile and constrained to a perfectly ordered arrangement. Soft matter sits in between: lacking the rigidity and order of a crystal, but also the freedom and (almost total) disorder of a gas. There are then fewer obvious simplifications: it’s not clear what you can afford to ignore in the details of the molecular encounters. It’s hard not to resist a political metaphor: all extremisms offer easy solutions by factoring out the messiness and contingency of the real world, thereby limiting their applicability to a few ideal and unrealistic scenarios.

But as McLeish shows, the mess need not be intractable, for it’s almost as if the universe shows mercy in pointing us towards a few salient aspects. In many soft-matter systems, key length scales emerge: a size range in which significant structure appears. A theory can then be “coarse-grained” to focus on this scale and not worry too much what goes on beyond it. Typically these scales are in the so-called mesoscopic range of around one to a several hundred nanometers (billionths of a meter): bigger than atoms and molecules, but still too small to see. That’s the scale, say, of the shells and sheets that self-assemble in water from soap-like molecules, or the layers and helical arrangements that appear in liquid crystals, typically made from rod-shaped molecules that jostle one another into a shared alignment. It’s the scale of the blobs into which some polymer molecules fold up randomly in a solvent. It’s the scale of the fatty globules suspended in milk, and because it’s also the scale of the wavelengths of visible light, milk and other emulsions scatter light strongly and look cloudy or opaque.

And it is the scale at which life itself first manifests – in the structures and processes of living cells. McLeish has space only to touch briefly on this topic at the end of the book, where he describes research on “active matter” in which the particles have their own means of propulsion and activity. Cells and tissues are the most sophisticated forms of soft matter, which makes it all the more peculiar that we have for the past several decades insisted on seeing them as mere machines assembled and governed by information encoded in genes. McLeish’s book shows that soft matter has its own rules and principles, and that these are indeed universal: biology doesn’t magically suppress them. In fact, biology seems to do just what you’d expect natural selection to do: make use of what’s on offer. It has become increasingly clear in the past decade or so that vital processes in cells exploit the laws of soft matter, involving the spontaneous formation of mesoscale structures that represent part of the “logic of life,” which genes sometimes just fine-tune rather than dictate.

Soft Matter is not an introduction for the faint-hearted. It engages with its subject at a level better suited to an undergraduate course than a lay readership. That is not complaint; in fact, the book made me reflect on what a dearth of material there is, in between popular science and the weighty textbook, that offers students an overview of their subject at the right level to prepare them for the harder stuff to come. What nevertheless comes across is McLeish’s passion for his topic, his wonder at how nature creates rhymes and variations in the poetry of the physical world, and his joy in the mysteries and delights to be found in the apparently mundane.

Philip Ball is a writer, author, and broadcaster, and was formally an editor at Nature. His writing on scientific subjects has appeared in places from New Scientist to the New York Times. He is the author of more than twenty books, including Invisible, Curiosity, and The Water Kingdom. He lives in London.