Given recent press coverage, you might be forgiven for thinking that all physicists were interested in was the Higgs Boson. But our research fields extend much further than the ultra-small particles under study at Geneva in the Large Hadron Collider or, at the other extreme, the enormous distances of remote galaxies and stars studied by astrophysicists. Physicists study the behaviour and interactions of systems (including particles, stars and molecules) across the entire range of distances.Â My own particular field of research is concerned with materials familiar to us from the everyday scale of our world; materials that are soft, as opposed to things like metals or diamonds, such as foods, ointments and creams, paint and the cells within our bodies. This world of soft matter physics â" where the materials appear squidgy or gooey and can be readily squeezed, compressed or stretched by our own hands at speeds that are easily achievable â" holds many fascinations.
At the heart of many of these materials are a class of moleculesknown as polymers, long chain molecules that can consist of millions of atoms in total. A polythene bag, for instance, consists of polythene (or more accurately polyethylene) molecules which are simply thousands or millions of repeats of a so-called monomer motif consisting of one carbon and two hydrogen atoms, where the carbon atoms are strung together to make a very long molecule rather like a string of beads.
In proteins the repeating monomer is much more complex, consisting of one of the 20 naturally occurring amino acid residues, but these are similarly strung together to make long molecules but with very specific sequencing. In polythene, the overall shape of the individual molecule is simply a random coil, but the functionality of a protein depends on it folding into a very specific shape, typically approximately globular. If this folding goes wrong, perhaps due to a genetic mutation or as a consequence of ageing, then function is lost and disease may result. Such diseases are often associated with the protein molecules sticking together in inappropriate ways that damage normal processes within the body. This is thought to be what underpins many of the catastrophic diseases of old age such as Alzheimerâs and Parkinsonâs Diseases.
But as a physicist I can start to think about those proteins which have lost their native globular shape as simply being long chain molecules akin to synthetic ones such as polythene, where it is the stringiness that matters not the specific sequence of amino acid residues. Equally, one sort of unfolded protein now looks like any other one, where what matters is how neighbouring molecules come together to stick in clumps or crystals; ideas developed during the study of one, perhaps simpler, molecule can help to inform the study of another. Â This is the approach I have been using to look at similarities between protein aggregation occurring after artificially-induced unfolding using heat to âdenatureâ the milk protein beta-lactoglobulin (a procedure used when milk is processed to make cheese or yogurt) and what may be happening in our brains as we age; the former may be able to give us clues about the latter.
What about the âgooinessâ? Long chain molecules have other interesting properties when dissolved in a liquid such as water. Small molecules such as salt or sugar dissolve but donât then physically interact at normal dilutions: the consistency of water or tea is very little altered by adding a few spoonfuls of sugar.
However, long molecules such as starch (which is simply a long chain molecule of repeating sugars) or gelatin (which is a denatured protein) when dissolved â" typically heat may be required â" can get tangled up together, like strands of spaghetti or skeins of wool. The resultant network of tangled chains means that the flow of the fluid is significantly modified and the viscosity is very different â" the liquid has become gooey. This underpins the making of white sauce with cornstarch; the same effect is used to thicken water in shampoos or shower gels, although other polymers are then typically used. In the case of gelatin a further change happens, as will be familiar if youâve made a jelly mould. After heating the gelatin to dissolve it, first the liquid thickens as it is cooled but ultimately the jelly âsetsâ. This again is due to the protein molecules clumping together into organised regions, knitting the tangled chains together into a more permanent network which can support its own weight â" at least for a while.
Foods, shampoo and brain disease all come together in soft matter physics. Itâs an exciting and rather young branch of physics which touches all our lives and opens up intriguing questions. Just because the materials are visible all around us, doesnât mean the physics is mundane. Iâll be talking about some of these aspects of âgooâ and the underlying physics on my soapbox at Soapbox Science today on the South Bank.Â This event is a wonderful occasion for us to share the excitement we each feel about our very different types of science, but also a time to celebrate womenâs contributions. As long as itâs not too wet, Iâm sure weâll all have a great time exchanging ideas with anyone willing to stop by and listen and â" I hope â" inspiring some of the next generation.
Dame Professor Athene Donald is speaking at ZSL and LâOrÃ©al-UNESCOâs Soapbox Science onÂ Southbank, 16 July 2012 www.zsl.org/soapboxscienceTagged in: physics, science, soft matter, women