The Perse School
 

The Science of Goo

Everyday science of soft matter

A new look at some household items

Professor Dame Athene Donald delivered the Michaelmas 2017 Community Lecture to a packed audience at the Upper on 4 October 2017.

What is ‘goo’? Professor Dame Athene Donald began her lecture by introducing us to these fascinating but familiar substances. In layman’s terms they are ‘soft, squidgy stuff’, technically ‘soft matter’, substances that change shape easily and on timescales we can perceive in our everyday life. We come across all kinds of ‘goo’ every day – shower gel, shaving foam, butter and cosmetics are all examples, while there is even ‘goo’ in our own bodies in the form of mucus.

An important feature of these substances is how runny, or how viscous, they are. The more viscous a substance is, the more difficult it is to spread. This is often essential to the function they perform in our lives. Most of us have probably never considered the problems we would face if our shampoo was so runny it slid straight off our head, or so thick it couldn’t be rubbed into our hair, but these are key considerations for manufacturers. Within our own bodies, the right viscosity of substances like mucus is crucial; in conditions such as cystic fibrosis mucus is too thick to be cleared easily from the lungs, leading to many health problems.

Viscosity is not a stable feature in many substances, and will often change under the application of stress. In a Newtonian fluid, like water, if you apply stress the viscosity will not change; when you stir your cup of tea, it does not get any thicker. However, this is not the case for many other familiar substances. Have you ever wondered why tomato ketchup seems impossible to get out of the bottle, and then rushes out all at once? This is because it is a Bingham Fluid. A little stress does not affect it, but once the level of stress reaches a certain point the rate of movement increases rapidly, and suddenly you have ketchup all over your chips. Shear thinning substances – ‘goo’ that becomes thinner and easier to spread when stress is applied – can be useful. For instance, non-drip paint starts off thick, so it sticks to the brush and does not drip when you apply it to the ceiling, but under stress it becomes spreadable so that you can move it around the surface you are painting. Shear thickening substances become more viscous when you apply stress, for instance custard or a white sauce will thicken when they are stirred.

Professor Donald then gave us a further insight into the viscosity of ‘goo’ by introducing us to polymers. These are long chain molecules, made of many repeats known as monomers. There can be many thousands of these repeats in a single chain, and if they are dissolved in a liquid the length of the polymer chain has a significant effect on its viscosity. If long polymer chains are dissolved at a high concentration, they are more constrained and more likely to become entangled, making the substance more viscous. This does not happen to the same extent with small molecules such as sugar or salt; a sugary cup of tea is not noticeably thicker than tea without sugar. Professor Donald helped us to visualise this with the analogy of two types of pasta. If you put a lot of spaghetti into a small pan, the pieces become entangled because they are long strands, whereas a short pasta like penne will not become tangled. This feature of polymers is important in the manufacturing of many familiar types of ‘goo’. For instance, it is not necessary to add many polymers to the water that forms most of your shampoo to create the right level of viscosity. However, the entangled polymers do not stop all flow completely. They are able to untangle themselves in a process which is called ‘reptation’ because it is reminiscent of the movement of a snake. It is not just polymers that do this. Professor Donald introduced us to the fascinating research of Stephen Chu, who demonstrated in 1994 that if you tie a knot in a strand of DNA, it will also disentangle itself through this process of reptation.

There are some cases in which the flow of polymers does stop, when chemical links are created between the polymer chains. This is what happens when liquid rubber is vulcanised to make the product that forms rubber bands. Spiders also use the arrangement of the molecules in their silk to change its viscosity, allowing them to create a range of silks for different purposes. When the protein fibroin molecules in the silk are lined up, they are less entangled so the silk is less viscous. However, the chains can crystallise to stabilise the solid silk, creating a substance that started off as ‘goo’ but ends up weight-for-weight stronger than steel. This technology has the potential to be adapted to a range of uses by humans, from clothing to surgical dressings.
Crystallisation also plays an important role in the way that different types of starch can be used as thickeners, one of the topics of Professor Donald’s own research. Any cook will be familiar with using flour or potato to thicken soups and sauces, and we were given a fascinating insight into the science behind this process. When starch granules are cooked in water, the granules themselves swell, and a substance called amylose leaks out of the granules. The swollen granules make the solution denser, while the amylose polymer chains become entangled, both of which increase its viscosity. The structure of the granule itself also plays a role. The granules contain semi-crystalline regions and amorphous regions. When the granules are cooked, water enters the amorphous regions first, which become swollen, while the semi-crystalline regions are affected and broken down last. Therefore, the internal structure of different types of starch molecule affects how they respond to the cooking process, and how they can be used as a thickener. Light scattering can be used to map this structure. If you shine a laser through evenly spaced holes in a solid object, if you make the holes closer together, the spaces between the points of light in the diffraction pattern created grow further apart. This principle can be used to detect the ‘gaps’ in the internal structure of starch molecules, which allows scientists to map the amorphous and semi-crystalline areas.

In one question from the audience, Professor Donald was asked about the ‘unknowns’ and future challenges in the study of soft matter. One obstacle is likely to be that many synthetic polymers are made from oil, so as this resource becomes scarce we may need to find alternatives. There is also the question of how we dispose of plastics, and whether other substances like starches could be used instead for packaging.

The science of ‘goo’ has far-reaching implications, from our understanding of everyday experiences to creating innovative technologies and treating diseases. Professor Donald gave us a taste of soft matter physics that was both accessible and thought-provoking, and no doubt many of us will never look at tomato ketchup or shampoo in quite the same way again.

This lecture forms part of our Community lecture series. Information about upcoming lectures can be found here.

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