Inside Science

Lockdown scientists have been turning their homes into labs, says Tom Mullin

Whether it be losses in the production process, rejection at inspection stages, or the over-consumption of raw materials, it is a fact that the production of goods produces waste.

As a researcher at the Mathematical Institute in the University of Oxford, I spend much of my time carrying out experiments and research regarding the flow of fluids in various circumstances.

The lockdown in 2020 meant I no longer had access to the observatory, and so instead, I created experiments at home, aiming to determine how physics, and an understanding of the motion of soft materials, can help to reduce waste in production processes.

During the last 20 years, theory and experiment have progressed our understanding of how rigid particles join together and form the basis of a solid material. My plan was to use these ideas to explore how soft solids formed from highly elastic particles will yield and flow.

For example, whipped cream from a canister is a soft solid; if a small amount is sprayed on a table it will retain its shape, but when pushed, it will flow and spread. A heavy ball (a rigid body) will sink into a bowl of whipped cream. But will it sink in a well-defined way?

Are there predictions one could make from the known laws of motion about the speed at which the ball will descend? Do the accepted laws of fluid motion apply to materials which have properties which can be attributed to both fluids and solids, such as whipped cream?

In a bid to answer these questions, I studied the fall of a rigid cylinder through collections of highly elastic particles. The cylinder fell under gravity down the centre of a container containing a mixture of water beads and water. My main aim was to try to find repeatable behaviour from this complicated mix of materials.

I correctly hypothesised that when a small number of water beads were used, the cylinder would fall as it would in a simple fluid such as water, with some slowing of the motion by the beads.

However, increasing the number of beads in the solution slowed the descent of the cylinder significantly, because when there are a large number of them, the beads act like an elastic body, such as jelly.

The capability of predicting this type of motion can have a significant impact on the manufacturing industry, helping to inform ways in which we can reduce waste in the production process.

For example, in cosmetics, there is a necessity for gels, creams and toothpastes to stay stable and mixed in their containers before use. If there is too much fluid, they will separate out, too little and they will become hard and unusable. An understanding of the physical laws which govern materials will enable much better predictions of how to do this efficiently and thus reduce waste.

Covid may mean we are restricted in our experimental equipment, but when it comes to science, we can continue to further our knowledge of physics with the simple tools we have at home.

Professor Tom Mullin is a visitor at the University of Oxford, and a fellow of The RSE. The RSE is Scotland's National Academy, which brings the greatest minds together to contribute to the social, cultural and economic well-being of Scotland.