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Everyday science

Physics of cooking perfect pasta, plant-inspired actuator cracks bricks

15 Apr 2022 Hamish Johnston
Sticky spaghetti
Sticky stuff: pasta noodles are shown fixed at the top and hanging vertically after cooling. They stick near the bottom. The stick length, which can be measured by a ruler, is directly correlated to how much the pasta is cooked. (Courtesy: Jonghyun Ha, Jonghyun Hwang, and Sam Tawfick)

For some reason physicists love pasta – and I don’t mean eating it, the physics of pasta is something that seems to fascinates scientist. So much so, that in 2014 two physicists at the UK’s University of Warwick invented a new type of pasta they called anelloni, which has a large ring shape. You can find a link to a recipe for anelloni and learn why it was inspired by polymer physics here: “A taste for anelloni”.

Anelloni is a fresh pasta, so it is easy to cook – just 3–5 min in boiling water is all it takes. With dry pasta, however, it can be much more difficult to gauge when pasta is ready. But the days of crunchy or gloopy pasta could soon be over thanks to a team of mechanical engineers at University of Illinois at Urbana-Champaign.

Sameh Tawfick and colleagues studied how pasta swells, softens, and becomes sticky as it takes up water. They measured parameters such as expansion, bending rigidity, and water content and this allowed them to develop a theoretical model of the swelling dynamics of starchy materials. His team normally studies how deformable materials such as hairs interact with fluids, and they realized that pasta provided a good opportunity to further their knowledge. The fact that many of them were working at home during the pandemic also pointed them towards pasta.

Salt matters

One thing the team found is that the amount of salt added to the water had a significant impact on how long it took the pasta to cook. This could explain why some people struggle to produce perfect pasta despite following the instructions on the packet.

The team also made some important observations about how pasta changes as it cooks – observations that could be very useful for budding chefs.

To study how cooked noodles interact with each other, they observed what happens when pasta is lifted out of boiling water. They found that the liquid surface energy creates a meniscus that sticks noodles together. The is opposed by elastic resistance from bending the noodles, but aided by the adhesion energy from the surface tension of the liquid.

Stick test

Not surprisingly, the degree to which the pasta stuck together increased with cooking time. They conclude that by observing what percentage of two strands of spaghetti stick to each other, a chef could work out if their pasta is cooked to perfection (see figure).

The team also found that as spaghetti cooks its length expands 3.5 times more than its girth. This could explain why I always seem to not put enough water in the pan when I cook pasta. As a result, measuring the increase in length of a noodle as it is cooked would be another way of getting consistent results when making pasta.

You can find out more about their research in: “Swelling, softening, and elastocapillary adhesion of cooked pasta”, which is free to read in AIP Physics Of Fluids.

Super seedlings

We put a new patio in a few years ago and ever since we have been busy pulling out little plants that sprout up from the mortar between slabs. After a seed gets into a tiny gap in the mortar, the delicate-looking seedling manages to crack and move the much harder material so that the plant can grow.

Inspired by this annoying phenomenon, Hyeonuk Na and colleagues at Seoul National University have created a hydrogel actuator that can crack bricks. Plants can grow through mortar thanks to turgor pressure – which is osmosis-driven hydrostatic pressure confined within a plant cell’s walls. Na and colleagues wrapped a hydrogel in a stiff yet flexible semipermeable membrane. This membrane controlled and confined the osmotic swelling of the hydrogel. This resulted in an actuation force of about 730 N, which is 1000 times greater than that achieved by existing hydrogel actuators. This was enough force for the device to be used to break a brick.

You can read more about the actuator in Science.

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