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3D printing testing theory from 150 years ago

2024-04-18 16:04

In 1871, the famous British physicist Lord Kelvin (named after him for the unit K of absolute temperature) proposed an imagined strange shape, which he called an "isotropic spiral.".

This shape is similar to a sphere with many fin like protrusions on its surface, and is isotropic from any angle. Kelvin believed that if it were submerged in water and allowed to sink, the shape should rotate like a small propeller. It sinks while rotating, and the kinetic energy of rotation comes from a portion of the vertical motion.
 

Kelvin designed such a spiral to better understand his early theory of atomic eddy currents. This theory depicts atoms as tangled, stable vortices in the ether, rather than as physical entities, as modern atomic theory was not widely accepted in his time.

Kelvin handcrafted isotropic helices and observed their behavior in various liquids.
 

In order to make the spiral rotate, there is a certain requirement for the viscosity of the liquid, and both the size of vertical kinetic energy and the proportion of this kinetic energy converted into rotational kinetic energy need to be considered. Firstly, the viscosity should not be too high, otherwise the vertical direction will have too much resistance and too little kinetic energy, and rotation will also be impossible to achieve due to too much resistance. Secondly, viscosity cannot be too small, as the conversion of vertical kinetic energy into rotational kinetic energy relies on the viscosity of the liquid.

Kelvin first placed them in silicone oil and then tried many liquids, but did not observe the spiral spinning. But he attributed the failure to his limited technological level at the time, which prevented him from producing a perfect isotropic spiral.
 

Now, with advanced 3D printing technology, two American physicists want to give it a try again. They 3D printed 5 isotropic helices and then placed them in silicone oil. They, like Kelvin, did not observe these spirals spinning.

But they found that Kelvin was actually right - when they sank, there was indeed a force that pushed them to rotate. But this force is too weak to make them rotate in silicone oil.

Physicists plan to design isotropic helices of different shapes to increase the interaction between vertical motion and rotation, and see if they can ultimately rotate in a liquid.