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[Jeremy McCreary]

We've all seen clear hollow tops with watery colored liquids inside.



Wish I had one, but they're hard to find nowadays at any price -- maybe because the liquid tends to escape the plastic ones over time.

Now I'm scheming to make a Terminator top: A toroidal version with liquid metal inside -- specifically, nontoxic liquid gallium (Ga), my new pet element.

Schematically, the top sans Ga would look something like this, but with a suitable clear non-LEGO toroid and a lower CM...



Why Ga?
1. Totally non-toxic, with effectively zero vapor pressure below 1,000°C.
2. Melts at 30°C, between room and body temperature.
3. Once molten, stays that way at all reasonable operating temperatures.
4. Viscosity only twice that of water.
5. Density over 5 times that of water.
6. Surface tension over 6 times that of water, so much less prone to leak through small cracks.
7. Affordable: 20 ml (100 g) of 99.99% pure Ga for $60 on Amazon.

I think the combo (clear toroidal tank/flywheel + liquid metal look + Ga properties) would make for good top performance and some spectacular visuals -- especially when you start or stop it suddenly.

A rheoscopic fluid (as in the Vortex Dome) might be another interesting fill.

[Jeremy McCreary]

https://youtu.be/g_VYf8MdVSw

[Joah]

What an epic idea! It would be so cool seeing gallium move inside a top. Maybe the top coul have inner and outer tube. The inner would hold the metal. The outer could have disks of of insulation and an inlet for a heat gun to quickly melt the metal...It would look like your prototype just with rings of insulation every couple of inches

[Jeremy McCreary]

Good ideas! The beauty of Ga is that once you melt it, it supercools without refreezing until far below room temperature. So no need for insulation.

This wide gap between melting and freezing points, very unusual for a metal, comes from the solid's unique bonding stucture. Instead of the usual atom-to-atom metallic bonds, Ga atoms first pair up in strongly bonded covalent dimers (diatomic molecules). Then the dimers bond weakly with each other to build the crystalline lattice. Very cool.

Might also try molding Ga weights to add to other LEGO tops in need of more AMI.

You're our metal molding expert. Any suggestions on clays for making the molds? (Video above uses plasticine.) Other tips?

[Joah]

Good ideas! The beauty of Ga is that once you melt it, it supercools without refreezing until far below room temperature. So no need for insulation.

This wide gap between melting and freezing points, very unusual for a metal, comes from the solid's unique bonding stucture. Instead of the usual atom-to-atom metallic bonds, Ga atoms first pair up in strongly bonded covalent dimers (diatomic molecules). Then the dimers bond weakly with each other to build the crystalline lattice. Very cool.

Might also try molding Ga weights to add to other LEGO tops in need of more AMI.

You're our metal molding expert. Any suggestions on clays for making the molds? (Video above uses plasticine.) Other tips?

Wow, what a cool metal just got a chance to watch the video you posted! That's awesome, definitely reduces the complexity of the build if you don't have to use insulation. Thanks I use plasticine too because it doesn't contain sulfur. Sulfur and mold-making silicone don't mix well. Although you can make a master for a mold out of polymer clay as long as you cure it in the oven...The brand I use for plasticine is from Sargent Art...so far it's super hardy and you can reuse it many times. Here's a link to the pack I got off Amazon: https://www.amazon.com/Sargent-Art-Plastilina-Modeling-5-Pound/dp/B00FR7TTBM/ref=sr_1_10?dchild=1&keywords=modeling+clay&qid=1629584940&sr=8-10

Yes! Weights for LEGO tops sounds like a great idea!!! I always wanted LEGO to make a top set that was STEM-based and not Ninjago. To make molds for bricks you could use both Easy Mold Silicone...the stuff I use to cast tops... or even 100% Silicone Caulking...here's a great tutorial on how to use this stuff as mold silicone: https://www.youtube.com/watch?v=8ekC1x0Coi4

Can't wait to see what you make

[mailman]

I've seen videos of casting lead using kinetic sand, as sold in toy stores.  It worked well....

[Jeremy McCreary]

Thanks for the tips, Joah and mailman! Will look into all of them.

Thinking that my 1st Ga mold mght be a solid torus with LEGO-compatible spoke mounting tabs on the inside. The rest of the top will be LEGO. Something like this, but with better spoke-ring connections...

[ortwin]

@Jeremy: You should seriously reconsider making shapes with Gallium using molds:
"Liquid gallium cannot be kept in a glass or metal container because it expands 3.1 percent when it freezes"
That could be a problem.
Also I am not so sure that Gallium would stay liquid in a top at room temperature: It is not perfectly pure, it is in contact with surfaces that could act as crystalization seeds, it is moving, ....Have a look into the alloy Galinstan, maybe that is better suited for the liquid metal experiments.

[Jeremy McCreary]

Yes, Ga shares with water the very rare property of expanding on freezing. But Plan A is to fill the clear toroidal tank -- if I can find such a thing -- by less than 50% to give the liquid metal plenty of room to do its thing during play.

So any freezing between sessions shouldn't pose a problem. Question is, will I be allowed to use my wife's fancy blow dryer to remelt it as needed?

Galinstan, a commercial Ga-In-Sn alloy, has a lower melting point than pure Ga. So does EGaIn, the Ga-In alloy on the market. Plan to look into both WRT cost, availability, and safety.

But my hunch is that the 99.99% pure Ga usually sold will work just fine.

PS: Found a Galinstan knock-off for about 67% more than Ga on Amazon.

PPS: The plasticine molds will be open above, so no problem with expansion there, either. Will rely on Ga's very high surface tension to form rounded upper surfaces spontaneously.

[Jeremy McCreary]

The Physics Today article responsible for starting this liquid metal madness...

https://physicstoday.scitation.org/do/10.1063/pt.6.1.20210420a/full/

Free if you register.

[ortwin]

...

You're our metal molding expert. Any suggestions on clays for making the molds? (Video above uses plasticine.) Other tips?

I thought Iacopo is really a  mold professional!
Spinning tops are mostly a hobby for him. 
Maybe he wants to make contributions here?

[Jeremy McCreary]

Yes! Weights for LEGO tops sounds like a great idea!!! I always wanted LEGO to make a top set that was STEM-based and not Ninjago.

Ditto. Seems odd that they never have.

But all is not lost. You can make visually interesting, long-spinning tops out of those old weighted Spinjitzu turntables. And  they're cheap on BrickLink.

https://youtu.be/zBQ6oT241WI

https://youtu.be/52Sw7xaJqQA

The lower Spinjitzu "burger" top with 2 turntables stays up ~270 s with a single twirl and oner 300 s with a starter.

[Joah]

Yes! Weights for LEGO tops sounds like a great idea!!! I always wanted LEGO to make a top set that was STEM-based and not Ninjago.

Ditto. Seems odd that they never have.

But all is not lost. You can make visually interesting, long-spinning tops out of those old weighted Spinjitzu turntables. And  they're cheap on BrickLink.

https://youtu.be/zBQ6oT241WI

https://youtu.be/52Sw7xaJqQA

The lower Spinjitzu "burger" top with 2 turntables stays up ~270 s with a single twirl and oner 300 s with a starter.

I will have to check the turntable parts out. At first glance I couldn't even tell you used Ninjago for those tops! Another beautiful set of tops well done!

[Joah]

Thanks for the tips, Joah and mailman! Will look into all of them.

Thinking that my 1st Ga mold mght be a solid torus with LEGO-compatible spoke mounting tabs on the inside. The rest of the top will be LEGO. Something like this, but with better spoke-ring connections...

Very cool I like this design even with the metal ring it looks like it's completely made from LEGO.

[ortwin]

...
Might also try molding Ga weights to add to other LEGO tops in need of more AMI.
...

Did you ever consider using tungsten putty for that task? Search amazon for "tungsten putty", it has a density of about 10 g per cubic cm. You could squeeze it in all that air space in LEGO parts, usually used for fishing.
Edit: I meant the tungsten putty is used for fishing, not the air in lego parts!