Fun With Gallium and Indium: Liquid Metal Alloy!
The term eutectic should not have any other meaning than
"fusible at a low melting point"
-The Engineering and Mining Journal (1903)
Gallium is a neat metal. It will melt in your hand! Magicians use it mold spoons, then they swirl the spoon in a glass of hot water and the spoon melts in front of the audience's eyes!
It takes a while of holding it to get it to melt in your hand. Once melted it stays a silvery metallic liquid for a while. It looks like liquid mercury.
If you bend a piece of gallium it makes a crackling sound. It "cries" when you bend it. Just like tin and indium do. It has to do with the crystalline structure of the metal "twinning" and breaking and then grating and vibrating against each other. If you keep bending you can break these metals-in which case you can melt them and let them re-solidify. Then you can bend them and make them cry all over again.
If you take some liquid gallium and put it onto a soda can, on the seam of the opening drinking hole, it will eat through the aluminum and pop open the can for you (after a while). Gallium embrittles steel and aluminum by forcing it's way (diffusing) into their grain lattice. A pretty cool way to open a pop can!
If you found a piece of gallium on a table you'd think it was a piece of lead or cooled solder. At least until it started melting in your hand. In certain applications gallium is a replacement for mercury in thermometers.
Indium is less interesting at first glance. It also "cries" if you bend it. It also looks like a piece of lead or cooled solder. To melt it you can use a blow torch and it'll turn to a liquid very easily, only to cool and solidify quickly. Okay, you'd think you'd have a huge blob of solder. Indium is used in some types of solder actually.
Sometimes indium is used to reduce the amount of mercury in dental fillings. Kind of boring huh?
Well, if you lightly rub a piece of gallium against a piece of indium THEY TURN INTO A LIQUID METAL ALLOY THAT STAYS A LIQUID! I tried it myself:
It was totally cool and really messy. I put the liquid into a glass container: mistake! This mixture "wets" glass, which means it coats it very thinly and gives it a tinted look. What good is that? Well it's how touch screens for tablets and phones are made and how newer solar panels are being produced!
A reason to care about relatively warm melting points? Liquid metal telescope mirrors! If you fill a container with a liquid and spin it, the liquid will have a low point in the center and raise up at the sides. This forms a parabloid shape that can be used as a telescope. It only has to spin at around 8 revolutions per minute-nice and easy.
[Our old nemesis Isaac Newton took credit for figuring this trick out. He had trouble spinning his container at a stable rate, so he didn't use it for telescopes.]
Of course a telescope mirror (even made of liquid metal) also has to be shiny. Mercury is used for this, but is toxic so gallium/indium liquid metal alloys are a great replacement. It's way cheaper to get a spinning vat of this liquid metal than to cast a HUGE glass mirror in a HUGE oven, grind it with tools to the curved shape, coat it with a reflective surface coating (usually aluminum) and then polish to 1/8 of a wavelength.
Of course you can tilt and swing a regular mirror telescope all around--can't really do that with a vat of liquid metal spinning around. You can look straight up with it really well though!
Looking straight up-like me! Meow.
The best place to have a telescope boringly aimed straight up would probably be at the equator. It's warm there too so the gallium indium mix would be fine and liquid.
A telescope like this at either pole might freeze...or would it? I guess that's were the "Melting Point Controversy" rears it's ugly head. I have no intent on freezing while staring at the North Star 24/7 so again, I don't care about that. Build that thing at the equator and it'll sit and spin as the stars curve by all night long. Just look carefully, because once something zings by you can't tilt a liquid telescope to follow it!
This new liquid alloy I created by rubbing gallium and indium together is referred to as a eutectic alloy. Many people mention "eutectic" liquid metal alloy in reference to any liquid metal, but eutectic refers to a mix of solid ingredients that melt at a single temperature together all at once, instead of at two different temperatures and the percentage of the elements mixed allows for the lowest melting point of those two materials possible**.
So, I wanted to find out if my blob of liquid alloy was actually also eutectic. Research was in order!
[If you liked the liquid metal blob and want to make your own, just hit Ebay or Amazon right now and forgot the oddness that follows below.]
So, I wanted to find out if my blob of liquid alloy was actually also eutectic. Research was in order!
**I APPARENTLY HAVE NO IDEA WHAT EUTECTIC MEANS...AND I'M NOT ALONE:
I thought I understood, but was amazed by the variety of definitions that have arisen in different fields of industry and science. So feel free to just breeze past the following:Of course if you're manufacturing thermometers or ultra-high tech sensors or touch screens with this material its actual melting point at your usage's pressure and temperature may be quite important.
A bucket filled with cheese and hunks of steel could be heated and cheese would melt at a different temperature than the steel. Even 5lbs of butter and 1 molecule of steel would result in the butter melting at a way, way lower temperature than the steel. At the melting point of steel the butter will be melted too; however it wouldn't be uniform: the butter would melt before the steel, not with it, and varying the proportions of butter vs steel wouldn't really affect the "alloy" of butter and steel. The same thing as it cooled: the butter and steel would harden at different times/temperatures. The cheese and steel would never fuse either.
A lot of non-technical dictionaries and encyclopedias say that "eutectic" just means the mixture with the lowest melting point possible. In that case a single molecule of steel and a bunch of butter would have the lowest possible melting point mixture of butter and steel...but that's true of pretty much anything, so I'm going to go out on a limb and say that ButtSteelium is NOT eutectic. It doesn't melt as a WHOLE entity-the butter melts and then later at a higher temperature the steel melts (even if it's just a single flimsy molecule of steel).
You don't get a eutectic mixture by throwing random things into a bucket and melting them together eventually. Although I've read were this is referred to as a "Simple Eutectic System" or "Simple Eutectic"...did they just mean a "binary alloy". Are they using the old meaning "easily fusible"? Would they consider ButtSteelium eutectic? Although simple eutectic refers solidifying points and to me at least, seems to mean differing points of solidifying for the components. I've read fruit juice freezing into a popsicle being called a simple eutectic: one liquid (fruit juice) freezing into two solids (ice and slushy water vs. juice). Seems the opposite of eutectic. Hmmm...I'll keep trying to understand...
The definitions I've found all seem to conflict with each other (solidifying at different times doesn't seem eutectic, or does it?). The confusion? Eutectic reaction is used to refer to easily fusing of ingredients that turn to a liquid. Eutectic point refers to that special, lowest temperature where everything liquefies as a whole.
Indium and gallium rubbed together are both a liquid at the same (room) temperature--which is all I was interested in. If you vary the percentages of indium and gallium you can make them liquefy (or solidfy) at the lowest temperature possible for indium and gallium.
For commercial eutectic mixtures of gallium and indium you usually add some tin (and sometimes some bismuth)! Even this is a subject of controversy. It's a liquid, and under the brand name "Galinstan®" it seems to have percentages of these (and more) elements and a much lower melting point than other gallium, indium and tin (stannum) eutectic alloys.
The company that owns Galinstan® (Geratherm Medical AG) markets it with a very low melting point-other people say generic versions made by other companies have a much higher melting point even though (it is thought) that they all have very similar percentages of ingredients.
A few wisely point out that eutectic mixtures of TWO metals are defined as having the lowest melting point possible when the mixture melts as a whole, which is achieved by varying the percentages of those TWO metals. Binary (two) ingredients have a "sharp" melting point.
Others mention a requirement that: the mixed alloy has a melting point lower than either of the ingredients by themselves. This is radically different than just having the percentage mixture of ingredients that melt together at a lowest temperature for that percentage mixture.
Welders and solderers use eutectic to mean: this alloy melts completely at an exact temperature. Non-eutectic alloys melt in a range of temperature as it heats up one ingredient melts faster and the alloy turns into a mushy paste as the ingredients are acting on their own, instead of as a whole, fused eutectic thing. For welders: eutectic means the ingredients all turn to a liquid at a single temperature. Non-eutectic means some of the ingredients start liquefying while others don't yet so you get mushy, paste-like, jelly glop called a slurry...which can be useful. When looking at eutectic alloys, welders look at how stuff melts, most scientists look at how stuff solidifies.
One can speak of the eutectic point of salt water. Add more and more salt and the water can get colder and colder before freezing. Many refer to this as "lowering the eutectic of water" which means that melting point of the water is the same as the salt dissolved into it? They'll solidify at the same time? Well, I guess when you're below the normal freezing point the salt water would turn to sludge (non-eutectic) and then freeze solid. Yeah, maybe?!?! They mean the freezing point is lowered. More on this in the second bullet point below.
"EUTECTIC" ACCORDING TO VARIOUS SOURCES:
- "Many mixtures of materials crystallize into two distinct materials when they solidify. First one component forms, then the other. A system of this sort is called a simple eutectic." Seems backwards no? One THEN the other? Does this contradict "as a non-eutectic mixture cools, each mixture's component will solidify at a distinct temperature, until all material is solid."
-
A mixture where both elements solidify at the same time/temperature (which is the 'V' point of a typical phase diagram from school). Even in school cooling was boring, it was the super lattice instant full meltdown mode that was neato! OK, that sounds like the eutectic point of saltwater again.
On top of these two definitions there are these that I've found in use. So, is a eutectic alloy:
These definitions encompass: eutectic, eutectic system, simple eutectic, eutectiod, eutectic temperature, eutectic point or points, eutectic transformations and/or solder specification.
Hmmm...for any definition I disagree with or that contradicts another I can find multiple "reliable" sources. These sources are college professors, technical data sheets, various shop manuals, manufacturer specs, sales specs and a whole lotta "I use eutectic item xyz everyday in my laboratory/welding shop/space station/etc. and it doesn't mean this but means that..."
I really don't care about all this, but I do find it interesting. For me in this instance: gallium and indium rubbed together becomes a liquid before your eyes. That is cool. That is fun. So yes, I have the luxury of enjoying liquid metal alloys without needing to know if they're 'truly' eutectic.
That was boring so I made a bunny out of cotton balls. It will be delicious! Meow.
