Monday, October 31, 2005
Microwaves and Mini Bubbles
Have you ever stuck a CD into a microwave and hit "start"? The resulting glowing crackle disintegration of the aluminum oxide layer on the CD is very cool. However, such foolishness just scratches the surface of what can be done in one of these amazing devices. EMPTY microwave ovens can build up rather large voltages. As you know from high school or college physics, most substances (such as air) have a certain breakdown voltage, at which point the material is no longer able to act as an electrical insulator. The material becomes conductive (or rather, the conditions are such that the material is conductive). To explore the fascinating world of microwave-related feats, I wholeheartedly recommend this excellent website. Please follow this guy's safety instructions, and don't use a microwave you don't want to destroy. (Translation: use a microwave you want to destroy).
There are some valuable tips to learn from that site. The most important involves the phenomenon of the superheating of water. "Superheating" refers to a condition where a material is heated above its boiling temperature. This is a metastable condition, meaning that some minor stimuli is needed to allow the system to move from metastable to stable, accompanied by a release of energy. In the case of water in the microwave, what happens is this:
1) you put some water in a mug and into the microwave, planning to make some tea or cocoa. 2) you heat up the water reeeeeal good, but you don't see any boiling action. 3) you think "Oh well, it's been in there for five minutes, I bet it's hot enough." 4) you remove the cup from the microwave and toss in some sugar (or cocoa powder, or even a tea bag or spoon). 5) the water erupts forth in a furious boil, possibly scalding you and/or innocent bystanders.
What happened? The water was heated above 100 C (or the boiling point at the altitude of your stove depending on weather conditions and ion content of the water -- salty water boils hotter). It didn't boil because, for some reason, bubbles couldn't form. When you put the cocoa powder or sugar into the cup, a bunch of tiny bubbles formed. The water could then "boil" as liquid water turned into vapor across the bubble-water interface. This is all explained quite well on the website I recommended. Check it out.
There are some valuable tips to learn from that site. The most important involves the phenomenon of the superheating of water. "Superheating" refers to a condition where a material is heated above its boiling temperature. This is a metastable condition, meaning that some minor stimuli is needed to allow the system to move from metastable to stable, accompanied by a release of energy. In the case of water in the microwave, what happens is this:
1) you put some water in a mug and into the microwave, planning to make some tea or cocoa. 2) you heat up the water reeeeeal good, but you don't see any boiling action. 3) you think "Oh well, it's been in there for five minutes, I bet it's hot enough." 4) you remove the cup from the microwave and toss in some sugar (or cocoa powder, or even a tea bag or spoon). 5) the water erupts forth in a furious boil, possibly scalding you and/or innocent bystanders.
What happened? The water was heated above 100 C (or the boiling point at the altitude of your stove depending on weather conditions and ion content of the water -- salty water boils hotter). It didn't boil because, for some reason, bubbles couldn't form. When you put the cocoa powder or sugar into the cup, a bunch of tiny bubbles formed. The water could then "boil" as liquid water turned into vapor across the bubble-water interface. This is all explained quite well on the website I recommended. Check it out.
# posted by stratovolcano @ 5:45 PM 
