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Using Microwaves to Drill Through Glass
Linux_ho writes "UPI is reporting that Israeli researchers have developed a drill that can melt a small hole in glass, ceramics, or concrete with no dust or noise. Nature.com reports that it doesn't work very well with good heat conductors or materials with very high melting points, but the researchers envision a wide variety of manufacturing applications, and possibly some medical uses as well."
From the article: "The device is limited to a penetration depth of a quarter of the wavelength of the microwaves used -- in this case, about an inch. If the drill bit is any longer, the microwaves are no longer beamed forward but instead radiate in every direction like an antenna."
If I understand correctly the drill works as a highly directive antenna - beaming microwaves towards the material to be melted. The drill needs to be short to achieve good directivity.
Different frequenciens have different penetration depths - that is, how deep the electric field or radiation energy can penetrate into the material which is being "drilled". The penetration depth also depends on the conductivity of the material, so different materials can have very different penetration depths for the same frequency.
I think the depth of "a quarter of its wavelength" is just a very approximate rule given to journalists. It is more of a comparison rule: the penetration depth is comparable to a quarter of the wavelength. (Although I'm not sure why, the penetration depth is proportional to the square root of the wavelength, if I remember correctly.)
After they have reached the penetration depth, they need to move the antenna/drill forward. So of course they can drill deeper holes than that, but not at a time.
What kind of an "antenna" are they using? To achieve good directivity, they would need to use "traveling wave antennas" (or whatever they are called in english), I'd imagine. Does anybody know any details of this?
Common mis-understanding about microwaves. Cooking "from the inside out" is misleading. They only penetrate 3-4 cms (Might want to check exact). This is why cooking large items in a microwave is problematic.
You risk burning the outside of the food while the inside is still under cooked. For things like roasts, etc it is still better to use the slow and steady approach as the temperature has more of a chance to distribute evenly throughout the food, ensuring an even consistency. But I digress.
Your warning is a good one, but generally you do feel "warm" if you step in front of an active feed horn, before any damage is done.
You are very correct about the blindness though, looking directly into a hot horn can blind you before you knew what happened. Your eyes resonate right around the microwave range and absorbe them readily.
I've had enough abrasive sigs. Kittens are cute and fuzzy.