Re: Looking to boil water with a co2 laser

Hi Zachary,

You won't see a chemical explosion. What some folks may have seen is a very rapid transition from liquid to vapor, which can occur so quickly as to appear like an explosion (and can be dangerous).

Water is a very tightly bound molecule. It therefore takes energy to break it up, which is exactly the opposite of an explosion. If you were to use electrolysis, you could break up the water into hydrogen and oxygen, and those could explosively recombine to form water, but a CO₂ laser won't cause that to happen.

A CO₂ laser emits light at a wavelength of 10.6 microns. Water molecules absorb quite strongly at many infrared wavelengths. The region around 10.6 microns, however, isn't one of the strong absorption peaks. Still, water absorbs light of that wavelength strongly enough for CO₂ lasers to be used in medical applications (humans are mostly water). Most of the laser energy is absorbed within a depth of 20-30 microns (you can find this discussed on emedicine.com).

That strong absorption has me wondering how people could see anything resembling an explosion. You may have heard of "exploding water" in microwave ovens. Water is a much more poor absorber at the wavelengths used by microwave ovens (that's important, in order for the food to be cooked uniformly). What has happened is that people have heated cups of water in microwaves, and then taken them out and added something like instant coffee. The coffee reduces the boiling point of the uniformly heated water, which then boils all at once, scattering steam and scalding water over a wide area. Not an explosion in the chemical sense, but a dangerous situation.

Because a CO₂ laser is absorbed more efficiently by water than microwaves, I believe that you would see a situation similar to that of a medical laser. Namely the surface would boil away (quite rapidly), while the water even a short distance below would not be directly heated. The surface boiling would be intense, however, and could very well splash heated water over a large distance. I would therefore strongly recommend staying a safe distance away, and placing shielding between yourself and the water.

This could possibly be used to convert water into steam for use in a turbine, but there would be some logistics issues. The water needs to be contained within a pressure vessel (the turbine operates because of high pressure steam released from boiling water). Using a laser to heat the water would then require a window into the pressure vessel that was transparent to the wavelength of the laser.

Assuming that these technical issues can be overcome (I wouldn't foresee any real issues there), the question would remain about the value of the process. The laser requires electricity, which could be generated by the turbine. However, the laws of thermodynamics dictate that you would get out less electricity than the laser required (you can't get something for nothing). If you are using the turbine for some other purpose, such as pressure generation, then it would be easier to simply drive it directly (as in a turbocharger).