Science Buddies: "Ask an Expert"

We're doing this backwards and need a little help. We have the experiment (see above), but we really don't know what we are proving. Any comments/weblinks would be appreciated.

I haven't been able to find that experiment. Can you cut-and-paste the link to the procedure for that experiment? Also, please let us know about any changes you plan to make to the procedure, and let us know how far you have gotten with the project. Looking forward to hearing more from you.
Chris

Fair question. Here's who it works:

Ingredients: ultrasonic cleaner which transmits at 43 kHz, 12 oz. soda in plastic bottle

1) Fill the holding tank of the ultrasonic cleaner with water (about 60 oz.).
2) drill 1/8" hole in soda bottle's cap
3) place soda bottle upright in holding tank
4) plug in cleaner, turn on cleaner
5) watch soda instantly shoot approx 15' into the air for approx 5 seconds

I assume what we are seeing is the frequency of the ultrasonic cleaner matching the distance of the two oxygen atoms in CO2, thus causing the soda to rapidly expand. That's all I got, would appreciate your comments.

Sounds like a fun procedure.
I think you may be having trouble figuring out what you are proving, because, if you run that procedure once, it would be more of a demonstration than an experiment. To make it into a more rigorous experiment, you could vary one of the conditions of the procedure to test some hypothesis. For example, you could run the experiment with several bottles of soda at different temperatures to see how that might affect the total volume released, and/ or the height of the soda geyser.

txtxyeha,

Your hypothesis about the frequency of the ultrasonic cleaner being on the same order of magnitude as the size of the CO2 molecule can be checked be further experiment if you are interested. However, I don't believe that is right.

First, how fast is the speed of sound in water?

http://hyperphysics.phy-astr.gsu.edu/HB ... uspe2.html

1482 m/s for pure water, for sugar water (soda pop) the density of the liquid is higher, which makes the speed of sound slower, so let's use 1400 m/s as an easy number to calculate.

At 43kHz and 1400 m/s the wavelength will be 1400 m/s divided by 43,000 cycles/second = 32mm, which is about eight order of magnitudes larger than the size of the CO2 molecule.

So what is going on? The same thing that happens when one shakes up a bottle of soda, small bubbles are created that are nucleation sites for further CO2 release.

Good points. Here's some further information on the physics of ultrasonic degassing:
http://home.att.net/~Berliner-Ultrasoni ... l#degassng

Candice - thank you for taking the time to disprove my theory (seriously). I enjoyed your calculation and introducing me to the word "nucleation".

Chris - Thank you for the ultrasonic degassing web site. This helped in understanding the creation of nucleation sites.

Let me try to re-state what is going on:

Ultrasonic waves make cavitation bubbles in the soda. The energy released by the colapsing bubbles cause the CO2 go combine and in turn make their own bubbles. However, the CO2 bubbles have enough energy to rise to the surface (and beyond in this demonstration).

Not sure if I'm happy with that explanation, therefore your further comments are encourage.

txtxyeha,

You are welcome.

The way to think about this is that the CO2 is in solution in the water. Under pressure, the rate of solution is higher than under low pressure. There is always some molecules of CO2 leaving solution and those entering solution. But this mostly happens at the surface at the top of the bottle. Sometimes a crevice or crack in the bottle surface serves as a nucleation site for tiny bubbles to form. When one opens the bottle, one sees a series of bubbles form at these spots, grow large enough to have the displacement volume be large enough that it begins to rise to the surface, to be replaced by another tiny bubble that grows. The bubbles grow because the rate of disolution is higher than the rate of solution at the surface.

When you create a huge number of tiny bubbles, with the ultrasonic generator, the very large surface area that the tiny bubbles presents allows a large volume of CO2 to come out of solution in a stort period of time, as the bubles grow, the surface area increases as well, to the system "runs away" in an ever increasing surface area leading to ever increasing disolution of CO2, WHOOSH!!!

In addition to providing additional sites for nucleation, sonication increases the rate of coalescence of bubbles:

"Degassing in an ultrasonic field occurs when the rapid vibration of gas bubbles occasioned by the passage of acoustic waves from the radiating surface through the liquid causes adjacent bubbles to touch and coalesce."

This is important in degassing, because bubbles below a certain radius (the 'critical radius') will shrink and redissolve unless they combine with other bubbles to become larger than the critical radius. Once they are larger than the critical radius, then they will grow spontaneously.

This is a topic that could keep you busy for the rest of your life.