Anyone who studies chemistry will inevitably happen upon the sonicator. These little metal baths are a godsend for anyone with a sparingly soluble compound and a nightmare for anyone with a sense of hearing. However, these noisy puddles of water can lead to joint damage with incorrect use, a detail which is often not made clear to undergraduates.
For the uninitiated, when asked what a sonicator does the general response may be “it shakes, heats, and solvates things”. While not incorrect, the underlying workings are far sexier than this.
So, let’s step through what exactly a sonicator does. It applies sound waves of intense ultrasonic frequencies (typically >20 kHz) to a water bath and whatever happens to be submerged in it. For reference, dog whistles emit sound in the range of 23 to 54 kHz and middle C on a piano is a measly 261.6 Hz. Now, these ultra-sonic sound waves don’t directly interact with chemicals. Instead, the affects seen arise from what is known as “acoustic cavitation”.
When a liquid is subjected to rapid changes in pressure, here arising from an ultra-high frequency acoustic wave, vapour cavities form. These are tiny, tiny bubbles of vapour within the liquid, often referred to as voids.
These voids form in the liquid, but suddenly realise they’re surrounded by an awfully high pressure. As expected, these voids then collapse. Or, more accurately, implode. This is so intense that temperatures of 5000 oC and pressures of 500 atm can be reached. Yes, a sonicator produces regions of temperatures nearly as hot as the photosphere of our Sun. For interest, this is the same effect used by pistol shrimps to stun their prey (https://youtu.be/QXK2G2AzMTU?t=119). Nature is crazy.
So, this suddenly makes the effectiveness of a sonicator a bit clearer. It simultaneously stirs the sample, disrupts intermolecular interactions and provides energy for stubborn reactions to proceed. And that’s what’s happening in the innocent looking water bath on the side of your lab bench!
This also makes the dangers of sonication a bit clearer as well. If you happen to stick your finger inside a sonicator, anywhere inside your finger that there is liquid (FYI, there’s a lot of liquid inside your finger) the sonicator will cause acoustic cavitation to occur. Not only can this cause immediate damage and pain, but it can also cause long term joint damage which is generally regarded as a bad thing. This can happen in just seconds of sticking your finger in there. So next time you find yourself in the proximity of one of these beasts, just stick some tinfoil in there for a few seconds and see what happens to it. Bad things, that’s what.
Tim Chisholm
methan3ol 