Sonoluminescence Cavitation



"One day, I was sonicating one of the most recent creations, a cryogenic sample holder needle after electrolyzing it in attempt to obtain a smooth, polished surface finish when I found some spherical depressions which appeared to have formed due to sonoluminescence bubble collapsing."

The cryogenic sample holder (0th generation coldfinger) uses a needle to calibrate the location of coldfinger and measure laser beam size. Since the needle itself was mechanically lathed, the surface roughness is quite high relative to the size of laser beam. Therefore this introduces significant errors towards our measurement data. The problem with 0th generation coldfinger needle is that the side of needle is "threaded" meaning that we cannot use the side surface of needle as sort of a guide to the tip when we graze laser beam on the surface. At this point, the only way to calibrate is to guess the beam location and just hope it to touch the tip by luck. 

It has been proposed that a new needle with smooth, unthreaded surface be made and that was when I took the job and had been working in the machine shop for several hours. The needle itself is made of non-magnetic stainless steel (304) and only about a quarter inch long. Nevertheless, the actual process of making it is just a brute force mechanical lathing/sanding. 

After few hours, the end collection has a 840 micrometer in diameter closer to tip and 860 micrometer closer to the base - therefore a little bit tapered - and the tip diameter measures around 30-40 micrometer. Not bad for mechanical sanding without a precision lathe. The only downside is the surface roughness is not very decent. Currently, my rough guess is about 2-5 micrometers which can easily contribute upto 20% in our beam size measurement. 

I hardly have any experience in electrolysis however I began to suspect the process of electrolyzing as a way to achieve this job since it may cause the surface atoms to ionize away creating relatively smooth surface. 

Using 8g of NaOH pallets mixed with 50ml of water, I dipped a bunch of stainless steel samples (not the actual needle since I had to test my hypothesis first before I light up the fire) and began electrolysis with 10V A/C current. Each of stainless sample had varying roughness which I created using different
grit sand papers.

When I took the samples out, I was clearly able to observe some visible reduction in surface roughness. However, it no longer exhibited the characteristic stainless steel shine implying the chemical composition of the surface somehow changed (which by the way is unacceptable for our cryogenic sample holder needle!). In fact, the surface color has become rather brownish-black. Next thing I did?

So I went to the other room where our awesome sonicators are located (they form (1,2,1) nodal structure in the water compartment almost always). Sonicating them for 5 minutes surely removed some of the "discoloring," but then it is when I began to suspect a permanent composition change. Maybe it reacted with the graphite rod I used or other impurities or should I have used a D/C current.

Now all that above was just a story for how I got the sample prepared. The above picture is from one of my stainless steel sample after sonication (click it to view 3D structure). One can clearly see bubble shaped depressions on the surface as if it was caused from sonoluminescence cavitation bubble? After all I was sonicating them for a while in distilled water. It sounds crazy but the shape of bubble depression suggested me otherwise back then. To be continued...

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