Over the past week I have had enough of the surface tension tests since I was getting nowhere. I redid the low surface tension (Ethylene Oxide) and the high surface tension (salt) tests to get some more resolute data. (I thought I might have just been either putting too much in at a time or something.) I found that when I reduced the surface tension this increased the amount of large bubbles being created and the amount that were sustained on the surface (surface bubbles for a long time). This meant that there were some semi-large bubble noise but some utterly huge signals when the large bubbles on the surface popped. Hence, not a good route to go down.
For the higher surface tension test with salt I found that the results were little better. What seems to happen here is that the production of many smaller bubbles is more apparent. Im assuming this is because the large bubbles that are created cannot withstand the pressure inside the water and break up into lots of small bubbles. This is quite good to watch, since the 1 large bubble that is created due to the drop seems to ‘blow up’ and scatter into many smaller bubbles; typically in a spherical arrangement. This also leads to quite a lot of small ‘foam’ on the surface. So the signals from the ‘exploding’ bubbles plus the signal from the drop hitting the foam is also pretty rubbish.
So I sat back and started thinking why oil works so well again. After a small conversation to Sergei we decided that the only other parameter to play with was viscosity. This reminded me of the idea that Kevin had that a change in viscosity may change the frequency of the escaping bubbles. So I thought I would give it a go; a double whammy if you will. Sergei pointed me to Glycerol, stating that it had good thickening properties, but it wasnt very good. Infact it was very similar to the ethylene oxide tried before. So I went back down my simplistic metal path and decided the easiest thing that I could get hold of to make things thicker was Jelly! At worst I would have a nice pudding for tea.
First attempt I didnt use enough jelly, but second attempt I got is spot on. (It is a bit annoying that I have to leave it overnight, but thats the price I pay!) With about a mixture of 3 parts water 1 part jelly-water (as mixed to the directions on the packet) I got a consistency very similar to oil. After a few data captures I knew I was onto a winner.
So, for once I think this almost categorically means that viscosity is the key when trying to minimise bubbles, not surface tension. A nice think liquid acts like a cushion not allowing a depression to form and also preventing the backlash of water that you get on the exit of the bubble.
The images below compare the oil results to the new jelly results. Note the similarity.
The jelly has 3 levels of zoom. The first is the normal 10 second data. The second is zooming in on the noise that you can see in the 10 second data. I think this is the actual oscillation of the liquid iteslf. I could quite easily test for this later. The third image is zoomed right in to the spike itself. The very quick oscillation you can see is reflections of the side walls. These tests are done in a 6×6″ bucket, and will be a lot less in real life. (So will the surface oscillation)
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