Do the cases state the length of the film?

Yep, they do. I like to think of my DVD as a collection of different works. Yes, the main film is 8 minutes long but there is also over 30 minutes of bonus content including additional underwater footage of marine life, extended time lapse sequences, the film's trailer and various other gems. So think of it as being like an anthology. 

Maybe pick better subjects?

There can be some subjectivity with regards to deciding which subjects are interesting and which are not. There's someone on another microstock photography forum who has some unusual views on what constitutes interesting subject matter. According to this guy:

Wildlife is not interesting
Boats are not interesting
Naval warships are not interesting
Plants and trees are not interesting
Mushrooms are not interesting
Planes are not interesting
Vehicles are not interesting
Waterfalls are not interesting
Road signs are not interesting
Construction is not interesting
Cities are not interesting (both daytime images and night / time exposures)
Monuments are not interesting
Beaches are not interesting
Food is not interesting

Gosh, in my 20+ years of photography, I honestly thought that those subjects had at least some appeal. I guess I was wrong.

Not an SLR as such but I have been happy using a Panasonic G6 Micro 4/3 camera for video. It offers full manual exposure control (including iso) during video mode. It records in HD video (later Panasonic models can record in 4k.) Another great thing about these mirrorless cameras is you can use heaps of old, manual film lenses with adaptors (35mm format, medium format etc.) I regularly use Canon FD lenses with my G6.

I have very small portfolios on the stock sites that I contribute to but in the beginning, things looked quite promising on Dreamstime (when the sales started.) At the time, I had a tiny, tiny port on Dreamstime and sales started tricking in. Meanwhile, iStockphoto was not doing well at all - I had just one sale there and it seemed pointless continuing to upload there. Then later things changed - iStock really took off - generating more and more sales. And Dreamstime's sales pretty much dried up. So now it's pretty much the opposite situation with both agencies.

Sales on Dreamstime were much better for me in 2016 compared to this year. DT in 2017 is just about dead. Though I still continue to upload to this site because I live in hope.

I did another google image search using different keywords and this time, I found some additional photos that are similar to my image. But not a whole lot more. Still not a lot of this sort of imagery on google. I also did a search on Shutterstock and Dreamstime and although they had a fair number of images that demonstrated surface tension, there was nothing like mine. Overall, there seems to be a bit of a shortage with these kinds of photos.

Yea does seem strange (a complete contrast to most stock sites submission procedures.) Though honestly, I wonder how many clients make use of the categories? Not many I gather but I could be wrong. I'm assuming that most clients are after something rather specific and for that, they'd use keywords rather than relying on categories.

Ive always had problems uploading to Fotolia. Only about half of my photos get submitted (actually slightly less than half.)

Sean, Ive just browsed your site. I like your portraits (especially from a lighting point of view.)

I've got quite a bit of medium-format film on SS - one of my best sellers among them - file number 68871745 shot on a Pentacon Six. Another was shot on a 1930s folding camera (B&W, that one, so no CA problems). So they will accept it.
The real issue with film is the quality of scanning. I spent a lot of time learning how to scan manually on an Epson V500. Automated scanning is no use at all, it simply doesn't deliver the best quality - even from one of the best specialised film processors in the UK (I've compared their scans with what I can achieve, and it's like a P&S vs a 6MP DSLR).

Ah nice! Mine was shot with a Koni Omega Rapid so likely 60s or 70s vintage. The scan is of Fuji Velvia 100 and it was accepted by SS and iStockphoto no problem. However, Dreamstime wouldn't accept it, stating that there was some technical problem with the file. It was scanned by a lab, not myself. Generally when I use medium format, I use slow speed and medium speed film. Though Ive also submitted some scans of 35mm film that happened to be 200asa and 400asa and they tend to be too grainy for the stock sites except for iStockphoto who accepted them.

maybe you could approach scientific journals etc? In the same way that if I were a less moral person if I had a pic of a celebrity in an embarrassing position I would probably go to the newspapers rather than putting it up for general sale ;-)

Ah I like that idea - the direct approach. Regarding scientific journals and magazines, I wonder if a submitted image by itself could lead to possible publication or whether it would need to be accompanied by a written article? I guess at the very least, I could write a body of text explaining the scientific principles that are demonstrated in the image (even though it's very basic science.) I guess journals would be after more higher level, complex scientific stuff (probably read by professionals etc.) Whereas a science magazine maybe targeted towards hobbyists and amateurs and might cover more general scientific themes (including basic science.) I wish my Dad was still around - he was a scientist. He might have known of some publications I could submit to. The next time I go to a newsagent, I'll look out for some science magazines.

you had to keep a note if the file had sold before, as this might restrict exclusive uses, which are extremely rarely asked for on Alamy (e.g. I've been asked twice to confirm that images hadn't sold before and even though I was able to assert that they hadn't sold anywhere, neither sale went ahead).

I'm considering maybe joining Alamy. There are two images that I currently have on microstock sites that I'm thinking of removing and submitting to Alamy. I'll probably leave the rest of my photos on micro. These two photos did not sell through microstock but they have each been sold in exhibitions with large physical prints being purchased. Also one of them was published in a magazine though not as stock (it won a photography competition.) Would any of this cause any issues?

you could well end up making a lot more than through a micro sale thanks to DACS...

Is that the British Journal of Photography? I'll have to look more into them.

how does 'other people having seen the image' dispute the possibility that 'it isn't really as interesting as you think it is'?

Interesting to who? Interest is subjective. The image is interesting to me as it is to the other people I have shown it to. Therefore it's likely that it will be interesting to other people too. Though it may not hold the same level of interest for everybody as we all have different tastes and preferences. Like I said, you can't please everyone. We're all individuals.

By the way, despite being an avid reader of photography books and magazines for a number of years, I haven�t come across too many images like mine. I do have two books which each feature a very similar kind of image � one of those being a very nice example. Even a google image search didn�t find too many of these kinds of images. And I couldn�t find any such image in the Science Photo Library (yes there were photographs in there that demonstrated surface tension but nothing like mine.) So what I have produced is certainly not common.

And regarding refraction, there is certainly nothing unique about how it's portrayed here but what i think makes it rather special, in my opinion, is that the refraction is temporary, or momentary. The refraction in this image only existed for a fraction of a second (resulting from a disturbance in the surface tension.) It was impossible to see this in real time and I just happened to trip the shutter at the right moment.