Your original image was saved in JPEG, which is quite lossy. You might want to post a higher quality image saved using PNG or BMP.
If the find circles routine is not good enough, there is another way to do it. All of the drops have a very distinctive feature - they have a black ring at the edge, surrounded by a white ring of background. You could generate a template for pattern matching that only searches for this feature, ignoring everything inside the drop. You would have to generate a template for each size of drop, but that could be done automatically since you are looking for high contrast circular objects.
Once you have your drops located, I would consider examining concentric rings within the drops. For a liquid drop, each concentric ring is about the same brightness with very little variation. For a frozen drop, there is quite a bit of variation. If there are noticable edges in a clean image (can't tell from JPEG image), concentric rake might pick up any edges that exist very easily.
If you are serious about hiring a professional to do this for you, I might be able to help you out. Send me an email (address is in profile) and we can talk.
Bruce
Bruce Ammons
Ammons Engineering
