As I had to realize during my first photo shoot that the internal “master flash” of the 7D is not very suitable for continous use, I have found a much better solution. I ordered some wireless flash triggers from Yongnuo and tested them already successfully, as you can see from the photos of the waterdrops.
The YN-622 wireless flahs trigger work fantastic and without any need to configure them. All who want to know more find this here and here. To photograph the waterdrops I used two 430 ex ii with only 1/32 of the maximum power. What I learned during this was that the flash duration depends on the power level and with only 1/32 of the maximum power the duration is as little as about 1/10.000 second! Therefore it is possible to freeze the motion of the waterdrops. With less flash power the duration would be even shorter.
Also outdoor the wireless flash triggers did already do a great job and I hope I can show you soon some first results of my second photo shoot. Meanwhile I hope you have fun with the macros of the waterdrops!