Thanks Rick.
Very well explained and a good solution.
The turbulent displace also gives a good result, probably not as good as the real drops, but it's ok.
My approach:
Step1: Create a shape layer with an elipse. That elipse should be bigger than comp size, so that it is totally filling the screen.
Step2: Add an extra elipse within the shape layer.
Step3: Add a Merge paths.
Step4: Animate each elipse so that the inner elipse is growing outside.
