I think the initial question was about droplets on a camera surface. Water droplets coalescing into larger droplets is more of a Poission points problem than a noise one. As two droplets come into contact they will generate a new droplet at the mean point between the two centers with a radius proportional to their two volumes. Making them run down the picture plane under gravitational accelleration means animating it's center and altering it's mass as it picks up surrounding droplets. Trails could probably be faked using a small heightfield that you decrement each frame.
The droplets themselves have a very specific shape depending on the surface tension and hydrophilic properties of the surface they land on, specified by the contact angle and the contact radius. These subtle curves around the contact edge of the droplet are important cues to interpreting what is being dropped onto what.
Trying to find a formula for the shape, ISTR it's a pretty regular analytic polynomial.
- Robin Green.
When it comes to morphing effects, your best bet is to start with noise.