This sounds like a great problem! I may need some extra information about your problem to answer though. You can parameterize geometric properties in time. We don’t have any example of this but if you look in the wave equation example you can see how to do transient simulations. Then if you see the 3D parameterized 3 fin example you will see that you can parameterize the geometry with arbitrary values. Basically you will just use the t Symbol to parameterize the geometry. This can allow you to have time dependent geometric boundary conditions.
It could be that you are looking to two way couple the droplet size to the thermal simulation. If this is the case then it is not entirely obvious how to do this in SimNet yet. This would actually require changing the geometric boundary condition based on the current simulation solution. This may not be possible in SimNet yet.