I don't know much about spray droplets, size, pressures, etc. But I'm sure someone does?
The curious thing I think I remember is the surface tension - is constant - except has a radius part of the function, so a smaller radius is a higher tension...? Then there is Surface energy: Never got my head around that... except I think there is more surface energy "per surface area" with a small radius than a larger one....
So those things (combined?) suggest to me that for smaller droplets, we need higher "pump energy = pressure - to get some shear conditions to create Aerosols instead of Droplets.
Then there is heat: A smaller droplet needs less heat to vaporise than a larger one, and has a higher ratio of surface area to mass, so smaller seems better than larger in fuel droplets, so the droplets vaporise quicker and the gas can then burn as more heat ionises it.
I guess that droplets are formed when the liquid is expelled from the nozzle/jet and the pressure gradient shears the jet of fluid? - I.E. It doesn't take any distance from the jet to convert the liquid at pressure at the jet into droplets without pressure but with velocity? - In which case, I can appreciate how the "spray" does not wash the piston, even though distances are very small.
There must be some science in it that I don't understand, so any advice is of interest. - I just "know nowt about owt" on this subject!
Thanks,
K2