You don't’ have to write a UDF. You should just create a report file before making your simulation.
Monitors --> Report Files --> Surface Report --> Create File Report (Report Type = Mass Flow (or other "Report Type" you want to follow) & choose the phase related to your droplets ).
But if you need an UDF, for whatever reason, some more information would be required - most importantly: which models do you use (single-phase vs. multi-phase) and is it a serial or parallel process?
Furthermore, do you need the concentrations on a surface, what Mounir assumed, or in a volume?