Electrohydrodynamic/Electrospray: How to find the spray current
 

I have an electrospray system (interFoam + electric equation).

I have droplet production but want to find out a way to solving/post-process the electrical current from volumetric charge density (C/m^3).

My system is 2D axisymmetric model.

I can find the charge from charge density but I am stuck at current, charge and time relationship. Any guidance?

Thanks in advance.