A drug delivery infusion pump is made out of thick synthetic rubber and has a very small outlet. The balloon pump is filled with 500mL liquid drug and is now inflated into a ball shape. The drug will exit from the very small outlet at a nearly (but not always) constant flow rate of 5mL/hr (takes 100hr to deplete). How can I derive the flow rate as a function of time? I'd also like to know how the temperature and drug viscosity affect the flow rate. Someone suggested me to use Poiseuille-Hagen equation, but I need to know more details, especially how I can relate the flow rate to time and establish a model. Your insight will be highly appreciated!