I am using syringe pump with the flowrate set to 5ul/min. When I flow water which is in plastic syringe through a 1mm 0.5mm tube and collect the liquid for 10min. Then I see that ~50ul liquid is collected. But if I flow water using the same setting through a 6um glass capillary the liquid collected is only ~44ul. This can happen only if the flowrate is reduced incase of micro capillary. But why does this happen? Is there any theory to calculate the loss?
A flow rate dependent pressure applies to the system according to the Hagen-Poisseuille law:
V∝Δp R^4
with V volume flow rate, Δp pressure difference, and R the capillary radius.
Thus, you are increasing the (back)pressure massively by using a microcapillary, against which the syringe pump has to pump. If you are using a plastic syringe it may happen that it is (slightly) deformed due to the pressure, and thus a reduced flowrate might be observed. I assume a more rigid syringe (e.g. made from glass) would provide a better flowrate.
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