How would you determine the flow rate of a liquid that flows through a narrow microneedle channel and then through the opening of a funnel, i.e. pouring liquid through the other end of a funnel?

I've chosen arbitrary diameters but the pressures are from literature. The diameter of the lumen/tip is 50 $\mu$m and the base diameter is 1 mm. The pressure at the tip is 25 mmHg and the other end is open to the air.

I've tried to split the problem into two parts: first determine the flow rate through the straight part of the needle, and then determine the flow rate through the funnel. I calculated the flow rate in the first part using Poiseville's equation but I think that there must be another model for narrow channels or fluids with capillary action coupled with initial pressure. The Reynold's number is clearly in the turbulent range for the first part and I'm not sure if the numbers should be this high for narrow channels.

Is there a fluid model that's better suited for microneedles?

  • $\begingroup$ Are you assuming zero viscosity? BTW, have you seen the famous pitchblende experiment? en.wikipedia.org/wiki/Pitch_drop_experiment $\endgroup$ Feb 16, 2017 at 16:18
  • $\begingroup$ No, I'm using the viscosity of blood in native plasma: 3.2 e-3 Pa s. That's interesting, I'll look into it. $\endgroup$
    – AnkilP
    Feb 16, 2017 at 21:10
  • $\begingroup$ Could you provide a sketch for the problem? Also you should write down all the parameters that are available inculding geometry. $\endgroup$
    – MrYouMath
    May 5, 2017 at 22:00

1 Answer 1


Apply Bernoulli's equation across the two ends. You have pressure values at both ends. Elevation level is same. And substitute velocity(v)=flow rate (Q)/area of flow. Assume head loss to be negligible


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.