I am working on a \"fluid flow\" lab where we connected a horizontal capillary t

Question

I am working on a "fluid flow" lab where we connected a horizontal capillary to a beaker of water using a plastic tube. We measured the time it took for a fixed amount of water to pass through the capillary and the height difference between the top of the water and the end of the capillary as an approximation of relative change in pressure (delta-p). By calculating flow rate Q as delta-V/delta-t we proved the linear relationship between pressure and flow rate stated in Poiseuille's Law. Why must the capillary in this experiment be positioned horizontally?

Explanation / Answer

One of the conditions for the valid application of poiseuille equation is that the flow of fluid (liquid) must be streamlined.For this condition to be satisfied , the pressure difference across any section of capillary should be constant and the flow shouldn't be accelerated.These conditions are well met in horizontal position of capillary than in vertical position.In vertical position, the liquid will be in free falling mode and hence will be accelerated under gravity.This won't give satisfactory results.So horizontal position is most viable option for a successful experiment .

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