Anomaly of excess pressure drops of the flow through very small orifices

Excess pressure drops are measured for the flow through very small orifices whose diameter ranges from the order of 1 mm to 10 μm using water, silicon oils, and solutions of glycerin in water: For larger orifices it is almost the same as, but for smaller orifices several times higher than, that of the numerical analysis of Newtonian fluid. Water gives the highest among the liquids used. Velocities are also measured along the centerline of a small orifice and found inconsistent with the result of the numerical analysis. A different mechanism may be dominant in the flow through very small orifices.