Water flows through microscopic orifices have attracted much research interest recently because of their many potential industrial, biological, and medical applications, as well as their importance in the physics of fluids. In this study, by using circular nickel micro-orifices with diameters of 5-100 µm and applying pressures from 0.5 Pa to 0.6 MPa, the following anomalies were revealed. The velocity of water flowing through the 5-25 µm micro-orifices was 1.4-2.5 times higher than that through macro-orifices under low pressures and was close to the velocity of inviscid fluid under high pressures. The pressure divided by the dynamic pressure was frequently independent of the thickness of the orifice and close to that of the infinitely thin orifice under low pressures, whereas it was almost unity under high pressures. Notably, the flow rate often decreased gradually or steeply to negative values under applied pressures, whereupon either nickel or nickel and organic substances were found in and around the orifice. The velocity through the circular polyimide micro-orifice was substantially lower than that through macro-orifices, and a polyimide-like substance developed in the orifice. The effect of the organic substances found in the orifice on the flow properties was discussed.
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