Anomalous phenomena in several types of liquid flows through small orifices in a range of low Reynolds numbers

• Flow properties of water, glycerol, and complex liquids passing through small orifices were investigated.
• Flow properties of water and glycerol did not agree with the predictions.
• The measured pressure drops of surfactant solution were similar to those for water.
• The interface phenomenon between the orifice wall and the liquid is discussed.
• The thickness of the assumed solid-like layer is estimated.

Micro- and nano-scale fluid mechanics is one of the most intriguing and attractive fields in modern fluid mechanics and engineering because it can be applied in various physicochemical, industrial, and biological fields. In the present study, pressure drops were observed in a number of applications for several types of liquid flows through orifices of various sizes at low Reynolds numbers. In the case of water and an aqueous solution of glycerol, although the resultant pressure drops and Euler numbers agreed almost exactly with the values calculated by using the Navier–Stokes equations for an orifice with a diameter of 50 μm, the values increased as compared with the calculated values for orifices with a diameter of 100 μm and 200 μm. The values for the pressure drops of surfactant solutions were similar to those for water. The measured values for some dilute surfactant and polymer solutions appeared to be uncorrelated with the Reynolds number. Several contributing factors are discussed, such as elasticity, cavitation, electric effect, and micro-aperture effect. The formation of a solid-like layer at the solid–liquid interface around the orifice wall is inferred at slow flows and flows with low Reynolds numbers, in agreement with previous studies.