We washed a soiled stainless-steel (SUS304) surface with attached starch paste as the soiled model because of investigating washing effect of microbubble mixtures generated by a swirling method (S-MB), a pressurized method (P-MB), and a pressurized method with a pressurized tank (NP-MB). Compared with the resultant washing rate of tap water alone, relatively high washing effect of microbubble mixtures was obtained. Furthermore, the washing effect of P-MB approximately agreed with those of S-MB. For understanding the experimental results, particle diameter change against elapsed time and static contact angle of interface between solid surface and droplet of test fluids was discussed. Particle diameters of all microbubble mixtures were changed as elapsed time. Elapsed time until collapse of P-MB higher than that of S-MB. Moreover, NP-MB exhibited highest elapsed time until collapse. Moreover, static contact angle of microbubble mixtures was lower than that of tap water alone. Correlation between washing effect and contact angle was obtained. The obtained knowledges were beneficial for many applications using fine bubble mixtures.

Owing to the many potential industrial and biological applications of microfluid mechanics, it has recently become an attractive research topic. However, researchers have mainly concentrated on microchannel flows and studies investigating micro-orifice flows are rare cases. In the present study, the results from experiments conducted on flows through micro-orifices with diameters of 100 μm, 50 μm, and 25 μm are presented. In these experiments, the thrust and diameter of observed outflow jets are measured. The resultant thrust and diameter of the jets for the 100 μm orifice flow agree with the numerical predictions obtained via the Navier–Stokes equations. Conversely, for an orifice with a diameter of 50 μm or less, it is found that the thrust is lower than that predicted and the existence of jet swell becomes apparent. With the estimated elastic stress proportional to squared mean velocity, a change in the elasticity of the water as it flows through a micro-orifice is strongly suggested.

Flows are generally divided into two types: shear flows and shear-free elongational (extensional) flows. Both are necessary for a thorough understanding of the flow properties of a fluid. Shear flows are easy to achieve in practice, for example, through Poiseuille or Couette flows. Shear-free elongational flows are experimentally hard to achieve, resulting in an incomplete understanding of the flow properties of fluids in micro-devices. Nevertheless, flows through micro-orifices are useful for probing the properties of elongational flows at high elongational rates; although these flows exhibit shear and elongation, the elongation is dominant and the shear is negligible in the central region of the flows. We previously reported an anomalous reduction in pressure drops in the flows of water, a 50/50 mixture of glycerol and water, and silicone oils through micro-orifices. In the present paper, we rearrange the data presented in the previous paper and reveal a simple relationship where the pressure drop is proportional to the velocity through the micro-orifices, independent of the orifice diameter and the viscosity of the liquids tested. We explain our observations by introducing a “fluid element” model, in which fluid elements are formed on entering the orifice. The model is based on the idea that low molecular liquids, including water, generate strong elongational stress, similar to a polymer solution, in the flow through micro-orifices.