It is well‐known that laundry waste water contributes to water pollution, and the need to reduce the amount of detergent used is widely recognized. Predominantly, research has focused on the washing effects of microbubbles and nanobubbles, and mechanical work was found to account for about 50 % of the washing effect on the cloth. In the present research, mixed nanobubble and microbubble water and four types of surfactants (including a commercial cleaning liquid) were investigated in an alternating flow system. The nanobubble water achieved a washing rate greater than that of ion‐exchanged water. However, the microbubble water had the same washing rate as ion‐exchanged water. Moreover, nanobubbles mixed with an aqueous solution of surfactant exhibited a washing rate that depended on the ionization of the surfactant: the mixture with nanobubbles and anionic surfactant exhibited a washing rate that was higher than that of aqueous anionic surfactant solution without nanobubbles. The surface tensions of nanobubble water and mixed nanobubble anionic surfactant were lower than those without nanobubble, respectively. Also, there was no advantage in mixed microbubble liquids. These results provide evidence of an enhanced washing effect by nanobubble mixtures in liquids.

Microfluid mechanics is one of the most exciting research areas in modern fluid mechanics and fluid engineering because of its many potential industrial and biological applications. In the present study, pressure drops ͑PDs͒ were measured for water, a 50/50 glycerol/water mixture, and a 0.1% aqueous solution of polyethylene oxide ͑PEO͒ 8000 flowing at high velocities through various sizes of micro-orifice. It was found that the measured PD of water and the glycerol/water mixture agrees with the prediction of the Navier-Stokes equation for orifices 100 and 400 m in diameter, but it is lower for orifices less than 50 m in diameter. In particular, the measured maximum PD was almost two orders of magnitude lower than the prediction for the 10 and 5 m diameter orifices. The glycerol/water mixture, possessing a viscosity ten times higher than water, provided nearly the same PDs as water when the reduction was generated. The solution of PEO produced a lower PD than water and the glycerol/water mixture except for the 400 m diameter orifice. Several factors, including orifice shape, deformation of orifice foil, wall slip, transition, cavitation, and elasticity were considered but the evidence suggests that the reduction in PD may be caused by wall slip or the elasticity induced in a flow of high elongational rate.

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