Tomiichi Hasegawa

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.

Flows through contraction geometries (contraction flows) are found in many fields involving fluids, but they are poorly understood because they have two inseparable aspects as a whole: shear flow and elongational flow. In this paper, we focus on the center line of the contraction flow where the shear component vanishes because of the symmetry of the flow field and clarify elongational properties of liquids using the data obtained to date. First, we show images of contraction flows of water and dilute polymer solutions, and then, the continuity equation is used to obtain expressions for the center-line velocity. The expressions represent either the exponential contraction flow (ECF) or linear contraction flow (LCF) and agree well with experimental results obtained to date. It is suggested that the Deborah number plays an important role in deciding ECF or LCF. For dilute solutions of polyethylene oxide and polyacrylamide, the ratio of the elongational viscosity (μe) to the shear viscosity (μs) is found to be high, of the order of 104 and 103, respectively. On the other hand, molten polyethylene and 5.0-wt. % polyisobutylene in hydrocarbon tetradecane have greatly different values of μe and μs, but their values of μe/μs do not differ much from four. We propose a novel relaxation velocity number (RV) and find that the exponent ai of the ECF velocity-profile is correlated with log10RV for water, polymer solutions, and molten polyethylene. Furthermore, the expressions agree with experimental data for wet spinning. We propose a novel spinning model for the contraction flow of polymer solutions.

Because of investigating effect of washing time for ultra-fine bubble (UFB) mixtures in a laundry cleaning, we measured relationship between washing rate and washing time of water, surfactant solutions, and their UFB mixtures in an alternating flow. Compared with water alone and anionic surfactant solution alone, higher washing rate of a mixed UFB water and anionic surfactant solution was obtained. However, washing rate of nonionic and cationic surfactant solution agreed with that of their UFB mixtures. For discussing the experimental results, we measured dynamic surface tension. Furthermore, relationship between washing effect and wettability was discussed.

In our previous study, antimicrobial effect of ultra-fine bubble (UFB) mixtures for fresh vegetables was observed in an alternating flow. The Viable Bacterial Count (VBC) for UFB mixtures was less than those before treatment within the experimental errors. The effect on the VBC for UFB including hypochlorite was strongly expressed with alternative flows. Higher efficiency was thus suggested. However, effect of washing time was not discussed. In this situation, we investigated them, and the discussed optimum time on the washing effect for UFB mixtures.

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.