Takatsune Narumi

Transient response of concentrated suspensions after shear reversal

We have examined the transient stress response under shear flow of concentrated suspensions of non-Brownian spheres. We focused on the experiment where the shearing is momentarily stopped and restarted in the opposite direction. We found that the normalized stress recovery curves for different values of the initial and subsequent shear rates could be collapsed quite well if plotted against the strain. This behavior agrees with the basic concept that the transient stress behavior is a function only of the imposed strain, as predicted by some recent constitutive models of concentrated suspensions. We also found that the transient behavior of the normal stress difference showed similar data collapse. Further, there appeared to be little qualitative difference in the behavior of particulate systems with a high degree of size monodispersity and those more polydisperse.

Huge reduction in pressure drop of water, glycerol/water mixture, and aqueous solution of polyethylene oxide in high speed flows through micro-orifices

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 (PDs) were measured for water, a 50/50 glycerol/water mixture, and a 0.1% aqueous solution of polyethylene oxide (PEO) 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 μm in diameter, but it is lower for orifices less than 50 μm in diameter. In particular, the measured maximum PD was almost two orders of magnitude lower than the prediction for the 10 and 5 μm 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 ...

Anomalous reduction in thrust/reaction of water jets issuing from microapertures

Thrusts and reactions of water jets issuing through small orifices and thin slots were measured and compared with the predictions from numerical solutions of the Navier-Stokes equations. Reasonable agreements were obtained between the experimental and numerical results for orifices and slots with openings of the order of 1mm size, but not with those of 10μm or less. The experimental results were found to be well below the predictions for apertures of the order of 10μm. The difference between the numerically calculated and the measured thrusts/reactions for the small apertures was found to be proportional to the square of the mean velocity. Several possible causes for the observed reduction in jet thrust/reaction in small apertures were examined, but none of them could adequately explain the flow anomaly.

Response of concentrated suspensions under large amplitude oscillatory shear flow

Concentrated suspensions of non-Brownian spheres dispersed in a Newtonian carrier liquid were placed under large amplitude oscillatory shear flow. It was found that the response wave forms consisted of a transient response after each reversal in the shearing direction, followed by purely viscous behavior. It was thought that rearrangements in the particulate microstructure could account for this transient response. Further, the characteristic strain for the microstructural rearrangement was found to be essentially independent of the oscillation frequency, and showed good agreement with the corresponding characteristic strain obtained from measurements of the transient response after shear reversal in continuous shear experiments. In addition, the fluidity in the oscillatory flow after the transient response was found to be higher than that in the steady flow case. This increase in fluidity was found to depend on the particle size dispersity, with the largest fluidity difference occurring with the monodisp...

Modeling and Measurement of the Dynamic Surface Tension of Surfactant Solutions

Surfactant solutions are usually used under conditions accompanied by transient dynamic surfaces, and therefore the dynamic surface tension (DST) is important in many industrial processes. Theories regarding DST have been developed exclusively on the adsorption theory that molecules are transported from bulk solution to the interface. However, the adsorption theory is not closed and requires another relationship between the interfacial concentration of the adsorbing molecules and the bulk concentration of molecules near and at the surface, which at present is based on assumptions. In addition, DST obtained by the adsorption theory contains several parameters that must be determined beforehand, and it is not simple to use for practical purposes. Here, we propose a new model based on the concept that surfactant molecules rotate during the process reaching the equilibrium surface state, which is different from the conventional adsorption theory, and we obtained a simple expression of DST as a function of the surface age. In addition, an experiment was carried out to determine DST by measuring the period and weight of droplets falling from a capillary. The expression by the proposed model was compared with the results of this experiment and with those reported previously by several other authors, and good agreement was obtained. Furthermore, the characteristic time in the model was shown to be correlated with the concentrations of solutions regardless of the type of solutions examined.

Experimental estimation of elongational stresses of dilute polymer solutions and a related examination of some constitutive equations

Abstract Elongational stresses of dilute polymer solutions have been estimated by utilizing the flow through small orifices under the condition of no vortex upstream of the orifice plane. The flow was approximated with a linearly converging flow towards an apex of a cone, its validity being partially confirmed by the measured center velocities, and the elongational stresses are determined from the measured thrusts of dilute polymer solutions. On the other hand, elongational stresses were theoretically obtained with the modified Maxwell model and the second order fluid. A comparison was made between the experimental and the theoretical results and the following points were clarified; below an elongational rate of 2 × 104 s−1 the modified Maxwell model gives elongational stresses close to the experimentally determined ones, but above that elongational rate it deviates from the experimental results. The second order fluid is not sufficient to describe the stresses in this kind of elongational flow and an acceleration term such as δ2eij/δt2 may be necessary in this case.

A simple expression for pressure drops of water and other low molecular liquids in the flow through micro-orifices

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 pr...

Effect of Microbubble Mixtures on the Washing Rate of Surfactant Solutions in a Swirling Flow and an Alternating Flow

Abstract Wastewater from laundry cleaning contributes to water pollution, and the amount of detergent used needs to be reduced. In the present study, water, four types of surfactants, and their microbubble mixtures were used, and washing rates were measured in swirling flows and alternating flows. The microbubble/water mixtures (average particle diameter: 25 μm; mixed with air at 1.5 vol % in water) achieved washing rates higher than those of water alone. Furthermore, microbubbles mixed with an aqueous surfactant solution had a washing rate that depended on the ionization of the surfactant: the mixtures with microbubbles and non-ionic and anionic surfactants had a washing rate that was higher than that of aqueous non-ionic and anionic surfactant solutions without microbubbles. The surface tensions of microbubble/water mixtures and mixtures of microbubbles with non-ionic and anionic surfactants were lower than those without microbubbles. These results provide evidence of an enhanced washing effect for microbubble mixtures in laundry cleaning.

Is the water flow more or less than that predicted by the Navier-Stokes equation in micro-orifices?

Micro-fluid mechanics is an important field in modern fluid mechanics. However, flows through microscale short tubes (micro-orifices) are not yet fully understood. Thus far, experiments on the flow through micro-orifices have been conducted by two methods: the pressure-given method (PGM), in which the pressure is given and the rate of flow is measured, and the flow-given method (FGM), in which the flow rate is given and the pressure is measured. According to conventional fluid mechanics, these two methods should give the same result; however, studies have found lower fluidity (lower flow rate) in PGM and higher fluidity (lower pressure drop) in FGM than that predicted by the Navier-Stokes equation, suggesting that the difference is caused by the method used. To clarify the cause of this difference, we examined the flow of ultra-pure water (UPW) with elapsed time by PGM. UPW was passed through Ni or Ti micro-orifices with 20-μm diameter at applied pressures of 50-1000 Pa. The difference in the shape and ma...

Solid‐like Properties of Liquid Crystal in Smectic Phase Controlled with Electric Field Applied

Electro‐rheological characteristics of a liquid crystal (8CB) in smectic‐A phase were investigated utilizing a parallel‐plate type rheometer under the stress control mode. Solid like behavior of the liquid crystal under DC or AC electric fields were mainly examined. Bingham‐like properties were observed and yield stress measured was affected with the electric field conditions. When the electric field strength was low, the yield stress was almost the same as that obtained under no electric field. Above a threshold of DC electric field, the yield stress increased and then became constant. An opposite tendency was observed under AC electric fields. In order to clarify the yield process in detail, the structure of the liquid crystal between two parallel plates was directly observed with a polarizing optical system. Under the DC electric fields, we found there were partially collapsed structures at the first stage of yield process and then largscale deformation was generated. We had also conducted a strain rec...