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Grease is used as a lubricant in a wide range of fields, including bearings, because it reduces friction, prevents harmful wear of components, protects against rust and corrosion, and acts as a seal to prevent the invasion of dirt and water. Although most of the research on grease has focused on the environment inside the bearing, there has been little research on the fundamental lubrication mechanism of grease. It is known that thickeners, which keep a complex three-dimensional structure in the grease, have a significant effect on the shear characteristics of grease, and it is assumed that this is due to the orientation of the thickener structure in the shear direction. In this study, the apparent viscosity of grease in a micro-order gap was measured using our original viscometer and compared with the apparent viscosity measured with a submillimeter-order gap rheometer because grease may show different rheological properties compared to conventional measurements. In addition, the dynamic response of viscous resistance that appeared when each grease was subjected to a change in the shear force was quantitatively evaluated using relaxation time. As a result, the apparent viscosity remarkably decreased in a micro-order gap compared to a submillimeter gap, and two types of shear thinning mechanisms were proposed based on the orientation of the thickener: one caused by the narrow gap and the other by the shear force. In addition, the behavior of viscous resistance due to changes in the shear force depended on the type of thickener. It was also confirmed that the relaxation time of each grease correlates with its oil film-forming ability and the entanglement level of the thickener’s structure. Furthermore, the mechanism of the dynamic response was proposed based on the reorientation of thickeners.

We developed a technique which can increase the yield of one-to-one particle encapsulation by applying the dielectrophoretic particle alignment technique using boxcar-type electrodes. Dielectrophoretic force generated by the boxcar-type electrodes accelerate and decelerate the particles periodically as they flow in the electrode region. Further, the dielectrophoretic force is turned on and off at constant frequency. The force exerted on the particle periodically over space and time can align them in the streamwise direction with even interval. In this study, the boxcar-type electrodes were installed in the microchannel in the region upstream of the flow-focusing channel in which the water-in-oil droplets were generated. By adjusting the on-off period of the applied voltage generating the dielectrophoretic force to the period of the droplet generation, each particle could be separately encapsulated in the droplets. The principle of particle alignment using periodic force was first described based on a one-dimensional model. The flow structure and the characteristics of the droplet generation in the flow-focusing channel was then discussed in relation to the surface tension of the fluids and the wettability of the wall. We measured the velocity distribution of the particles flowing in the boxcar-type electrode region to evaluate the effects of the droplet generation on the motion of the particles and the alignment performance. The results showed that the particle could be aligned in the fluctuating flow caused by the droplet generation, and each particle can be encapsulated in different droplets. This was further demonstrated by measuring the probability function of the droplets containing specific number of particles, which showed that 100% yield of one-to-one particle encapsulation can be achieved under the investigated condition of particle number density of 0.4. Moreover, the throughput increased 46% compared to the case of having the particles supplied randomly.

We developed a technique which can increase the yield of one-to-one particle encapsulation by applying the dielectrophoretic particle alignment technique using boxcar-type electrodes. Dielectrophoretic force generated by the boxcar-type electrodes accelerate and decelerate the particles periodically as they flow in the electrode region. Further, the dielectrophoretic force is turned on and off at constant frequency. The force exerted on the particle periodically over space and time can align them in the streamwise direction with even interval. In this study, the boxcar-type electrodes were installed in the microchannel in the region upstream of the flow-focusing channel in which the water-in-oil droplets were generated. By adjusting the on-off period of the applied voltage generating the dielectrophoretic force to the period of the droplet generation, each particle could be separately encapsulated in the droplets. The principle of particle alignment using periodic force was first described based on a one-dimensional model. The flow structure and the characteristics of the droplet generation in the flow-focusing channel was then discussed in relation to the surface tension of the fluids and the wettability of the wall. We measured the velocity distribution of the particles flowing in the boxcar electrode region to evaluate the effects of the droplet generation on the motion of the particles and the alignment performance. The results showed that the particle could be aligned in the fluctuating flow caused by the droplet generation, and each particle can be encapsulated in different droplets. This was further demonstrated by measuring the probability function of the droplets containing specific number of particles, which showed that 100% yield of one-to-one particle encapsulation can be achieved under the investigated condition of particle number density of 0.4. Moreover, the throughput increased 46% compared to the case of having the particles supplied randomly.

We have developed a technique which can control the particle interval (spacing), velocity and timing in microchannel flow by exerting dielectrophoretic (DEP) force on the particles periodically over time and space using the boxcar electrode. Controlling the interval of the particles to align with even space in the microchannel can improve the performance of the sensor, sorter, and encapsulation system for particles and cells in the microfluidic devices. One of the concerns about this technique is whether we can align the particles when several particles are located in the same periodic area. In the present study, we conducted numerical simulations and measurements for the motion of 12μm polystyrene particles flowing in the boxcar electrode region to evaluate how the particles separate when they are located in the same periodic area, and understand their motion characteristics during the separation process. The result showed that the two particles approach each other, change their position, and separate due to the DEP forces, flow velocity distribution, and collision between the particles. The particle located in the downstream side then moves to the upstream periodic region in a repetitive manner until it fits in a vacant region. We measured the frequency distribution of the number of particles in the periodic area at the inlet and outlet of the boxcar electrode and observed only 0 and 1 particle in each periodic area at the outlet. This represents that particles entering the boxcar electrode region with random interval separate and align with even interval by the time they reach the outlet.

A fluid temperature measurement technique based on fluorescence polarization is developed and applied to measure the two-dimensional temperature distributions in microchannel. In this measurement method, the fluorescence depolarization due to rotational Brownian motion of the fluorescent molecules in the solution is measured and converted to fluid temperature. Since the fluorescence polarization degree is independent to fluorescence intensity, the measurement is less influenced by the fluorescence quenching effect, which is an issue in laser-induced fluorescence (LIF) method. Experiments were performed using a microchannel with fluorescent molecules solved in water. The effects of the fluorescent molecule concentration, fluid pH and fluid temperature on the fluorescence polarization degree are discussed to evaluate the influence of the quenching effects and to derive the correlation curves. Furthermore, the proposed method was applied to measure the temperature distribution with linear gradient generated in the microchannel. The results showed that the fluorescence polarization is considerably less sensitive to quenching factors compared with the fluorescence intensity measurements. A linear correlation between the polarization degree and the fluid temperature was obtained. This relationship agreed well with the theoretical one. Further, measurement of two-dimensional temperature distribution in the microchannel agreed well with the values obtained by the thermocouple measurements. These results confirmed the validity of the measurements and feasibility of the proposed method.

Silica glass is frequently used as a device material for micro/nano fluidic devices due to its excellent properties, such as transparency and chemical resistance. Wet etching by hydrofluoric acid and dry etching by neutral loop discharge (NLD) plasma etching are currently used to micromachine glass to form micro/nano fluidic channels. Electro-osmotic flow (EOF) is one of the most effective methods to drive liquids into the channels. EOF mobility is affected by a property of the micromachined glass surfaces, which includes surface roughness that is determined by the manufacturing processes. In this paper, we investigate the effect of micromaching processes on the glass surface topography and the EOF mobility. We prepared glass surfaces by either wet etching or by NLD plasma etching, investigated the surface topography using atomic force microscopy, and attempted to correlate it with EOF generated in the micro-channels of the machined glass. Experiments revealed that the EOF mobility strongly depends on the surface roughness, and therefore upon the fabrication process used. A particularly strong dependency was observed when the surface roughness was on the order of the electric double layer thickness or below. We believe that the correlation described in this paper can be of great help in the design of micro/nano fluidic devices.

The present study proposes a velocity measurement based on thermal tracing by Raman imaging and investigates its applicability focusing on the error in temperature measurement, towards the establishment of a non-intrusive and micro-scale velocimetry. In order to realize fluorescence-free measurement, two-wavelength Raman imaging was employed to measure the temperature field in a channel flow. This technique exploits the contrasting temperature dependencies of hydrogen-bonded (HB) and non-hydrogen-bonded (NHB) OH stretching Raman bands of liquid water, and enables the determination of planar temperature distributions from the intensity ratio of the HB to NHB images. A calibration experiment showed a linear relationship between the temperature and the Raman intensity ratio in the range 293-333 K with temperature sensitivity of -0.56% K-1. It was also confirmed that the spatial variation of the intensity ratio led to a large measurement error (approximately 9.1 K). Afterwards Raman images were acquired with various measurement conditions, and the influence of each parameter on the measurement error was quantitatively investigated. The apparent temperature variance was considerably reduced by increasing electron-multiplying gain and binning factor for spatial averaging, whereas an increase in the size of the measurement area resulted in a quadratic increase in the temperature variance due to the inhomogeneous excitation intensity. Finally, the requirements of the thermal flow conditions for the present methodology to be applied were quantitatively examined according to the measurement results.

The present study developed a near-wall pH measurement technique based on nanoscale laser-induced fluorescence to evaluate the spatio-temporal distributions of pH in the vicinity of a glass-solution interface. Fluorescein sodium salt, a pH-sensitive dye, was dissolved in working fluids and excited by an evanescent wave generated by total internal reflection of a laser beam. Near-wall pH distributions were determined based on a ratiometric calibration using a high-pH buffer solution (pH  ⩾  10) as a reference. The proposed technique was applied to both pressure-driven and electroosmotic flows to investigate the effects of interfacial potential and external electric field on proton concentration. The near-wall pH in pressure-driven flow was lower than the bulk pH by ~0.2 due to the local excess of protons attracted towards the negatively charged glass wall. This pH shift was especially noticeable in solutions with low ionic strength, which indicates that the near-wall pH is closely related to the thickness of the electric double layer. The pH shift in electroosmotic flow was smaller than that in pressure-driven flow by ~0.02 and decreased with electric field strength. In addition, a time-series measurement clearly visualized the temporal change in non-uniform pH distributions in electroosmotic flow at a spatial resolution of 5.2  ×  5.2 µm. The measurement uncertainty was estimated to be 0.16 pH unit at 95% confidence level. These results demonstrated the feasibility of the present technique to evaluate spatio-temporal proton concentrations within the space 10–100 nm away from a solid–liquid interface. The proposed technique will therefore contribute to quantitative investigations into electrostatic interactions and ion transport in developing nanoscale electrochemical transport techniques and devices in, for example, biochemical analysis and water purification. [ABSTRACT FROM AUTHOR]