Two-dimensional fluid viscosity measurement in microchannel flow using fluorescence polarization imaging

This study describes the development of a noncontact and two-dimensional fluid viscosity measurement technique based on fluorescence polarization microscopy. This technique exploits fluorescence depolarization due to rotational Brownian motion of fluorophores and determines fluid viscosity in microchannel flow by measuring steady-state fluorescence polarization. The main advantage of the technique is that planar distributions of fluid viscosity can be visualized by noncontact optical measurement, while commonly-used mechanical viscometers measure the viscosity of bulk liquids. Moreover, steady-state polarization measurements are realized using a simpler experimental setup compared to other noncontact techniques such as time-resolved fluorescence lifetime/polarization measurements. The relationship between the fluid viscosity (μ) and the fluorescence polarization degree () was experimentally obtained using casein molecules labeled with fluorescein isothiocyanate as a fluorescent probe. The fluid viscosity was controlled within the range of 0.7-3.0 mPa s, which is the range often encountered in biological materials, by mixing sucrose or glucose with the solution. The fluid temperature was maintained uniform at 30 °C during the measurement. The calibration result showed that 1/ linearly increased with 1/μ which qualitatively agreed well with the theoretical prediction. The measurement uncertainty was 7.5%-9.5% based on the slope of the calibration curve. The viscosity gradient generated by the mass diffusion between the two solutions co-flowing in the Y-shaped microchannel was clearly visualized under uniform temperature conditions by applying the calibration curve. Finally, the influence of the temperature change on was experimentally evaluated. The results supported the applicability of the present technique for visualization of the viscosity distribution induced by temperature change. These results confirmed the feasibility of the present technique for analyzing microscale viscosity fields associated with mass transport or temperature change

Timing and Spacing Control in Microchannel Flow by Applying Periodic Force over Space and Time

This study develops a technique to control the timing, spacing (interval), and velocity of particles in a microchannel flow by periodically exerting forces on the particles over space and time. The periodic force was produced by dielectrophoretic force using boxcar-shaped electrodes on the channel wall. We could define the timing, interval, and velocity of the particles by configuring the on–off cycles of the applied voltage. Controlling the particle spacing and timing when it crosses a position in the channel and the focusing effect in the cross-sectional position could improve the performance and throughput of microfluidics, particularly for sensing, active sorting, and encapsulation of particles and cells. The proposed technique was first evaluated by a one-dimensional analysis based on a perturbation theory. We conducted a numerical simulation to solve the dielectrophoretic force distribution and the equation of motion of the particles to understand the relationship between the force and the particle motion in the boxcar-electrode region. We measured the velocity and position of the micro-particles flowing over the boxcar-electrode region in the microchannel and demonstrated the performance and accuracy of the proposed technique for alignment and timing control. The probability density functions (PDFs) of the period between the particles, particle velocity, and timing, concentrated at the target value with minimal variation. Furthermore, the measurement of particles with diameters of 8, 10, and 12 μm resulted in the same PDFs, which showed the applicability to a reasonable variation of particle diameters

Particle and rigidized red blood cell concentration distributions in microchannel flows

The motion and concentration distribution of particles and cells in flow are important factors which affect the fluid properties, flow structure, and mass transfer of biological and chemical species in blood vessels and channels. In this study, number density distributions of particles and rigidized red blood cells (RBCs) in a microchannel whose size is comparable to the sizes of the particle and RBCs are measured. Measurements were conducted at several streamwise locations for suspensions of particles and RBCs with hematocrits of the order of 10% and particle sizes of 5 and 8 µm. Analysis of the migration and resulting concentration distribution of the particles and RBCs was conducted using a model that considers the particle–particle collision and fluid dynamic force. As the size of the microchannel is small, the wall effect on the collision and migration of the particles and RBCs was significant. The wall reduced the overlapping area of the particles in collision and their displacement after collision (mobility), which varied the number, location, and magnitude of the maximum peaks observed in the number density distribution. Furthermore, the rotational motion of the rigidized RBCs in the channel flow reduced the effective lengths of the overlapping area and displacement, whereas it produced additional migration at the wall. With these terms added in the model, the number density distributions of the particles and RBCs showed reasonable agreement with those of the measurement. Especially, the number of peaks and their location for the maximum values in the model and measurement matched well

Increase of one-to-one particle encapsulation yield using dielectrophoretic alignment technique with boxcar-type electrodes (Translated)

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

Effects of Micromachining Processes on Electro-Osmotic Flow Mobility of Glass Surfaces

micromachine

Association Between Smoking and Hypertension in Pregnancy Among Japanese Women: A Meta-analysis of Birth Cohort Studies in the Japan Birth Cohort Consortium (JBiCC) and JECS

Background: Recent literature suggest the effect of maternal smoking on risk of hypertensive disorders in pregnancy (HDP) and preeclampsia may differ by ethnicity; however, studies on Asians are limited. Methods: We investigated the association of maternal smoking with HDP and preeclampsia using a common analysis protocol to analyze the association in six birth cohorts participating in a Japanese consortium of birth cohorts (JBiCC). Results were compared with-published results from cohorts not included in this consortium, and, where possible, we produced a meta-analysis including these studies. Results: Meta-analysis of four cohort studies including 28,219 participants produced an odds ratio (OR) of 1.24 (95% confidence interval [CI], 0.88–1.87) for the effect of smoking beyond early pregnancy compared to women who did not smoke during pregnancy. These results combined with those from the Japan Environment and Children’s Study (JECS) yielded an OR of 1.19 (95% CI, 1.00–1.43, P = 0.056). Meta-analysis results for categories of smoking volume were insignificant, but when combined with JECS yielded an OR of 0.86 (95% CI, 0.65–1.12) for smoking 1–4 cigarettes, 1.25 (95% CI, 0.98–1.60) for smoking 5–9 cigarettes, and 1.27 (95% CI, 1.04–1.54) for smoking 10 or more cigarettes per day. All effects were insignificant for preeclampsia. Conclusion: Our results suggest that the protective effects of smoking longer and smoking more on HDP and preeclampsia repeatedly observed among Europeans and North Americans likely do not hold for the Japanese