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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.