| Droplet generation and its application under microfluidic technology are being caused more and more attention in many areas, such as chemical and life sciences, due to their need of the miniaturization of regents'flow and the discrete manipulation. Many previous works have shown that a microfluidic channel structure formed by certain microfabricated technique has an important influence on behavior of droplet generation. This thesis investigated the capability of microdroplet generation in a cross-over microchannel device with through-hole(s) fabricated out with PDMS microwire-molding technique.Firstly, the capability of microdroplet generation in a cross-over microchannel (diameter 20μm,40μm,60μm,80μm, respectively) with one single through-hole was investigated under the fluidic manipulation to two-phase working media, in which pure water acting as gas phase while pressurized-nitrogen gas as liquid phase. When the pressures ofΔP1 andΔP2 were applied on the inlets of the two cross-over microchannels, respectively, the former for liquid-filled microchannel for pushing-forward the liquid section while the later for gas-filled microchannel, bothΔP1 andΔP2 were increased as the diameter of microchannels decreased. It was found thatΔP2 could cause the through-hole to squeeze and cut off the liquid section cross along the hole in the liquid-filled microchannel. Moreover, the microdrop'size (below some threshold) of thus-generated via the hole was determined only by the pushing-forward pressureΔP1, independent of the cut-off pressureΔP2. When hydrophilic in the inner surface of microchannel, the minimum volume of microdroplet generated in such microdevice could be 0.157pL, which was correspondence with 5μm in microchannel diameter 20μm, however, when lipid solution was microflowing coated on the inner surface, the microdroplet volume could be significantly decreased to 0.064pL, which was 2μm length in the above same microchannel. In addition, the break-up of a microdroplet under such pressure-driven condition were also discussed.Not only could such a cross-over microchannel device with one single through-hole generate droplets on-demand discretely, it could also generate microdroplets (of oil or of water) continuously under certain pressure-load conditions. The requirement is that the PDMS inner wall of the downstream of gas-filled microchannel and the through-hole should first be coated a layer of lipids as surface modification. When two-phase flow were applied within the crossed microchannels, gas-filled and liquid-filled respectively, with a flow ratio of 2/3 ~ 9/10, the device presented a picture of microdroplets continuously generated. The sizes of droplets were increased as the flow ratio decreased, and their generation frequency could be reached more than moderate throughput.Secondly, for the case of the multi-cross-over microchannle device in which parallel multi-through-holes were fabricated in along one single channel, the results showed it could also generate microdroplets in each through-hole, similar to that of the above-mentioned one single cross-over. However, the emerge of first droplet from each through-hole to its downstream microchannel was delayed sequentially along the pressure drop in the gas-filled channel, the time difference between the neighboring parallel microchannels depended on the spacing between the neighboring through-holes and their geometric conditions; when the spacing of 100~200μm, the time difference was estimated about 0.5s.Thirdly, the behaviors of microdroplet generation in such a cross-over microchannels with one single through-hole were further studied under the manipulation of two-phase of liquid-liquid. In experiments, one channel was injected into with lecithin (oil) solution, the other with pure water. The results showed that in the 4 groups of microchannel devices ( with cross-over diameters of 20μm, 40μm, 60μm, 80μm, respectively), only the groups of 60μm and 80μm could generate droplets at their 1.5mm downstream of the through-hole, with a means of oil-in-water. In general, to the robust generation of microdroplets in such case, the flow rate of oil phase should be about 1μL/min larger than that of water; meanwhile, the droplet volume, homogeneous enough thus-generated, strictly depended in proportional to on the oil/water flow ratio. However, when the oil/water flow ratio fell into the range of 10/9 ~ 15/14μL/min, tiny droplets (oil-in-water) could be seen rapidly generated with a diameter of 3~5μm.Finally, a rudiment software interface to the integration of several processes related to the microdroplet generation and analysis was developement, based on the observation of droplets generation with microscopic video systems, microparticle image velocimetry (Micro-PIV) technology and the measurement of contact angle in microchannel with image analysis process. This programmed interface could be helpful for future microdroplet manipulation and analysis. Futhermore, as a preliminary application of the microdrople generation with the microchannel devices two biological examples were tested, one was used to generate DNA solution droplets for DNA micro-analysis; another was the manipulation of microdroplets to separate one single cells from solution volumed in microliter for future experiments for single-cell analysis.In summary, the results show that the cross-over microchannel with a through-hole, supported by appropriate gas/liquid flow control mode, could effectively generate microdroplets of nanoliters even picoliters on-demand. Its significant technique effects could be included in the following:①the easily simple fabrication of microchannel and its micro-structure;②the low pressure needed;③virtually on-demand, the volume as low as picoliters, even to femtoliters;④under two-phase of gas/liquid, the generation frequency of microdroplets on-demand could reach more than moderate throughput; and the homogeneous of droplet size is good enough.
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