Deseription Of Microfluidic Transfer Process Based On Raman Spectroscopy And Its Application

Micro-Chemo-Mechanical systems (MCMS) have attracted great attention because of their high efficiency heat and mass transfer and compactness. However, problems associated with the transition from macroscale to microscale have limited the establishment of microscale transfer theory. As the characteristic scale is miniaturized, either inertial and electromagnetic forces that are directly proportional to the high power of the size, or surface tension and viscous, elastic and electrostatic forces that are directly proportional to the low power of the size, probably lead to the significant difference of the microscale transfer phenomena. The researches on these scale effects are crucial for the improvement of the microscale transfer theory. Noninvasive measurement with high spatial resolution and high accuracy is the key to carry out those researches. In this context, a method to accurately measure the microfluid temperature and concentration fields based on confocal Raman microscope was proposed in this paper. The single-phase convective heat transfer process and the liquid diffusion were investigated. Based on the feature of the microfluid enhanced mixing, a high-efficiency micromixer employing two chaotic convective mechanisms was also developed. The main work and results of this paper are as follows:(1) Effect of laser induced heating on microfluid Raman measurementIn the microfluid measurement based on confocal Raman microscope, if metallurgic objectives are adopted, the refraction will occurr when the laser penetrates the top surface of the microchip and focuses on the liquid sample in the microchannel, leading to the focus distortion. This drastically reduces the spatial resolution, especially the depth resolution. Experiments found when the distorted focus covered the channel bottom of the silicon chip, the chip absorbed a great portion of the laser energy. This was termed laser heating effect and could result in the temperature-rise. The laser heating effect in the Raman measurement was studied for the first time in this paper. Via omputational fluid dynamics (CFD) simulations, the sensitivity of 11 factors was investigated with orthogonal analysis (OA), and the surface power and illuminated area diameter were found to have the most significant influence on the temperature-rise. The siginicant influence of the surface power on the heating effect was also observed from experiements, and simply lowering the surface power was not the efficient approach to eliminate the heating effect. Finally, a correlation to evaluate the maximum local temperature-rise in the microfluid Raman measurement was proposed.(2) Research on microchannel wall axial heat conduction under single-phase convective heat transfer condition based on Raman temperature measurementThe water temperature measurement in microscale based on the confocal Raman microscope was proposed, and the experimental system for microfluid single-phase convective heat transfer was established. The experiments were combined with CFD simulations to investigate the axial heat condution in microscale. It was found that the axial heat condution led to a maximum heat flux at the channel inlet, and the nonlinear development of fluid and wall temperatures. In some locations, the fluid temperature was even higher than the wall temperature, and heat would, therefore, transferred from fluid to the channel wall. In this case, a singular point emerged in the local Nusselt numer (Nu) curve, and would move toward the inlet as the Reynolds number (Re) was increased. The Nu increased with the increased Re, which was caused by both the entrance effect and axial heat conduction caused.(3) Microfluid liquid diffusion coefficient measurement and interface control based on Raman concentration measurementWith the optimum microchip design obtained from CFD simulations, the microfluid concentration measurement errors caused by the decreased resolution were reduced. A method for measuring the liquid diffusion coefficient at various temperatures based on the Raman concentration measurement was established. This method reallized the continuous diffusion coefficient measurement at various temperatures, and was more convenient, efficient and accurate, compared with conventional methods for diffusion coefficient measurement. Based on the Raman concentration measurement, the laminar flow fluid interface control method was proposed, using a trial-and-error control program. The accurate diffusion interface control of the micro fluids without sample property information was reallized, using this method.(4) High efficiency micromixer employing two chaotic convective mechanismsA high efficiency stainless steel micromixer employing two chaotic advective mechanisms was developed for MCMS. The micromixer featured a square-wave structure and periodic cubic grooves. CFD results revealed that the square-wave structure induced the laminar recirculation mechanism, and the periodic cubic grooves produced the flow stretching mechanism. The configuration of grooves locating right after the turns of the square-wave structure yielded the relatively higher mixing quality and minor pressure drop increase. At low Re, the flow stretching was dominant. As Re rose, the laminar recirculation contributed increasingly more and the two mechanisms became jointly influencing on the mixing so that the mixing was remarkably enhanced. A parallel competitive reaction was utilized to evaluate the performance of the proposed micromixer. The optimum Re range for its operation was 30-220. Within this range, the high mixing quality and low energy consumption could be ensured. Compared with commercial micromixers, the present micromixer was more easily-fabricated to facilitate the large-scale production. Besides, it is competent for applications with adverse circumstances, such as high temperature and pressure, and strong corrosion.