A planar micromixer with rhombic microchannels and a converging-diverging element has been proposed for its effective mixing. Both CFD-ACE numerical simulations and experiments were used to design and investigate the effect of three parameters (number of rhombi, turning angle and absence or presence of the converging-diverging element) on mixing. Mixing efficiency is dependent upon Reynolds number and geometrical parameters. Through the results of numerical simulation, it is evident that smaller turning angle (a), higher Reynolds number and increasing number of rhombi will result in better fluid mixing due to the occurrence of larger recirculation. The large recirculation is beneficial for both the increased interfacial contact area between two species and the convective mixing. In the numerical simulations, mixing efficiency of 99% was achieved with a most efficient system consisting of threerhombus mixer with a converging-diverging element at a = 30°and Re = 200. An experimental mixing efficiency of about 94% has been obtained with the same design parameters. As expected, it is lower than the theoretical efficiency but is still very effective. A micromixer with such design can be potentially useful in the future applications of rapid and high throughput mixing.
The etching rate in silicon deep reactive ion etching (RIE) is related to pattern geometry and a frequently seen defect, RIE lag, appears in feature sizes up to hundreds of micrometers. Different feature dimensions of rectangles, squares and circles/doughnuts are designed to realize how the geometrical pattern affects RIE lag in the inductively coupled plasma (ICP) etching process. Experimental results reveal that the primary dominating factor in RIE lag is feature width and secondary factors are feature area, shape and length-to-width ratio. Etching rates of rectangular trenches are sensitive to width while ring trenches are sensitive to both width and area. Process parameters are also adjusted to control RIE lag magnitude and realize its mechanism. The inverse RIE lag phenomenon appears at a much higher pressure of APC (auto pressure control) 75% at constant area features. The formation and removal of passivation film at the trench bottom will delay Si etching by F radical density, which will start earlier in a small width than a large one. It will be more obvious at higher pressure and lead to the reduction of RIE lag. This indicates that the cause of RIE lag in ICP etching is primarily attributed to the formation and removal of passivation film at the bottom of the trench, together with feature geometry. The RIE lag-eliminated trenches with constant area are obtained at a higher pressure of APC 70%. Deep and high aspect ratio silicon microstructures can be controlled by ICP etching with different pattern geometry.
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