Design and Evaluation of a Highly-Efficient Miniature Mixing System

With the increasing need for molecular reactions with small liquid volume in biomedical and chemical analyses, miniature mixers have captured considerable attention. Due to the nature of laminar flow in a millimeter-size and blow flow channel, mixing, or the molecular mass transfer in miniature mixers is usually dominated by molecular diffusion. The long diffusion time and mixing distance make rapid mixing unachievable. To address low mixing efficiency, various active or passive micro mixers have been proposed and examined [1-10]. Miniature mixers can be divided into two types: active and passive. Active mixers are equipped with various driving sources based on different mechanisms to create fluctuations within the fluid to achieve high mixing efficiency [11-20]. For example, Moctar et al. [21] developed a mixer using electro-hydrodynamic force (EHD) to drive two types of fluids with different electrical properties. The two fluids were brought into contact in a channel and were optimized by controlling the Reynold number (Re) at 0.0174. Thus, mixing could be achieved in less than 0.1 sec within a short distance. Ahmed et al. [22] studied an acoustically driven mixer with an air bubble trapped inside the mixing channel. The air-liquid interface could be excited to resonance by acoustic excitation to induce streaming for mixing; it took only a few milliseconds to complete the mixing process. Liu et al. studied a pulsed mixing method based on a Y-shaped micromixer driven by two piezoelectric micro-pumps. Using two out-of-phase sinusoidal waves, the contact area between two solutions could be increased. They successfully used this device to synthesize gold nanoparticles using HAuCl4 and Na3C6H5O7 solutions [23]. A good synthesis was achieved with a Y-entrance angle of 60 degrees and flow rate of 4 ml/min. Passive mixers usually utilize molecular diffusion and chaotic advection. The liquid flowing in traditional T-shaped and Y-shaped mixers is basically laminar flow, leading to long mixing distance and time due to weak molecular diffusion and strong advection (Pe >> 1). To enhance transverse advection, passive mixers with various complicated geometries have been developed, including the tesla micromixer [24], the zigzag micromixer [25], the split-andrecombine (SAR) micromixer [26], the slanted groove micromixer, and the curved channel micromixer [27-29]. Schönfeld et al. [30] & Li et al. [31] designed and implemented different SAR micromixers. The special sub-channels in this type of mixer split the flow stream and destroy the laminar flow characteristics within the stream to increase the tendency of chaotic advection. Lynn and Dandy numerically studied a micromixer embedded with microgrooves and described the dependence of mixing efficiency on the channel aspect ratio, the groove depth ratio, and the ridge length [32]. By changing the geometric parameters of the mixing channel, the variation of the transverse advection flow can be as large as 50%. Nguyen [33] & Jeong [34] gave good reviews of mixer design and applications; their studies suggested that the geometric parameters and the flow rates are essential to the design of good mixers. In this study, a miniature mixing system is developed by combining a passive mixing chamber and two miniature pumps like that reported in our previous studies [35]. A circular-shaped mixing chamber was adopted for easy assembly, with different mixing channels designed within the chamber to investigate the optimal mixing effectiveness. A simulation was conducted first to study the feasibility of the mixing system based on a serpentine channel structure and different corner space design within the flow channel. Experimental studies were conducted to verify the design. Finally, the method to optimize this design is discussed. Volume 4 Issue 1 2018