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.