A Review of Enhanced Micromixing Techniques in Microfluidics for the Application in Wastewater Analysis

Owing to new methods in agriculture, modern techniques and innovations in domestic activities and mushroom industrialization, rigorous studies are required to understand the wastewater ingredients and their toxicities so that minimization of harmful effects can be achieved. Micromixers may provide insight for analysis in efficient and reliable water treatment methods. Due to the high interfacial-area-to-volume ratio of fluids in micromixers, the study of interaction/reaction between several mixtures and chemicals can be performed, for in-depth, microscale wastewater analysis. This involves various applications, including analysis of wastewater for monitoring and evaluation of heavy metal removal, diclofenac detection, consumption assessment of the illicit drug, tobacco, alcohol and cocaine within local communities and treatment of palm oil mill effluent. However, achieving adequate mixing performance is considerably difficult in microfluidic micromixer, as the flow is always associated with unfavourable laminar flow and dominated by molecular diffusion. As an effort to address the issue, active and passive mixing configurations have been proposed in previous studies. Active mixing mechanism operates on the basis of external energy input to actuate relevant actuators, to enhance mixing by stretching and folding of the fluid mixture by several modes such as pressure, thermal effect, magnetohydrodynamic, electrohydrodynamic, vibration, acoustic and microstirrer. Passive mixing mechanism can be categorized based on different fluid stream types which ascribe to geometric design modifications such as lamination, chaotic advection, droplet, collision of jets and special effects. In the prospect of reducing energy consumption, the passive flow driving mechanism does not require energy as it mostly depends on the effects of surface tension, capillary action and gravity rather than using powered syringe pumps