# Numerical and Experimental Investigations of a Micromixer with Chicane Mixing Geometry
## 1 Introduction  
Nowadays, micromixers and microreactors are used in a wide range of chemical reactions, from simple homogenous mixing to catalytic and biochemical processes [1]. Some pilot plants based on microstructured systems have been reported [2], which indicates the potential for novel microstructured systems in the chemical industry. Their advantages are based on extremely small dimensions which lie in the range from some micrometers to a few millimeters. This reduces the diffusion length and the mixing time in comparison with conventional reactors. The volume of micromixers varies from microliters to milliliters. This provides better process control (e.g., temperature) and better safety when working with dangerous or explosive substances. Moreover, the microstructured systems operate continuously, providing wide scale-up possibilities, e.g., parallel running of several microreactors in order to increase the setup performance. In non-micro systems such as in huge pipes or vessels, an increase in mixing rate is mainly reached using turbulent flow regimes and relatively high values of Reynolds number (Re > 2400). However, this leads to an instability in flow, as well as to higher energy costs. In contrast to conventional reactors, Reynolds numbers in microfluidic systems are relatively small (Re < 10) and the flow is laminar. The mixing effect is stronger in comparison with conventional reactors due to a significant decrease in diffusion length and a significant increase in the interfacial area between the species. More complex and effective geometric structures, such as micromixers based on multilamination focusing and splitting-and-recombination effects were developed to increase the interfacial area and stimulate the mixing process [1; 3; 4; 5; 6]. There are two main types of micromixers: active and passive. Active micromixers use external energy sources to accelerate the mixing process (acoustic, periodic pressure, temperature fields, etc.) [34]. In contrast to active micromixers, passive ones do not need external energy sources, but use special geometric structures of mixers and channels. The passive micromixers are also divided into two types depending on the primary mixing process: based on the molecular diffusion (laminar micromixers) and based on the chaotic advection (convective micromixers). The laminar micromixers, which use diffusion as a primary mixing process, obey [PERSON]'s law and are limited by molecular diffusion lengths. Consequently, geometry, residence time, and dimension of channels of such microfluidic devices play an especially significant role. Small characteristic lengths and low flow rates lead to low values of Reynolds number, which normally equals Re < 10. In some cases, with sample geometries such as a T-mixer, the diffusion continues to be the main mixing process with Reynolds numbers up to Re = 75 [7]. This laminar flow regime with parallel streamlines without vortices is well-known as \"stratified flow\", whereas the main mixing process is via diffusion. However, diffusion is rather a slow process with mixing times of up to several minutes. Special microreactor geometries (interdigital [8], split-and-recombine [9], focusing [10], etc.) were designed to focus, stretch, or recombine the flow and as a result decrease the characteristic mixing length and improve the mixing process. Microreactors based on chaotic advection provide intensification of the mixing process due to vortices generation. The guided flow streaming lead to hydrodynamic instabilities and can significantly improve the transverse mass transfer. Convective micromixers largely operate with high values of the Reynolds number Re > 10, but this value depends directly on the geometry and varies from Re = 0.02 (for a spiral channel micromizer [11]) up to Re = 50 (for a T-mixer [12]). In convective micromixers, mixing stimulation is achieved by vortices, which appear according to domination of inertial to viscous forces.