A dynamic micromixer for arbitrary control of disguised chemical selectivity

Abstract

A new type of dynamic micromixer combining the concepts of parallel multi-lamination and hydrodynamic focusing was developed for arbitrary control of disguised chemical selectivity.Microreactors are microscopic reaction settings that exhibit unique properties versus traditional macroscopic environments: advantages of microreactors include reduced reagent consumption, high surface-area-to-volume ratios and improved control over mass and heat transfer. [1][2][3][4][5][6][7][8] In addition, fluidic physical phenomena in the microreactors are dramatically different from those observed in the macroscopic reaction apparatuses. [9][10][11] For instance, the inertial effect in the microfluidic environment is negligible (due to a low value of Reynolds number, Re), and thus fluid transport is dictated by fluid viscosity, leading to laminar flow in microchannels. A microfluidics-based reactor is known to be a turbulence-free system, in which diffusion-driven mixing is relatively slow. As a result, one of the most challenging issues in the field of microreactor design is to facilitate mixing in stand-alone devices. Over the past decades enormous efforts have been devoted to develop micromixers (one type of microreactor) that can be categorized into either active or passive ones. 12 Unlike active micromixers, passive ones13 do not require external power, are often composed of robust channel geometry or microstructures, and are developed upon straightforward concepts of increasing contact surfaces and/or shortening the distance for diffusion among different streams. For example, devices based on parallel14 -16 multi-lamination 12 have been developed to achieve efficient mixing. † Mixing may impact reaction outcomes. 17,18 A good example is the diazo coupling reaction 19 between 4-sulfono-benzenediazonium (1) and 1-naphthol (2) under alkaline conditions at room temperature. This reaction is known as a fast competitive consecutive reaction 20 (Scheme 1), where the selectivity between the two major reaction products (i.e., monosubstituted product 3 and disubstituted product 4) is not only determined by intrinsic reaction kinetics 21 (k 1 = 1.2 × 10 7 M −1 s −1 and k 2 = 2 × 10 3 M −1 s −1 ) but also affected by the mixing rates. When reactants 1 and 2 are mixed in a macroscopic setting, the reaction solution is first fragmented by stir-induced turbulence. No matter what mixing approach (e.g., hand mixing or mechanical stirring) is applied, in the best case scenario, the mean radii of the fragmentized solutions are approximately in the range of 100 to 1000 µm, 22,23 leading to a mixing time ranging from a millisecond to seconds. Consequently, the fast reaction between the 1 and 2 happens prior to complete homogeneous mixing to give a complicated reaction mixture containing reactant 2 and products 3 and 4. This phenomenon is known as the disguised chemical selectivity. 20,24,25 In order to avoid the undesired disguised chemical selectivity, micromixers exhibiting fast mixing rates have been developed. 4,20 In this paper,...

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