Design and experiments of a short-mixing-length baffled microreactor and its application to microfluidic synthesis of nanoparticles

Conventional microreactors with a meander micromixer were generally operated at low flow velocity together with long mixing length in nanoparticle synthesis. In this article, we demonstrate a simple micromixer with short-mixing-length baffles in order to obtain an effective mixing for a wide-range flow rate in silica nanoparticle synthesis. The planar baffled micromixer can greatly enhance the mixing behavior through the diffusion-dominant mechanism at low flow rate and the convection-dominant mechanism at high flow rate. The 3D numerical simulations and experimental confocal flow visualization evidence that a good mixing efficiency at either lower or higher flow rates can be achieved using the baffled micromixer with three mixing units with a gap size of 50 μm in the 400 μm wide channel. The efficient micromixer was then used for silica nanoparticle synthesis through tetraethyl orthosilicate (TEOS) solution. The size of particles can be adjusted by means of flow rate, reaction time and temperature. An average particle diameter of 250 ± 50 nm can be obtained at a low flow rate of 0.1 ml/h. At high flow rate of 35 ml/h, small particles of 88 ± 11 nm in diameter were formed at a reaction time of 12.5 min at 25 °C and the size was reduced to 46 ± 7 nm at 55 °C. The particle size decreased with increasing flow rate and the reaction temperature. Enhancing fluid mixing at high flow velocity with a short mixing length is beneficial for the possibility of obtaining high throughput of microreactor.