Sub 100 μS Studies of Fast Chemical and Biological Reactions Using a Free- Jet Micromixer

Full text: Short time-scales are fundamental for gaining insight in fast chemical and biological reactions like the nucleation and growth of nanoparticles or the conformational changes of proteins. In the present communication we describe first the design, the fabrication process, and the system engineering of a microfluidic mixer aimed to studies of fast chemical and biological reactions with a time resolution lower than 100 μs. The mixer is based on hydrodynamic focusing. It works in the laminar regime, and mixing occurs only by diffusion that can be predicted and controlled. The mixing is completed inside the device before the nozzle, and a free liquid jet in air exits from the device. The time evolution of the reaction is separated spatially in the steady state flow along the liquid jet exiting from the device. Measuring the signal form the jet at different distances from the nozzle exit corresponds to measuring the reaction at different times from the beginning of the mixing. The geometry and dimensions of the device have been optimized by Finite Element Analysis, and are stringent: outlet channel 500 μm long and 10 μm wide, and exit nozzle 8 μm wide in order to produce a stable jet. The channels depth is 60 μm. Even if the device has been optimized for synchrotron SAXS measurements, the choice of a free jet as observation region overcomes the problem of the material selection for the microfluidic chamber. This enables to use the same device with different investigation techniques (i.e Light Scattering and Raman Spectroscopy). The device usually works with a total flow rate of 300 μl/min, leading to a jet speed of 13 m/s, and to a mixing time of 45 μs. Anyway it can bear flow rates larger than 1 ml/min, corresponding to a nominal time resolution of 26ns/μm. The micromixer can be used to study a wide variety of fast chemical and biological reactions by adequately tuning the operating conditions. The maximum pressure inside the device is less than the denaturation limit for proteins. The shear rate is not sufficient to destabilize a typical small protein of ∼100 amino acids in water, but it could have influence on bigger proteins or when using viscous solvents. Therefore, the possibility to use the device with biological reagents must be evaluated for each specific case. Latest results of SAXS characterization of the jet itself, and of the early stages of nucleation and growth of nanoparticles will be presented. (author)