Take a look at microfluidic mixing research.
Mixing is a fundamental aspect of fluid manipulation for both chemical and biological microfluidic applications and presents an enduring challenge. At the micro scale laminar flow conditions dominate and mixing is primarily achieved by diffusion.
To increase the rate at which fluids mix the interface area between the fluids must be in increased. CFD modelling of the critical fluid injection points enables optimisation of physical micro-structures for improved mixing.
Rapid mixing can be achieved under the laminar conditions within microfluidic structures by temporally interleaving the injection of the liquids. The interface regions between the interleaved slugs of liquid are rapidly increased by the parabolic flow profile across the channel, thus allowing mixing to occur rapidly by diffusion.
At the University of Hertfordshire we have developed high speed piezoelectrically actuated time-interleaved mixers which can operate at higher frequencies than conventional micromixers enabling significant reduction in volume of the injected liquid slugs significantly speeding up the mixing process.
Simple adjustment of the injection timings can then be used to vary the concentration of the resulting mixed fluid.
Another approach is to physically separate the fluids into thin laminae which are alternately located and then brought back into contact, thus reducing the time required for diffusion to occur. This technique requires minimal external control however it requires more complex fluid architectures and precise alignment of substrate layers.