Table 1.

Summary of mixing enhancements presented in the review.

Methods	Pros	Cons	Suggestions/Comments
Passive micromixers
Stream splitting and recombination	Basic, but more effective than pure diffusive mixing	May require long channels for complex micromixer network	Pay careful attention to the method of recombination; adequate mixing may result without additional elements.
Slanted wells or patterned grooves	Easy to fabricate (no moving parts), CFD code available for optimization	Requires additional fabrication to adjust the level of mixing	Useful for mixing enhancement when precise control is not required.
Hydrophobicity or surface modification	Additional mixing enhancement over other passive mixers	May be complicated to define surface modification zones	Combining with other passive mixers to maximize mixing potential
Multiphase mixing enhancement	Low fabrication cost, good mixing with little sample dilution, and large mixing property change by adjusting flow rates	Requires additional purification step as immiscible carrier liquids or gas must be introduced into the device	Consider for use if introducing an additional fluid into a device is possible without hindering device operation.
Active micromixers
Microstirrers	Versatile; excellent mixing with precise control	Difficult to fabricate (moving parts); may require multiple stirrers	Useful for applications requiring precise control over the level of mixing; magnetic actuation easier than mechanical actuation. Utilizing magnetic particles may offset some of the fabrication difficulties.
Acoustic waves	Nearly instantaneous mixing; easy to operate	Need a bubble or air interface	Consider acoustic mixers if quick mixing is needed
Flow pulsation	Easy to implement using micropumps or electric fields	Requires fine-tuning to achieve optimal mixing enhancement	Convenient when pumps are integrated or do not compromise operation
Thermal enhancement	Simple to integrate into a device	Requires heaters	Consider thermal micromixers where heat does not affect samples/reagents
Electrokinetic mixing enhancement	Can be utilized effectively at relatively low voltages. Can be very effective over short mixing lengths.	Requires integrated electrodes	Consider when electrodes are integrated into the device. Utilize ac voltages to avoid bubble formation.