Moving and Mixing Fluids on a Chip

1 Moving and Mixing Fluids on a Chip Moving Fluids on a Chip 

Dealing with real fluids on the microscale is difficult. Bubbles, dust and surface tension can cause real problems due to the small size the fluid encounters. In fact, a whole science called microfluidics has built up and books have been written on the subject Challenges to all microfluidic systems include  

Liquid flow can be laminar or turbulent in nature. In laminar flow, fluids move in straight lines in effect the adjacent layers slide over one another. Hence, different liquids can flow side by side and the only way for them to mix is by diffusion. With turbulent flow, there are unstable stream-lines and it is unpredictable but mixing is more easily achieved. The type of flow that will predominate can be predicted mathematically according to the Reynolds number. In microchannels, such as in pTAS, the Reynolds number is low and so laminar flow dominates. Hence, diffusion in these narrow channels is practically the only way for mixing of the fluids to occur. The smaller the system and the narrower the channels, the faster the diffusion. Under typical conditions, flow rates in the microfluidic channels are about IpL/s. At these 

A difflculty arises from the fact that liquids are typically transferred in microlitre or millilitre volumes, whereas microfluidic devices work with nanolitre or picolitre volumes because of the size of reaction chambers and channels, which typically have microscale dimensions. 

Other possible means of fluid flow include capillary (surface tension), centrifugal -ultrasonic, electromagnetic, electro-hydrodynamic and pneumatic force. 

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