Microfluidic volume definitions
Most of the time, it is useful to know the total volume of one’s fluidic circuit. Volumes in microfluidics can be different from other areas. The chip has its own volume, tubing has an internal volume. Fittings like unions, adapters or tees can have an enclosed volume that will contribute to the total volume of a system. This is referred in specification sheets as “internal volume”.
The internal volume is the sum of two different volumes: the “swept volume” and the “dead volume”.
Microfluidic swept volume
The swept volume is the portion of the internal volume that is directly in the flow pathway: fluids are bound to flow through this volume when flowing through the fitting. It is generally best to have this swept volume as low as possible.
Microfluidic dead volume
The dead volume is the portion of the internal volume that is out of the flow path. It means that the liquid that will go in this area – or the molecules that will diffuse there may not be recovered, or may be recovered later on. It is a kind of “buffer tank”. Of course, all manufacturers of microfluidic parts try to minimize the dead volumes in their fluidic products. It can be an issue when several samples need to flow through the same path but mustn’t contaminate one another.
Microfluidic internal volume
The dead volume and the internal volume have to be differentiated, as it is sometimes the case. The internal volume is the sum of the swept and the dead volumes.
If the connection with the tubing is not optimized, an additional internal volume can be created. To help the liquid to flow directly and completely into the chip from the tubing, ensure that all tubing is fully seated and tightened at all times. A good practice is also to try to match the tubing ID as closely as possible to the diameters of the inlet/outlet ports of one’s chips.