The purpose of the fluidics system is to transport particles in a fluid stream to the laser
beam for interrogation. For optimal illumination, the stream transporting the
particles should be positioned in the center of the laser beam. In addition, only one
cell or particle should move through the laser beam at a given moment.
To accomplish this, the sample is injected into a stream of sheath fluid within the flow
chamber. The flow chamber in a benchtop cytometer is called a flow cell and the flow
chamber in a stream-in-air cytometer is called a nozzle tip. The design of the flow
chamber causes the sample core to be focused in the center of the sheath fluid where
the laser beam will then interact with the particles.
Based on principles relating to laminar flow, the sample core remains separate but
coaxial within the sheath fluid. The flow of sheath fluid accelerates the particles and
restricts them to the center of the sample core. This process is known as
hydrodynamic focusing. For an illustration of hydrodynamic focusing in each type of
flow cell, see Figure 2-1 and Figure 2-2.