In microbiology, hydrodynamic focusing is a technique used to provide more accurate results when using flow cytometers or Coulter counters for determining the size of bacteria or cells.
[1] Cells are counted as they are forced to pass through a small channel (often referred to as a flow cell), causing disruptions in a laser light beam or electricity flow.
The standard channel size used in most production flow cytometers is 250 by 250 micrometers.
A wide (hundreds of micrometers in diameter) tube made of glass or plastic is used, through which a "wall" of fluid called the sheath flow is pumped.
If the two fluids differ enough in their velocity or density, they do not mix: they form a two-layer stable flow.