How is the sample stream focused in a flow cytometer?

An outer sheath fluid is used in a flow cytometer to force the cells in the core stream to travel as single cells at the same speed. This technique is called hydrodynamic focusing. The sheath stream is usually just a saline solution, which runs at a fast flow rate in the laminar flow manner. The laminar flow maintains the core stream and makes sure that the core stream and the sheath fluid do not mix. Once the cells are injected, they are focused into the tighter core stream, travelling along the same axis at approximately the same speed.