"The “flow cell” is the place in the cytometer where the sample stream is injected into the sheath stream. After joining the sheath stream, the velocity of the cell suspension (in meters per second) either increases or decreases so that it becomes equal to the velocity of the sheath stream. The result is that the cross-sectional diameter of the core stream containing the cells will either increase or decrease to bring about this change in the velocity of flow while maintaining the same sample volume flow rate (in mL/s). The injection rate of the cell suspension will, therefore, directly affect the width of the core stream and the stringency by which cells are confined to the center of the illumination beam."

How is it possible that after joining that both the streams don't mix when they velocities are equal?

As it stated above the diameter is altered so that both have same velocity but then are the both streams in contact with each other?

This is achieved my laminar flow? how is that possible?