What component surrounds the sample stream in hydrodynamic focusing?

In hydrodynamic focusing, the sample stream is surrounded by sheath fluid. This configuration is essential for ensuring that the sample is propelled through the flow cytometer in a narrow stream, allowing for precise measurement and analysis of the individual particles or cells contained within the sample. The sheath fluid streamlines the sample, reducing the spread and ensuring that the cells or particles are aligned in a single file as they pass through the detection area.

The proper functioning of hydrodynamic focusing relies on the differential flow rates of the sheath fluid and the sample, which helps to minimize the impacts of any cellular interactions that could cause clumping or scattering, thus enabling accurate cytometric assessments. This method is a fundamental principle behind many flow cytometers, optimizing the flow dynamics necessary for precise data collection.