Hydrodynamic Focusing

Most flow cytometry measurements are made optically, and it is important to keep particles well positioned in the flow stream in order to make accurate optical measurements. Flow cy-tometers use a fluidic method called hydrody namic focusing to control the position of particles in the flow (Fig. 1.2.2). In this technique, a flow of carrier fluid, called the sheath fluid, is established in the cytometer. The sheath fluid, which is usually normal physiological saline with perhaps a few additives, originates from a supply tank under pressure and flows through tubing to a sensing region where the detectors are located.

Just before its arrival at the sensing region, the sheath fluid flows into a chamber of relatively large diameter, then out through a tapered conical section that reduces the diameter of the flow to the dimensions of the sensing region.

The sample-containing fluid is introduced into the middle of this chamber through a tube positioned on the central axis of the sheath flow. Under laminar flow conditions, the sample and sheath fluids do not mix but join together to form a coaxial flow. This combined flow then passes through the tapered section, which reduces the diameter (and increases the velocity) of both the sheath and sample flows simultaneously before they reach the sensing region. This technique confines the cells to a very narrow central core so that the path cells follow through the sensing region is very consistent. The central area of combined flow that originated from the sample flow is called the sample core.

It should be noted that the size of the sample

(f sample sheath pressure sheath fluid __►_Pi r hydrodynamic / (Ô)-^ |"| focusing section1

sheath tank

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