Why is sheath fluid applied in dynamic image analysis when using the wet method?

In dynamic image analysis, when using the wet method, sheath fluid is used to provide a stretching and focusing force on the sample flow. In the presence of the sheath fluid, the sample flow is “locked” on the focal plane of the camera, with which the sample particles can be clearly captured, ensuring the accuracy of particle size and shape analysis. Suppose that the sheath fluid is not utilized, blurred particle images will be caused due to the defocussing of the sample flow,  leading to an incorrect particle identification and analysis result.

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