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Across a cell of radius $a$, a pressure gradient $G_i$ changes the viscous shear stress by orderIn the two-dimensional model, the force per unit out-of-plane depth on one side is $O({\sigma }_{i}a)$ and its lever arm is $O(a)$. The net moment per unit depth is thereforeA fully three-dimensional force estimate adds one power of $a$. Since $Q_2>Q_1$, the stronger stress on the high-flow side gives a definite rotation that tips a deformable cell off the ideal point-particle separatrix and into daughter 2. This local finite-size mechanism biases cells toward the higher-flow branch and is consistent with the Zweifach--Fung effect.

The Zweifach--Fung effect, also called plasma skimming, is the tendency at an unequal microvascular bifurcation for the high-flow daughter to receive a disproportionately large fraction of the red blood cells. The low-flow daughter consequently has a lower discharge haematocrit than the parent.