Experimental results and theoretical analysis are presented concerning transverse hydrodynamic focusing of erythrocytes in a narrow channel flow of diluted suspension of cells. The characteristic width of plane and round channels was 40?500 microns, flow velocity at the channel axis was 1?10 cm/s, channel Reynolds number was 0.5?50. Transverse profiles of erythrocytes concentration in a flow were registered using integral Doppler anemometry. Two patterns of focusing the erythrocytes were observed, namely, non-central and axial focusing. A reversible transition between these two focusing regimes was possible under variation of flow velocity. A phenomenological theory of hydrodynamic focusing of deformable particles was developed neglecting particles diffusion. A general-type analysis of transverse focusing of erythrocytes was done in connection with the flow velocity, channel width, cell size, and erythrocyte deformability index averaged over cell orientations in a flow. It is shown that under small flow velocities the axial focusing of erythrocytes occurs even in comparatively wide channels, which is associated with high deformability of erythrocytes. Under flow velocities exceeding some critical value, the axial focusing regime changes for the noncentral one.
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