Comment on "Pinched flow fractionation: continuous size separation of particles utilizing a laminar flow profile in a pinched microchannel".

In a recent paper Yamada et al. [1] propose the novel concept of ”pinched flow fractionation” (PFF) for the continuous size separation and analysis of particles in microfabricated lab-on-a-chip devices. In their description of the basic principle they claim that especially the width of the pinched and broadened segments will affect the separation performance. In the following we comment on the physics behind this statement. Considering the steady-state laminar flow of an incompressible Newtonian liquid, the flow field v is governed by Stokes equation