Inertial focusing is the migration of particles in fluid toward equilibrium,where current theory predicts that shear-induced and wall-induced lift forcesare balanced. First reported in 1961, this Segre-Silberberg effect isparticularly useful for microfluidic isolation of cells and particles.Interestingly, recent work demonstrated particle focusing at high Reynoldsnumbers that cannot be explained by current theory. In this work, we show thatnon-monotonous velocity profiles, such as those developed in curved channels,create peripheral velocity maxima around which opposing shear-induced forcesdominate over wall effects. Similarly, entry effects amplified in high Reynoldsflow produce an equivalent trapping mechanism in short, straight channels. Thisnew focusing mechanism in the developing flow regime enables a 10-foldminiaturization of inertial focusing devices, while our model correctslong-standing misconceptions about the nature of mechanical forces governinginertial focusing in curved channels.
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