Opposing shear-induced forces dominate inertial focusing in curved channels and high Reynolds numbers

Abstract

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

High-Efficiency Small Sample Microparticle Fractionation on a Femtosecond Laser-Machined Microfluidic Disc

“…On the other hand, a focusing efficiency of 91.39% was achieved for the 5 µm particles. Increasing the rotational speed of the microfluidic disc to achieve higher efficiency for smaller particles might not be the best approach, since glass microfluidic discs are brittle and might break when subjected to high centrifugal force; furthermore, higher flow rates causes the emergence of a second velocity maximum point, causing multiple particle focusing positions [58], while, in other cases, increasing the flow rate doesn't cause a change in the particle equilibrium position. A new design with smaller features would be a good approach to specifically target particles smaller than 5 µm in diameter.…”

High-Reynolds Microfluidic Sorting of Large Yeast Populations

“…These forces increase as a function of particle diameter to the 2 nd power 20 , 21 , and thus predominantly work on smaller particles.

Figure 2 Microfluidic high Reynolds inertial sorting ( A ) Analytical calculation of shear-induced lift forces acting on 15 μm particle, in a horizontal cross-section of a rectangular channel, as previously shown 14 . Stable equilibrium points are indicated by black arrows.

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“… Microfluidic high Reynolds inertial sorting ( A ) Analytical calculation of shear-induced lift forces acting on 15 μm particle, in a horizontal cross-section of a rectangular channel, as previously shown 14 . Stable equilibrium points are indicated by black arrows.…”

“…Microfluidic cell isolation using inertial focusing is an appealing process due to its low cost and continuous operation of the disposable polydimethylsiloxane (PDMS) chip 10 , 14 . However, current approaches predominantly sort particles in low flow rates, increasing the risk of clogging and resulting cross-contamination.…”

“…Recently, we showed that large particles are pushed to the concave edge of curved channels in high-Reynolds flow due to opposing shear-induced lift forces 14 . The phenomenon permits a 100-fold miniaturization of microfluidic devices, operating at high flow rates thereby eliminating clogging while increasing throughput.…”