Fundamentals of inertial focusing in microchannels

Abstract

Inertial microfluidics has been attracting considerable interest in recent years due to immensely promising applications in cell biology. Despite the intense attention, the primary focus has been on development of inertial microfluidic devices with less emphasis paid to elucidation of the inertial focusing mechanics. The incomplete understanding, and sometimes confusing experimental results that indicate a different number of focusing positions in square or rectangular microchannels under similar flow conditions, have led to poor guidelines and difficulties in design of inertial microfluidic systems. In this work, we describe and experimentally validate a two-stage model inertial focusing in microchannels. Our analysis and experimental results show that not only the well-accepted shear-induced and wall-induced lift forces act on particles within flow causing equilibration near microchannel sidewalls, but the rotation-induced lift force influences the position of these equilibria. In addition, for the first time, we experimentally measure lift coefficients, which previously could only be obtained from numerical simulations. More importantly, insights offered by our two-stage model of inertial focusing are broadly applicable to cross-sectional geometries beyond rectangular. With elucidation of the equilibration mechanism, we envision better guidelines for the inertial microfluidics community, ultimately leading to improved performance and broader acceptance of the inertial microfluidic devices in a wide range of applications, from filtration to cell separations.

Particle Focusing in Curved Microfluidic Channels

“…For example, these side streaks emerge at certain Re C values that scale with ,l 0.4 (,105 for l 5 0.066, ,140 for l 5 0.149 and ,180 for l 5 0.225). The existence of these side streaks also concurs with the decrease in positive lift coefficient found at increasing Re C [33][34][35] . The formation of these side streaks also indicates the limited distance over which the lateral wall effect forces are dominant.…”

“…It is curious that there seems to be such a large set of operating conditions where this single point focusing behavior is prevalent. Due to the complex positional dependence of inertial lift and Dean drag forces within a single channel cross section 18,33,34 a more complete description of the mechanism behind the complex 3-dimensional motion and the transition from inner to outer focusing requires more precise vertical measurement of particle position or full numerical simulation.…”

Continuous separation of blood cells in spiral microfluidic devices

“…Inertial microfluidics takes advantage of hydrodynamic forces that act on cells to focus them within the flow. 5,6,[15][16][17][18][19][20]38 These forces cause cells to migrate across streamlines and order in equilibrium positions based on their size, leading to label-free cell separation, purification, and enrichment in a microfluidic device. Inertial migration was first observed by Segre and Silberberg 39 in 1960s who experimented with neutrally buoyant particles in capillaries and observed a narrow annulus formation at $0.2D from walls of a capillary of diameter D. This migration behavior is believed to be caused by the balance of lift forces arising from the curvature of the parabolic velocity profile (the shear-induced inertial lift) and the interaction between particles and the channel wall (the wall-induced lift).…”

“…5,6,[15][16][17][18][19] We 24,38 and others 16,17,33,38,42 used straight channels to order cells into multiple equilibration positions for size-based separations and flow cytometry. Although straight rectangular channels can be used to sort multiple cell types in single pass, separating more than two cell types causes drastic reduction in sorting efficiency and throughput, especially in high aspect ratio channels.…”

Integrated optical waveguides and inertial focussing microfluidics in silica for microflow cytometry applications

“…However limited attention has been paid to a suitable microfluidic particle focussing strategy which can be easily fabricated in these materials. One particle focussing strategy which shows promise for integration in glass is inertial particle focussing [18,19]. In this technique, shown in Figure 1, microparticles flowing in a straight, rectangular cross-section channel at a sufficiently high velocity are subjected to lateral lift forces which cause them to migrate across the flow streamlines to equilibrium positions.…”