Enhanced size-dependent trapping of particles using microvortices

Abstract

Inertial microfluidics has been attracting considerable interest for size-based separation of particles and cells. The inertial forces can be manipulated by expanding the microchannel geometry, leading to formation of microvortices which selectively isolate and trap particles or cells from a mixture. In this work, we aim to enhance our understanding of particle trapping in such microvortices by developing a model of selective particle trapping. Design and operational parameters including flow conditions, size of the trapping region, and target particle concentration are explored to elucidate their influence on trapping behavior. Our results show that the size dependence of trapping is characterized by a threshold Reynolds number, which governs the selective entry of particles into microvortices from the main flow. We show that concentration enhancement on the order of 100,000× and isolation of targets at concentrations in the 1/mL is possible. Ultimately, the insights gained from our systematic investigation suggest optimization solutions that enhance device performance (efficiency, size selectivity, and yield) and are applicable to selective isolation and trapping of large rare cells as well as other applications.

Vortex-aided inertial microfluidic device for continuous particle separation with high size-selectivity, efficiency, and purity

“…These results are 2 Â higher than previously-reported results in non-continuous vortex-trapping devices. 15,16,28 This high efficiency comes from two factors: (1) design of the focusing channel length that enables equilibration of target particles into two bands which is favorable for vortex capture; 15 and (2) The inducing of the sheath flow shifts the separation boundaries (d b ) closer to the particle focusing position (d p ) indicating shorter migration distance for isolation. In addition to high separation efficiency, there is no limit on trapping capacity due to the continuous release from the chambers.…”

“…The focusing channel is 10 mm long with a 50 lm  100 lm (w  h) cross-section; each capture chamber is 500  500 lm 2 . 15,29 The length of outlet channels can vary to modulate fluidic resistance ratio (r/R) of the side (r) and main (R) outlets for optimizing the device performance. To understand the design principle, we will first briefly review our two-stage inertial focusing model 29 and then discuss each system component in detail.…”

“…The formation of the vortex creates a virtual wall that reduces the wall-induced lift force F w , leading to particle lateral migration into sheath flow undergoing the shear-gradient induced lift force F s . 15,16 Asmolov 30 reported the shear-gradient induced lift force scaling as F s / C L a 4 , where C L is the size-dependent lift coefficient. Numerical work by Loth and Dorgan 31 suggested lift coefficient scaling as C L / a À2 , which was confirmed experimentally in our recent work.…”

“…5,6 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 with designed geometries. 7 Applications for separation of cells (erythrocytes/leukocytes, [8][9][10] neuronal cells, 6 cancer cells 11 ), flow cytometry, [12][13][14] and rare cell enrichment [15][16][17] have been developed achieving passive cell and particle manipulation with extremely high throughput.…”

“…In a parallel independent work, we reported size-based selection of particles in rectangular channel expansions. 15 These devices offer multiple promising potential applications, including extraction of plasma from blood and isolation of rare cells. While these devices are successful in capturing target cells at ultra-low concentrations ($100 cells/ml), their non-continuous two-step operation is complicated and requires a complex fluidic setup, with the release/washing step inevitably decreasing (diluting) the capture efficiency.…”