Doctor of Philosophy
School of Mechanical, Materials and Mechatronic Engineering
Zhang, Jun, Particle inertial focusing and separation in serpentine microchannels: mechanisms and applications, Doctor of Philosophy thesis, School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, 2015. http://ro.uow.edu.au/theses/4492
Inertial microfluidic technology, which manipulates (e.g. focusing, separation, trapping etc.) micro-particles by the intrinsic fluid dynamics and inertial effects, has attracted significant interests over the past decade. Compared with other microfluidic manipulation technologies (e.g. dielectrophoresis, magnetophoresis, acoustophoresis etc.), inertial microfluidics possesses superior advantages including simple structure and extremely high throughput. High throughput is especially useful for the isolation of rare target cells, where large volume of biological sample needs to be processed in a short time. The functionality of inertial microfluidics relies on the inertial migration phenomenon and/or curvature-induced secondary flow.
To date, two major classes of curving channels in inertial microfluidics have been presented, including 1) spiral channels, with consistent direction of curvature, and 2) serpentine channels, with alternating curvature. In spiral channels, curvature is along one direction, where secondary flow reaches steady state after long enough distance, and becomes almost uniform within each cross-section. So analysis of inertial focusing in spiral channels can be simplified by static superposition of inertial lift force and secondary flow drag force within a typical channel cross-section. Therefore, its mechanism has been extensively investigated and uncovered. However, in serpentine channels with alternating curvatures, the mechanism becomes more complex and has been rarely reported. For example, with alternating curvatures, secondary flow may not approach steady state at each curvature, and accumulation of this unsteady induces non-intuitive and unpredictable phenomenon. Furthermore, the varying secondary flow in the cross sections of alternating curvatures makes the inertial focusing process more complicated.
The main objective of this research is to conduct a comprehensive investigation on the inertial focusing and separation in symmetric serpentine channels, including mechanism and applications. In our work, three basic focusing patterns were observed: (i) single focusing streak at the channel centre; (ii) two focusing streaks at two sidewalls; and (iii) transition patterns. The mechanism of these three focusing patterns were studied and discussed. Disagreed with previous argument that single focusing streak cannot be obtained in symmetric serpentine channels due to the complete counteraction of symmetric secondary flow during opposite curvatures, we verified that sufficient single focusing streak could be achieved in symmetric serpentine channels. Through numerical simulation, analytical analysis and experimental validation, the positive effects of Dean flow on particle focusing were uncovered, besides its well-known mixing effects by the counter-rotating streamlines. This finding extends our understanding on the effects of Dean flow on the particle focusing in the curved channels, revealing the new possible explanation to uncover other non-intuitive inertial focusing phenomenon.