Process intensification of particle separation by lift force in arc microchannel with bifurcation

https://doi.org/10.1016/j.cep.2009.12.006Get rights and content

Abstract

The present study demonstrates a novel hydrodynamic lift force sieving attained in an arc microchannel with a bifurcation at the downstream end. The fluorescent polystyrene particles with diameter of 10 or 20 μm are dispersed in water or NaCl aqueous solutions so that the particles are completely neutrally buoyant, lighter or denser relative to medium. The slurry is fed into the arc microchannel whose radius, width and depth are 20 mm, 200 and 150 μm, respectively. The fluorescent trajectories of flowing particles are recorded at the bifurcation. It is found that the 20-μm particle is sharply focused to an equilibrium position somewhat distant from the outer wall regardless of the given density difference. As a result, all 20-μm particles report to the outer branch of bifurcation. On the other hand, the 10-μm particles dispersed mostly across the channel width are always recovered from the both branches. The results imply that the arc microchannel with a bifurcation will intensify the process of particle separation since the particles completely neutrally buoyant as well as denser and lighter particles can be simultaneously separated or classified without membrane. Finally, a separation process in a series of arc channels is proposed and the process efficiencies are discussed.

Introduction

Just recently, the issue of separation and purification has entered in the focus of microfluidic investigations, aiming at establishing such micro unit operations for later integration in complete microfluidic systems [1], e.g. for multi-step synthesis [2]. Apart from enabling to perform chemical reactions in this holistic way, functional material synthesis by particle generation has become a major application of microfluidic devices [3], [4]. With the introduction of microfluidic devices for particle/phase separation new paths for applications are opened, in particular in the fields of biological analysis [5] and chemical processes [6]. The developments do not only target screening in pharmaceutical industry and piloting in fine chemistry, but tend also at the large throughputs needed for bulk chemistry [7], [8].

In such a microfluidic device, the intensification of separation function can be considerable as compared to conventional devices owing to the scale effect. For instance, a microvortex can cause a centrifugal separation of particles small in size and density difference with a huge acceleration of 107 m/s2 in a tiny cavity [9]. This is a case of intensifying conventional centrifugation by means of miniaturization. So far, on the other hand, there have been developed many microfluidic devices for particle separation, in which flowing path of particles to be separated is differentiated by physical contact (or strong interaction) between particle and side wall or between particle and objects in microchannel [10], [11], [12]. Such devices often require precise control of flow-rate ratios between branch streams and hence may be sensitive to possible flow-rate disturbance propagated from up- or downstream of the fluidic system. Moreover, particle agglomerates will behave differently from a primary particle in the device due to enlarged size and non-spherical shape leading to non-intentional interaction with the side wall or the micro-objects. The interaction could also foul the device if the dispersed phase is liquid.

Recently, microfluidic devices with curvilinear geometry, such as arc [13], spiral [14], [15] and sinusoidal [16], are paid much attention. Since the particle is focused into an equilibrium position in the curvilinear devices due to a hydrodynamic force balance, the operation is free from such drawbacks due to the wall interaction. Here, it should be mentioned that the separation principle of curved devices is not centrifugation. The equilibrium position can be inside [14], [15] or center [16] against the centrifugal force in the curved channel at lower flow rates. Even when the equilibrium position is somewhat outside of the channel center at the higher flow rates, the separation principle cannot be centrifugation since even a neutrally buoyant particle reaches the equilibrium position against the centrifugal force [13].

The present paper demonstrates a novel hydrodynamic lift force sieving attained in an arc microchannel with a bifurcation at the exit, which was first proposed by Ookawara et al. [17]. It will be shown that, regardless of a given density difference, viz., neutrally buoyant, lighter or denser relative to the continuous phase, all large particles can be recovered from the outer branch while the smaller particles report to the both branches. The results imply that the reflux from outer branch enables hydrodynamic sieving, which can be understood as an effect of lift force based on the previous numerical simulations [18], [19], [20]. Finally, the possible device configuration for a simultaneous operation of purification and condensation processes is developed utilizing the hydrodynamic sieving and the efficiencies will be discussed.

Section snippets

Device fabrication and experimental setup for visualization

Fig. 1 shows (a) a design and (b) a primary component of microdevice examined in this study. A main part of the device is an arc microchannel with an arc angle of 180° and radius of 20 mm. At the upstream arc-end, a straight channel is connected. A symmetrical bifurcation is formed at the downstream arc-end for the purpose of separation. The channel width and depth are 200 and 150 μm from the inlet to the outlet except the downstream width of the outer branch that is doubled to adjust the

Sieving effect induced by lift force in arc microchannel

Fig. 2 shows the fluorescent trajectories of 20-μm particle flowing around the bifurcation, which is (a) lighter, (b) completely neutrally buoyant (CNB), and (c) heavier relative to each prepared medium. The region, where higher fluorescent intensity is observed, looking whiter in the figures, corresponds to the position where more particles pass through. The broken lines in white indicate the channel side-wall. The view of figure is indicated by the box on the bifurcation in Fig. 1(a). The

Conclusions

The present paper has visualized a hydrodynamic lift force sieving attained in an arc microchannel with a bifurcation at the downstream end. Regardless of particle density relative to medium, viz., lighter, completely neutrally buoyant or denser, all the 20-μm particles can be collected from the outer branch while the 10-μm particles are recovered from both the inner and outer branches. The operation associated with reflux from outer branch to inlet enables the hydrodynamic sieving, that is,

Acknowledgement

The present research was partially supported by a Grant-in-Aid for Scientific Research (A) (no. 17206079) provided by Japan Society for the Promotion of Science (JSPS).

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