Particles Separator Based on Inertial Force

Activities

While in Year 1 to Year 2 we demonstrated a standing surface acoustic wave (SSAW) based particle separator, there are two drawbacks: 1) low throughput because the channel size has to be comparable to the wavelength, and 2) relatively large power consumption (up to 0.5W) for applying RF excitation signals. To overcome the drawbacks, in Year 3 and Year 4 we fabricated and tested a new prototype particle separator based on inertia force. In comparison to the SSAW separator, it has higher throughput because the channel size can be made much larger; the power consumption is low because the particle separation is based upon inertial forces created by microchannels, with no need for applying an external force filed.

1.1 Particle Separator Design and Fabrication

Fig. 1. Illustration of working mechanism of particle separation in the microchannel with a series of repeated symmetric sharp corner structures. (a) Dominant forces exerted on the suspended particles. (b) 3-D illustration of particle separation in the microchannel

The device design is shown in Figure 1, consisting of a series of repeated symmetric sharp corner structures. Initially, particles are uniformly distributed at the inlet. In the expansion structure of the microchannel where the streamline changes direction smoothly, a particle in suspension is primarily subjected to the shear-gradient lift force and the wall-effect lift force; the net force of the two forces is the inertial lift force which dominantly drives micro particle toward two equilibrium positions near the walls of the microchannel. On the other hand, in the sudden contraction flow region induced by symmetric sharp corner structures, streamlines are dramatically curved. In addition to the inertial lift force, particles are also subjected to the centrifugal forces (Fc) because of the change of motion directions. The trajectories of suspended micro particles in the microchannel are determined by the competition between the inertial lift force (Fi, driving particles towards the side equilibrium positions near the walls) and the centrifugal force (Fc, driving particles towards the centerline). At appropriate flow rates, the centrifugal force becomes large due to high flow velocity; particles will be centrifuged to the center of the channel. At the same flow rate, the centrifugal forces on large particles are stronger than those on small particles. Hence large particles are driven to the center at a lower flow rate while small particles start to migrate to the center at a higher flow rate when the centrifugal forces are sufficient. Therefore at appropriate flow rates, large particles are eventually focused at the center of the microchannel and small particles are focused in the two side equilibrium positions.

1.2 Separator Fabrication

The microchannel was fabricated using a standard soft lithography technique. First, SU8-2075 photoresist (MicroChem Inc., USA) was spin coated on a silicon wafer with a thickness of 50μm. Then photolithography was applied to SU8 layer to form the microchannel mold. 10:1 PDMS (Sylgard 184, DowCorning, USA) was poured over the mold, degassed and cured, to transfer the patterns onto the PDMS layer. Next, the surfaces of the PDMS layer and a pre-cleaned glass slide were treated in air plasma for 40 seconds at 100W (Harrick PDC-32G). Finally, the device was completed by bonding the PDMS layer and a pre-cleaned glass slide.

1.3 Separator Testing

Fluorescent polystyrene particles 7.32μm (FS06F/9559, Dragon green, 480/520nm) and 15.5μm (FS07F/9277, Dragon green, 480/520nm) in diameter were used for the particle separation experiments (Bangs Laboratories, Inc., USA). The densities of both polystyrene particles are 1.062g/cm3. The standard deviation in diameter distributions is 7.2% and 9.8%, respectively. Two particle solutions were individually prepared with 7.32 and 15.5μm particles. The 7.32μm particles were suspended in DI water with a solid concentration of 3.5-5.5×103 particles per μL while the 15.5μm particles were suspended in DI water with a solid concentration of 0.3-0.5×103 particles per μL. 0.5 wt% of detergent/surfactant (Tween 20; Sigma-Aldrich Co., USA) was added into the particle solutions to avoid aggregation of particles. Next, the ultrasonic bath was used to sufficiently disperse the particles in suspension for at least 30min.

The solution prepared with 7.32μm fluorescent polystyrene particles was first injected into the inlet of the microchannel using a syringe pump (KDS LEGATO270; KD Scientific Inc., USA) equipped with a 10mL BD syringe at flow rates ranging from 40 to 160μL/min. The syringe was connected to the microchannel with a PEFE tube. An inverted optical microscope (IX-71, Olympus Co., Japan) was used to observe the migration of particles with a fluorescence mirror unit (U-MNB2, Olympus Co., Japan). Images of particle trajectories were captured with a CCD camera (Qimaging fast 1394, Qicam, USA). Similar processes were conducted on the 15.5μm fluorescent polystyrene particles solution.

The acquired images of 7.32 and 15.5μm particles were individually analyzed with ImageJ software (NIH, USA) and Adobe Photoshop (Adobe Systems Inc. USA). Because the 7.32 and 15.5μm particles fluorescent particles had the same florescence color (Dragon green), to differentiate 7.32 and 15.5μm particles, ImageJ software was used to add cyan florescence to the images of 7.32μm particles and green florescence to the images of 15.5μm particles. To analyze the florescence intensity, first the threshold was set for each image in ImageJ. Then threshold images were converted to the binary ones. Finally, the fluorescence intensity of particles was quantified with the menu command Analyze. Experiments were also conducted by mixing the 7.32 and 15.5μm particles in bright light field at selected flow rate to demonstrate the particle separation.

Findings

Migration behaviors of two-size particles (7.32μm and 15.5μm ) were tested as various flow rates (from 40 to 160μL/min). To show the device’s feasibility for two-size particle separation, distributions of 7.32 and 15.5μm particles at outlet at various flow rates were captured by a fluorescence microscope and are shown in Figure 2.

Fig. 2. (a) Fluorescence distribution images of 7.32μm and 15.5μm particles at various flow rates to show the feasibility of particle separation (cyan florescence for 7.32μm particles, green florescence for 15.5μm particles). (b) Bright-field image of separation of the two-size particles at a flow rate of 140μL/min.

Figure 2 shows at low flow rates (40 to 120μL/min) 7.32 and 15.5μm particles could be partly separated at outlet of the present microchannel. Complete particle separation can be achieved at the flow rate of 140μL/min, where all 15.5μm particles were focused at the center while the 7.32μm particles were focused at the two side streams. However, at high flow rates (>140μL/min), complete two-size particle separation is infeasible because part of the 7.32μm particles were focused at the center of the microchannel.

To quantitatively analyze the particle stream band for 7.32 and 15.5μm particles at outlet, we calculated the fluorescence intensity (FI) of the two-size particles at various Reynolds numbers from images shown in Figure 3 using ImageJ software. It is obvious that the present microchannel could tightly focus particles of different sizes in narrower streams than the prior Multi Orifice Flow Fraction (MOFF) microchannel. Furthermore, 15.5µm and 7.32µm particle streams were distinctively separated at Re=51.8 (140µL/min). The separation distance between the central particle stream (15.5µm) and side particle streams (7.32µm) in our microchannel is approximately 140μm at the outlet. In comparison, in the MOFF microchannel, different-size particle streams were partly overlapped with each other. The result demonstrated that using the microchannel with a series of symmetric sharp corner structures, two-size particles can be completely separated and be collected at the outlet of the microchannel.

While due to experimental difficulty and safety issues we did not conduct particle separation experiments in lubricant oil at operating conditions (~120°C), our analysis showed that it is potentially possible to use the separation to separate different-sized wear debris in lubricant oil at the operating temperature; such a separation device will cause improvement in the sensitivity.

The testing results demonstrated that the inertia force based passive microfluidic particle separator can be used for complete separation of micro particles, with no need for applying external force. The particle stream width is narrower; the throughput is larger than that the multi orifice flow fraction separator and the standing surface acoustic wave (SSAW) particle separator. In addition, because there is no need for applying an external force field, the power consumption is negligible. With these advantages, we believe this particle separator can be used for many particle detection sensors such as oil debris sensor and cell flow cytometer.