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Continuous particle trapping, switching and sorting utilizing a combination of dielectrophoresis and alternating current electrothermal flow Haizhen Sun, Yukun Ren, Likai Hou, Ye Tao, Weiyu Liu, Tianyi Jiang, and Hongyuan Jiang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05861 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019
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Analytical Chemistry
Continuous particle trapping, switching and sorting utilizing a combination of dielectrophoresis and alternating current electrothermal flow Haizhen Suna, Yukun Ren*ab, Likai Houa, Ye Taoa, Weiyu Liua, Tianyi Jianga and Hongyuan Jiang*ab a
School of Mechatronics Engineering, Harbin Institute of Technology, Harbin, Heilongjiang, PR China 150001
State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin, Heilongjiang, PR China 150001 b
ABSTRACT: We propose a simplified multifunctional traffic control approach that effectively combines dielectrophoresis (DEP) and alternating current electrothermal (ACET) flow to realize continuous particle trapping, switching and sorting. In the designed microsystem, the combined DEP and ACET effects, which are symmetrically generated above a bipolar electrode surface, contribute to focus the incoming colloidal particles into a thin beam. Once the bipolar electrode is energized with an electric gate signal completely in phase with the driving alternating current (AC) signal, the spatial symmetry of the electric field can be artificially reordered by adjusting the gate voltage through field-effect traffic control. This results in a reshapable field stagnant region for precise switching of particles into the region of interest. Moreover, the integrated particle switching prior to the scaled particle trapping experiment is successfully conducted to demonstrate the feasibility of the combined strategy. Furthermore, a mixture of two types of particle sorting (i.e. density, size) with quick response performance is achieved by increasing the driving voltage with a maximum gate voltage offset, thus extending the versatility of the designed device. Finally, droplet switching and filtration of the satellite droplets from the parent droplets is performed to successfully permit control of the droplet traffic. The proposed traffic control approach provides a promising technique for flexible manipulation of particulate samples and can be conveniently integrated with other micro/nanofluidic components into a complete functional on-chip platform owing to its simple geometric structure, easy operation and multifunctionality.
Development of microfluidic traffic-flow is of great importance in a variety of chemical and biological applications ranging from diagnostic to therapeutic practices.1-3 Particle manipulation in the micro-traffic system represents an essential step prior to any subsequent analysis for concentrating, switching, sorting and detecting samples, such as droplets, cells and colloids.4-6 Hence, incorporating multiple functions on a single platform allows faster analysis and the possibility to perform multiple continuous processes running in parallel to accomplish all tasks with reduced cost. Therefore, researchers have searched for possible techniques that extend the versatility and feasibility of microsystems.7 In the microfluidic traffic system, continuous particulate samples trapping, precise position switching and highresolution sorting are always vital for the performance of the subsequent process steps.8-10 Various techniques have been exploited by flexibly controlling the particle traffic flow in microchannels either passively or actively. In general, passive manipulation of particles exerts internal hydrodynamic forces on samples, which tends to depend on the flow conditions and the geometry of the
microchannel.11-13 Accordingly, such a control is relatively simple but less versatile. By contrast, active manipulation techniques, such as dielectrophoresis,14 optophoresis,15 magnetophoresis,16 and surface acoustic waves,17 typically need only a simple microstructure to work, but require an external integration of powered components. Importantly, electrokinetic techniques, including electroosmotic flow (EOF),18 induced charge electroosmosis flow (ICEOF),19 alternating current electrothermal (ACET),20 electrophoresis (EP) and dielectrophoresis (DEP),21 are readily available ways to manipulate particles and fluids in microfluidic chips. In particular, the sample manipulation principle for the EOF, ICEOF and ACET approaches is that the electric field first acts on the fluid, which results in bulk motion in the channel, and then the fluid indirectly drags particles to the desired region. In contrast, the momentum induced by EP and DEP directly drives the particles moving in the medium. Fu et al.22 and Pan et al.23 exploited EOF to drive a microfluid to a desired region. However, because a relatively high-voltage direct current (DC) signal was applied to the exciting electrodes and additional external
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pumps were also used to provide the sheath flow, it would be difficult to integrate EOF with other devices. Ren et al. demonstrated particle trapping on a wireless electrode with ICEOF , but the critical dependence on the buffer composition (very low conductivity) would hinder the manipulation of cells.24 In addition, EP merely affects the translational movement of insulating charged particles in steady DC fields, which limits the feasibility of samples.25 Moreover, both ACET and DEP can work in a medium with high conductivity.26-28 DEP has been demonstrated to be an effective technique for repetitive particles/cells trapping,2930 switching31 and sorting,32-33 and ACET occurs concurrently with DEP at high medium conductivities and high frequencies and can exert significant viscous drag forces on particles, thus affecting their transport patterns inside a nonuniform electric field.34-36 Heeren et al. demonstrated that DEP is a short-range interaction; hence, only the particles in the immediate vicinity can be influenced and fixed,37 while ACET exerts a body force density to the fluid bulk and obeys the no-slip boundary condition, which implies that particles depositing on the planar electrode surface would not be effectively affected by the ACET fluidic drag. In these cases, the combination of DEP and ACET is expected to be able to affect particles suspended throughout almost the entire channel. Herein, we developed a unique microfluidic traffic-flow control system which effectively combined DEP and ACET effects to realize particle trapping, switching and sorting. In the proposed platform, particles can be continuously focused into a single or two scaled beams following bipolar electrode patterns with a synergetic combination of DEP and ACET. The resultant beam position can be controlled by regulating the gate voltage, yielding precise switching of particles into different downstream branch ports. By increasing the applied voltage to a certain value, a sufficient ACET-based fluidic force will be generated to carry the light particles away from the stagnant region, but will not be sufficient to lift the heavy particles which would be stably trapped in the stagnant region, thereby realizing a continuous particle sorting into the outlet of interest. DEVICE AND METHODS Device design and preparation. The device used in our experiments is expected to realize three functions: particle trapping, particle-beam switching and particle sorting. In order to focus particles into a thin beam, a bipolar electrode was inserted in midchannel to weaken the chaining phenomenon resulted from particle dipoledipole interaction. The position of the resulting particle beam can be flexibly adjusted by energizing the bipolar electrode with a regulatable gating ac signal, thereby enabling multichannel particle switching (Figure 1(c)). By increasing the ac voltage applied to the bipolar electrode, it can also sort particles with distinct properties (i.e. density, size) into individual outlet (Figure 1(b)). The specification of characteristic dimensions of the device employed in the study is shown in Figure S2. Details of the processing steps of the chip are given in previous reports.38
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The theoretical basis and 3-D simulation model are described in detail in SI (Section S2).
Figure 1 A schematic illustration of configuration of the experimental system. (a) A three-dimensional schematic of the microfluidic device. (b)-(c) Particle behaviors above an electrode: particle switching and sorting simultaneously occur at a specified condition. A1 and A2 represent the voltage amplitudes, ω is the angular frequency of the signals, and α is the initial phase of the applied ac signal. (d) A photograph of the device. RESULTS AND DISCUSSION Continuous particle trapping. Negative DEP repels particles near the edge of the electrode to the center of the main channel, and this effect decays rapidly away from the electrode edge owing to its dependence on the gradient of the square of the electric field. The ACET flow also transports particles toward the channel center owing to the formation of a stagnant region with a symmetric microvortex pair in the bulk solution. The ICEO effect can be neglected owing to the high conductivity (0.1 S/m in our experiment) and higher frequencies have been used previously.39 Thereby particles suspended in the whole channel will be stably focused into a beam by the combination of DEP and ACET in the microsystem. The effect of a bipolar electrode in the electrolyte solution has been reported in many studies, especially in induced charge electroosmosis,40 electrochemical reactions,41 and dielectrophoresis.42 When there is no bipolar electrode, the electric field lines go directly from positive to negative terminal, thus polarized particles in suspension aligned along the electric field direction attract one another to assemble into arrays of particle chains. However, both the electric potential and direction of electric field were changed once a bipolar electrode was inserted at the bottom of the channel, as shown in Figure 2(a). In this situation, the electric lines were perpendicular to the bipolar electrode surface, resulting in a weak induced interparticle dipolar interaction. Additionally, the electric field and its gradient were simultaneously enhanced because of the bipolar electrochemistry (Figure 2(b)). To predict the velocity profiles induced by DEP and ACET, we used a coupled electricity-thermal-fluid model (Figure S3). Figure 2(c)-2(f) represents a typical transverse velocity distribution resulting from the 3D simulation. As 2
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Analytical Chemistry
shown in Figure 2(e) and 2(f), a synergetic stagnant region was formed at the electrode center induced by DEP and ACET simultaneously, in which particles could be trapped. To assess the focusing effects of the two forces, the velocity profiles at a height of 20 μm were obtained from the simulation results (Figure 2(c) and (d)). Figure 2(c) shows that both the DEP and ACET induced velocities gain much higher value owing to the enhanced performance by the bipolar electrode (Figure S6), and reached the maximum magnitudes in the vicinity of the bipolar electrode edges. Obviously, the effective interaction-range of the ACET flow is wider than DEP action, indicating that ACET flow dominated particle motion at a position far from the
electrode with a negligible DEP force (Figure S5). Figure 2(d) shows that the driving voltage magnitude plays an essential role in AC electrokinetics induced particle motion performance because it significantly affects the DEP and ACET simultaneously. However, the ACET velocity increased more dramatically than the DEPinduced velocity with respect to the increment of AC voltage magnitude, because it is proportional to the fourth power of the electric field (vacet ~ E4), whereas DEP induced velocity grows quadratically with the electric field (vdep ~ E2). As a consequence, the bipolar chemistry, DEP force and ACET flow are expected to synergistically enhance the particle trapping.
Figure 2 Numerical comparison results between with-bipolar electrode and without-bipolar electrode. (a) Surface potential on the bottom of the main channel and the electric-field distribution in the main channel (the spheres represent polarized particles in suspension). (b) Changes in the electric field and its gradient at a height of 20 μm. (c) Comparison study of the transverse velocity between the two electrode configurations, vacet represents the ACET flow velocity, and vdep represents the DEP induced velocity. (d) The changes of maximum velocity with the applied voltage. (e) ACET-based microvortex pair and the temperature field due to Joule medium heating with a bipolar electrode. (f) Surface and arrow plots of the DEP velocity field. To quantify the trapping performance with the proposed method in the experiment, a silica particle was used as an experimental sample, and the beam width ratio WR has been introduced to assess the focusing performance. WR = S/W3 (7) Where S is the width of the focused particle beam, and W3 is the width of the floating electrode. When the driving electrode is energized by an appropriate AC signal, a relatively fast induced velocity at the edges of the floating electrode transports particles to the central stagnant region, which enables particle focusing (Figure 3(b)). The width ratios increase monotonically as the field frequency increases at a particular field intensity (Figure 3(c)), and decrease with increasing voltage amplitude at a specific frequency (Figure 3(d)). As the frequency increases (or the voltage decreases), the particle beam tends to further broaden owing to the wider stagnant region
forming along the electrode centerline as a result of the bulk charge relaxation (or reduced smeared structural polarization).43-44 The width ratio is also affected by the inlet flow rate. When the flow rate increases, the width ratio decreases because of the reduced action time at a same axial distance. The width ratio increases slowly with increasing the frequency without a bipolar electrode because of reduced dipole-dipole interactions at higher frequencies, which result from a decreased value of CM factor (Figure S1), The beam width becomes narrower than that with a bipolar electrode at frequencies higher than 4 MHz owing to the stronger field intensity right above the centerline of the channel bottom surface in the absence of a gate electrode strip. The width ratio decreases to 0.11 (about 16.5 μm) above the bipolar electrode surface with increasing the applied voltage to 20 V at a flow rate of 0.9 μL/min and a frequency of 1 MHz. 3
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Figure 3 Experimental results of focusing effects. (a) Strong induced interparticle dipolar chaining without bipolar electrode. (b) Weak induced interparticle dipolar interaction above a bipolar electrode. (c) Frequency dependence of the width ratio at a voltage of 14 V and an inlet flow rate of 0.9 μL/min in aqueous solutions with a conductivity of 0.1 S/m. (d) Voltage dependence of the width ratio at a frequency of 1MHz. motivated us to adjust the bipolar electrode potential by Precise particle-beam switching. When a bipolar actuating the bipolar electrode with another AC signal to electrode is symmetrically disposed along the channel serve as a gate voltage. This was expected to break the centerline, it serves as an equipotential body with a natural spatial symmetry of the DEP velocity field and ACET vortex induced voltage, and particle trapping occurs above the pair to remold the stagnant region away from the central bipolar electrode center. This interesting mechanism line, and thus control the particle switching behaviors.
Figure 4 Numerical simulation analysis on combined field-effect control with biased gate voltage at 5 V, 10 V, 15 V respectively. (a) Simulation results of electric field and surface plots of DEP velocity field. (b) Simulation plots of the ACETbased microvortex pair and isotherm distribution. (c) Comparisons between ACET flow field and DEP velocity field at various gate voltages. As the bipolar electrode is energized with an AC signal, various field variables, including electric field, flow field and DEP velocity field, are all changed, and these changes can be flexibly controlled by adjusting the gate voltage. Figure 4(a) shows the variation tendency of the electric field by regulating the gate voltage A2 to obtain dissimilar types of gate electric polarity at a driving AC signal of A1=20 V and f=1 MHz. It is clearly that the electric field was arbitrarily altered, which was determined by the effective gate voltage difference with respect to the referential floating potential. If the effective gate voltage A2 was equal to the natural floating potential A1/2, there was no gate voltage offset (zero gate electric polarity), thus the electric
field was symmetrically distributed in the transverse direction of the main channel. Once the effective gate voltage was biased higher than (A2>A1/2 for positive gate electric polarity of bipolar electrode) or lower than (A2
