Flexible continuous particle-beam switching via external-field

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Flexible continuous particle-beam switching via external-fieldreconfigurable asymmetric induced-charge electroosmosis Haizhen Sun, Yukun Ren, Weiyu Liu, Xiangsong Feng, Likai Hou, Ye Tao, and Hongyuan Jiang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b02332 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

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Analytical Chemistry

Flexible continuous particle-beam switching via external-fieldreconfigurable asymmetric induced-charge electroosmosis Haizhen Suna, Yukun Ren*ab, Weiyu Liuc, Xiangsong Fenga, Likai Houa, Ye Taoa 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 c School of Electronics and Control Engineering, Chang’an University, Middle-Section of Nan’er Huan Road, Xi’an 710064, Shaanxi, P. R. China b

ABSTRACT: Continuous sample switching is an essential process for developing an integrated platform incorporating multiple functionality with applications typically ranging from chemical to biological assays. Herein we propose a unique method of external-field-reconfigurable symmetry breaking in induced-charge electroosmosis above a simple planar bipolar electrode for continuous particle-beam switching. In the proposed system, the spatial symmetry of a nonlinear electroosmotic vortex flow can be artificially reordered to achieve an asymmetric electrically-floating-electrode polarization by regulating the configurations of the external ac signals, thus contributing to flexible particle beam switching. This switching system comprises an upstream flow-focusing region where particles are pre-focused into a beam on the bipolar electrode by transversal electro-convective mass transfer, and a deflecting region in which the resulting particle beam is deflected to generate a steerable lateral displacement to enter the desired region via the action of an asymmetric polarization-induced reshapable electroosmotic flow stagnation-line in a controllable background field gradient. A lateral particle displacement on the order of hundreds of micrometers can be achieved in a deterministic manner by varying the voltage, frequency and inlet flow rate, thereby enabling multichannel particle switching. Furthermore, the versatility of the switching mechanism is extended by successfully accomplishing fluorescent nanoparticle beam switching, yeast cell switching, five-outlet particle switching and simultaneous switching of two particle types. The proposed switching approach provides a promising technique for flexible electrokinetic sample preconcentration prior to any subsequent analysis and can be conveniently integrated with other micro/nanofluidic components into a complete functional on-chip platform owing to its simple electrode structure.

Development of integrated microfluidic platforms incorporating multiple functions is of great importance in many chemical and biological applications such as sample preparation, chemical reaction and cell incubation.1-3 The advantages of such microfluidic devices mainly include infinitesimal sample consumption, increased testing sensitivity, reduced processing time, and device portability; which make it possible to carry out the entire analysis on a single platform with multiple continuous processes and tasks running in parallel.4 However, the method whereby the objects of interest are transported to the desired position is a key step,5 provoking researchers to search for possible switching methods that achieve the above requirements. Various switching techniques have been exploited by flexibly controlling the particle flow in microchannels either passively6-8 or actively.9-14 In general, passive particle switching methods using internal hydrodynamic forces have good integration characteristics but are still restricted by the small number of branches,15 flow conditions,16 and the geometry of the microchannel.17 Compared with passive methods, the active switching process typically needs only a simple microstructure to work, but requires an external force field. For the latter, a magnetic field is convenient but suffers from its dependence on the magnetic susceptibility of the particles or external magnetic labeling on cells.18-20 Optical force is a powerful tool for

fast and high-throughput switching but is restricted by an external sheath flow and an expensive optical system.21-24 Standing surface acoustic waves (SSAWs) have been reported as a versatile, non-invasive, and highly controllable technique, but the SSAWs electrode fabrication is very complex and makes the cost relatively high.25-30 Finally and importantly, electrical force is a flexible way to manipulate particles and fluids in microfluidic chips. Dielectrophoresis (DEP) is one of the most widely used electrical techniques for manipulating particles, cells, and other objects; and has been employed as an effective tool to switch particles to the desired region.31-34 However, because the DEP force is mainly governed by the electric field gradient (for one specific particle), the force action region is quite small and is confined to the vicinity of the electrode array. Electroosmotic flow (EOF) is another electrical switching method,35-38 but because the exciting electrodes are both the carrier of high-voltage dc signal and extra flow generation, additional external pumps are also required to provide the sheath flow. Thus, EOF is difficult to integrate with other devices. As a unique form of field-effect flow control in microfabricated fluidic networks, induced-charge electroosmosis (ICEO) has received considerable attention from the microfluidic community because of its simple electrode setting, easy integration and wide action range. The ICEO arises from an ap-

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plied field acting on its own induced diffuse charge in a thin boundary layer on gating polarizable surfaces in direct contact with electrolytes.39 An ICEO-based microvortex pair has been employed to efficiently focus particles to a single outlet,40 but typically the ICEO-based particle switching into multiple outlets remains a challenge. The mechanism for focusing particles into a beam on the electrically-floating electrode center is the action of the symmetric microvortex pair, giving rise to a stagnant region owing to the symmetric induced polarization of the bipolar electrode. The particle will remain stable when the upward ICEO flow component reaches a balance with any other vertical forces (i.e. DEP force, buoyancy, see Figure S-3).41 This interesting mechanism motivates us to break the symmetry of the vortex pair to remold the stagnant line away from the central line and thus control the particle behaviors. Therefore, we developed an asymmetric ICEO-based switching technique using two pairs of driving electrodes excited with different ac signals arranged on both sides of the

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main channel. With this design, particles were pre-focused into a narrow beam and then deflected away from the central line of the floating electrode with a controllable lateral displacement. Thus, the particle flow locus could be flexibly controlled by the configuration of the driving electrodes and the external-field-reconfigurable signals applied, yielding successful switching of particles into different downstream branch channels. Unlike conventional broken symmetries around a three-dimensional structure including irregular shapes, field gradients and nonuniform surface properties,42 the proposed asymmetric polarization can be adjusted to produce different transverse streaming flows on a single simple planar electrically-floating electrode. The versatility of the switching mechanism was demonstrated via multipleoutlet particle switching, fluorescent nanoparticle beam switching and simultaneous switching of two types of particles. This electrokinetic switching technique holds great potential for integration with other tasks to realize multiple functions on a single platform.

Figure 1 A schematic illustration of configuration of the experimental system. (a)-(c) Geometrical configuration of the experimental chip: (a) A three-dimensional schematic of the microfluidic device. (b) Top view of the structures in the focusing region and the deflecting region. (c) Side view of the main channel. (d)-(f) Working configuration and mechanism of the device. (d) Particles are pre-focused into a beam and then directed to the central outlet utilizing the standard ICEO. (e) Particle beam is deflected to enter the upper outlet as two ac signals (A2>A1) are respectively applied to the lower left and lower right driving electrodes with the same upper left ground electrode. (f) Particle beam is switched into the lower outlet when ac signals are changed to the opposite side. DEVICE AND METHODS Device design and preparation. The device used in our experiments consists of (i) a focusing region; (ii) a deflecting region, as depicted in Figure 1(a)-2(c). By convention, ‘right’ and ‘left’ sides of the microfluidic channels are referenced to the flow direction. Particles suspended in the stream of buffer solution are introduced to the main channel and focused into a beam on the floating electrode surface by the first ICEO microvortex pair in the focusing region, and then deflected to the desired branches by the second microvortex pair in the deflecting region with an adjustment of the ac signals applied to the two pairs of the electrodes as shown in Figure 1(d)-1(f). The specification of characteristic dimensions of the device employed in the study is listed in Table S-1. Details of the processing steps of the chip are given in previous reports.43 The sample preparation and system setup for the experiments are described in detail in SI (Section S1). The numerical calculations in this study is conducted with FEM software, COMSOL Multiphysics 5.3.

Asymmetric microvortex pairs based on ICEO. The standard ICEO due to an electric field applied along the channel polarizing the floating electrode results in the symmetric vortex pair flow whose time-averaged slip velocity is directed towards the center of the floating electrode, driving two rolls of fluid in the bulk above the electrode surface. In terms of this situation, ions in solution are driven along the electric field lines, with positive ions driven towards one half of the floating electrode and negative ions driven along the other half. Under ‘ideally polarizable’ conditions, no current passes through the electrolyte/metal interface, and instead the ions that arrive form an induced double layer. At steady state, the field lines outside the diffused layer are fully expelled generating a nonlinear electrokinetic slip given by the Helmholtz-Smoluchowski formula44 uslip = −

εζ Et η

(1)

where ε and η are the electrolyte permittivity and viscosity, respectively; Et is the tangential field component on the electrode surface, ζ is the induced zeta potential. The spatially

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averaged flow over the floating electrode in the transverse direction can be given as ε Q = ∫ uslip dA = − Et ∫ ζdA η

(2)

Given that the electrode and the solution are symmetric, and an ac electric field is applied, the standard ICEO flow Q should approximately be equal to zero. However, if the line-

averaged zeta induced potential is nonzero, that will cause a nonzero flow Q over the floating electrode, which is defined as the asymmetric ICEO flow with asymmetric microvortex pair. ICEO appears as a micro-vortex flow above the bipolar electrode and transports particles to the flow stagnation region, thus flexibly manipulates particles in the microchannel.

Figure 2 Numerical simulation (β-α=0°) for understanding the switching mechanism of ICEO-based fluid motion. (a) Surface plot of the slip velocity in the y direction. (b) Comparison study between line-averaged zeta potential on various electrode surfaces: F-F-ζ represents the line-averaged zeta potential (I-I line) on the floating electrode in the focusing region; D-F-ζ represents the line-averaged zeta potential (II-II line) on the floating electrode in the deflecting region; F-D-ζ represents the line-averaged zeta potential (I-I line) on the driving electrode in the focusing region; D-D-ζ represents the lineaveraged zeta potential (II-II line) on the driving electrode in the deflecting region. (c)-(f) The left side shows the electric field lines outside the double layer are fully repelled and act on the induced surface charge to actuate the ICEO slip flow at steady state and the right side shows the corresponding streamline plots of ICEO-based microvortexes at selected positions when the particle beam is directed to the desired outlet: (c) Mechanism of asymmetric microvortexes formed above the floating electrode (I-I section) when the particle beam is directed to the upper outlet A; (d) Mechanism of asymmetric microvortexes formed on II–II section when the particle beam is deflected into the upper outlet A; (e) Symmetric ICEO flow emerges above the floating electrode (II-II section) to lead particles entering the central outlet B; (f) Particle beam is switched into the lower outlet C. Herein, to investigate the particle behaviors in the designed device, a series of numerical simulations of the ICEO fluid dynamics was performed to predict the particle motion. This was done by decoupling the electrostatics problem from the mechanics problem by appropriately ignoring the convection current in front of the conduction current (Table S2). The ycomponent of the time-averaged slip velocity within each harmonic cycle on the bottom surface is shown in Figure 2(a), indicating the asymmetry of the ICEO flow over the floating electrode rendered by breaking the charge equilibrium of the double layer. The parameters α and β are the initial phases of the applied ac voltages in the focusing and deflecting regions, respectively. Taking the phase difference β-α=0° as an example, the line-averaged zeta-induced potential shown in Figure 2(b) is nonzero when the particle beam is directed to the upper outlet A. The absolute value of the line-averaged zeta potential decreases with increasing frequency. The magnitude of the zeta potential on the driving electrodes is larger than ζ on the floating electrode. Further, the sign is opposite on the floating electrode between the focusing region and the deflecting region in this situation. The line-averaged zeta potential can reflect the distribution of the induced bipolar double-layer diffuse charge, which can help us better understand the mechanism of the formation of the asymmetric ICEO microvortex above the floating electrode, as shown in Figure 2(c)-2(f). The field-induced diffuse screening charge on the floating electrode shown on the left of the images is nonuniform-negative

(ζ>0) in the focusing region (I-I line), which leads to a vortex stagnation line close to the driving electrode E1 (right side of Figure 2(c)) and positive (ζ 97% and WR < 0.07) at a voltage of 5 V. Thus, an ac signal with a voltage of 5 V and a frequency of 200 Hz was chosen to energize the driving electrode in the focusing region. This choice was owing not only to the low width ratio of the focused particle beam, but also the sufficient operational allowance for the other driving electrode in the deflecting region. Figure 5(b) presents the changes of the width ratio and focusing efficiency as a function of inlet flow rate for an ac signal voltage of 5 V and frequency of 200 Hz. It can be seen that the width ratio increases slowly with flow rate, and the focusing efficiency decreases to about 81% for an axial flow rate of 11 nL/s.

Figure 4 Particle beam switching at A1=5 V, A2=10 V, f=200 Hz and an inlet flow rate of 5 nL/s (Movie S1). (a) A photograph of the device. (b) Diagram of particle beam transfer based on ICEO. (c) Particle beam is directed to the central outlet B with the ICEO-based symmetric microvortex pair. (d)-(g) Particle beam is flexibly controlled to enter the upper outlet A with the ICEO-based asymmetric microvortex pair varying with the phase difference(β-α) between the two driving electrodes. (h)-(k) Particle beam is flexibly switched to enter the lower outlet C. ICEO-based particle beam switching. One significant advantage of the asymmetric ICEO-based switching system is its ability to flexibly switch the resulting particle beam. This capability is of great importance in particle traffic-flow control because it considerably improves the utility of the particle switching system. Figure 4 shows the results for flexible switching of the particle beam: the particle beam was deflected to the desired outlets by activating the driving electrodes with different phase angles. As can be seen in Figure 4, particles were directed to the central outlet with the ICEO-based symmetric microvortex pair induced by an equivalent ac signal for both driving electrode pairs (Figure 4b). Further, the particle beam was deflected to the branches on both sides with an asymmetric microvortex pair generated by turning the electrode E3 or E4 into an electrically-floating state in the deflecting region. The flow stagnant line in the focusing region varies with the phase difference (Figure 4(d)-4(g) and 4(h)-4(k)) such that the focusing line is away from the ground electrode at β-



α=0° and 90° and moves to the side of ground electrode at βα=180° and 270°. This result agrees with the numerical simulation of particle trajectories (Figure S-6). Since the slip velocity uslip is proportional to the electric field Et,33 the particle beam lateral velocity is uslip, y and thus the lateral displacement produced by the asymmetric vortex can be easily controlled by regulating the voltage of the ac signal. Figure 5(c) shows the lateral displacement as well as the change of the driving voltage in the deflecting region when the particle beam is switched into the upper outlet. The trend of the lateral displacement agrees well with the theoretical prediction (see Figure 3). The lateral displacement y1 in the focusing region remains substantially unchanged at β-α=90° and 270° and increases with increasing voltage at β-α=0°. Though the deflection of particles is a minimum at β-α=0°, a displacement y2 on the order of hundreds of micrometers (~100.1 µm) can also be obtained with the voltage A2 exceeding 12 V. The lateral displacement y2 of the particle beam in the deflecting region increases slowly with increasing input voltage, and there exists an equal migration y2 that can reach ~114 µm under an ac signal of A1=5 V, A2=12 V, f=200 Hz at β-α=90° and 270°. When the phase difference is 180°, y1 already has a considerable offset value, and y2 initially increases gradually to 130 µm (the corresponding transient switching range from one side outlet to another side outlet reaches about 280 µm on the floating electrode, see Section S7) and then slowly reduces to a stable value at ~123 µm. The strength of the electric field at the side of the ground electrode is weakened owing to the opposite voltage polarity at βα=180° applied on the two driving electrodes, rendering a deflection to the ground electrode in the focusing region and inducing an optimum displacement in the deflecting region. As is known, the lateral displacement relates to the time t of transverse acceleration by asymmetric ICEO fluidic drag as determined by the inlet flow rate. Therefore, this phenomenon was further explored in terms of the inlet flow rate, as shown in Figure 5(d). When the axial flow rate provided by the pump

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increases, the lateral displacement decreases. It should be noted that the lateral particle displacement in the proposed system is on the order of hundreds of micrometers and is much larger than that in the other switching systems, which can be well controlled by altering the experimental parameters such as the voltage amplitude and phase, field frequency, and inlet flow rate. In addition, the operable particle concentration can be much higher than other typical methods.

Figure 5 Characterization of particle beam behaviors based on ICEO. (a) Frequency dependence of the width ratio of silica at various focusing voltage when particle beam is directed to the central outlet B with an inlet flow rate 5 nL/s. (b) Trends in focusing efficiency and width ratio with flow rate at focusing voltage A=5 V, focusing frequency f=200 Hz when particle beam is focused to enter the central outlet B. (c) Voltage dependence of particle beam lateral displacement in the focusing region and deflecting region respectively with an inlet flow rate 5 nL/s when particle beam is deflected to enter the upper outlet A. (d) Deflection displacement of particle beam varying with flow rates at different phase difference.

Figure 6 Experimental results of sample diversity (a-c) 200 nm fluorescent polystyrene particles can also be focused into a beam and deflected to the desired outlet by modulating input voltage amplitude, frequency and phase difference (Taking A=10 V, A1=5 V, A2=10 V, f=200 Hz, β-α=0° as an example) (Movie S2). (d) Normalized fluorescent intensity profiles at a cross section IV–IV. (e-j) Experimental results of yeast switching at A=5 V (for the situation that yeast are focused into the central outlet), A1=5 V, A2=10 V, f=200 Hz, β-α=0° and an inlet flow rate of 3 nL/s. (e-g) The corresponding images captured using a 10× objective lens in the focusing region (Movie S3). shown in Figure 6. In these experiments, when two sets of Switching of fluorescent nanoparticles and yeast cells. To further demonstrate the multifunctionality of our electrode pairs with equivalent ac signals A=5 V, f=200 Hz asymmetric ICEO-based switching method, 200 nm were powered on, the sample beam did not experience lateral fluorescent polystyrene particles and yeast cells were tested, as acceleration and thus followed the incoming laminar stream to


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the center outlet (Figure 6(b) and 6(i)). When the electrode energization was asymmetrically changed over, the fluorescent polystyrene beads and yeast cells could be focused into a beam and switched into the side branch (Figure 6(a), 6(c), 6(h) and 6(j)). The fluorescence images in Figure 6 were produced by overlaying video stills that captured the continuous particleswitching process. The fluorescent polystyrene particles were not focused perfectly (i.e., the darker stagnant region in the focusing region owing to the weakened sedimentation of the nanoscale objects shown in Figure 6(a) and 6(c)). However, the other driving electrode in the deflecting region can still play a role in strengthening the focus of nanoparticles spreading close to the side of driving electrode into the beam and then deflecting them to the desired outlet. The transverse concentration distribution of the fluorescent polystyrene particles was evaluated by calculating the normalized intensity I=(Ii-Imin)/(Imax-Imin) (where Ii is the intensity at the ith pixel, Imin and Imax are the minimum and maximum intensity respectively.) from the average intensity over an image.47 Figure 6(d) shows the corresponding normalized fluorescent intensity profiles taken at the cross-section IV-IV to further demonstrate the focusing and the switching efficiency. The switching of yeast cells shown in Figure 6(h)-6(j) suggests that the external-field reconfigurable asymmetric ICEO-based switching method can also manipulate biological cells to perfectly enter the desired region, which is conducive to integrated biochemical analysis. The Figure 6(h)-6(j) images captured using a 10× objective lens clearly display the focused states of yeast cells in the focusing region, from where the yeast cells are directed into the upper branch, central outlet and lower branch, respectively, with two in-phase ac signals. The yeast stream moves over the surface of the floating electrode and the majority of the yeast cells maintain a compact beam (Figure 6(e)-6(g)), whereupon the cells trajectory is reshaped by regulating the power supply configurations. In this sense, the switching mechanism based on asymmetric ICEO flow can be used to sort different types of biochemical beads ranging from the microscale to the nanoscale. Multichannel particle switching. In terms of particle traffic control, multichannel particle switching is of significant importance because it enhances the functionality and utility of the particle switching system. The ability to switch particles into multiple outlet channels may be necessary in applications where post-processing at various conditions is required downstream. Because of the wider influence range of the ICEObased microvortex pair, the asymmetric ICEO-based switching scheme can be extended to channels with multiple outlets. Another particle-switching chip with a specific configuration for a longer deflecting region, wider tail for the floating electrode and five outlets was adopted to test the performance, as shown in Figure 7(a)-7(e). Particles were focused and directed to enter the central outlet with A=5 V, and then switched into the four other outlets via reconfiguring the ac signals to A1=5 V, A2=5 V, f=200 Hz and A1=5 V, A2=10 V, f=200 Hz. Our device effectively switched particles into five individual outlets on a single microfluidic chip. This could potentially be extended from the current five outlets to a higher number of outlet channels to increase the functionality of the device for applications ranging from cell culturing to antigen-based cell diagnostics.

Simultaneous switching of two types of particles. It is also worth noting that particles with distinct properties (i.e., density, size) are apt to be lifted to distinct vertical positions under the action of the ICEO-based drag force, thereby forming vertically-segregated stagnant lines. In fact, the closer the stagnant line is to the center of Poiseuille flow, the faster the particles on that line tend to move axially. Hence, the duration of lateral asymmetric ICEO fluidic drag varies with the laminar layer height, leading to the lateral deflection variation between various types of target samples. Silica particles (size: 4 µm) possessing a density (2.2 g·cm-3) greater than the buffer solution, and polystyrene particles (size: 20 µm) possessing a density (1.05 g·cm−3) similar to the electrolyte were chosen to demonstrate the simultaneous switching of two types of particles via the proposed switching mechanism of external-fieldtunable asymmetric nonlinear electroosmosis. The experimental results clearly show that the axial motion of polystyrene particles is faster than silica (Movie S5), indicating that silica experiences greater lateral migration. Thus, the simultaneous switching of two types of particles was achieved with certain coupled parameters. Mixed particles were focused into the central outlet using a standard ICEO flow, as can be seen in Figure 7(h). Figure 7(f) and 7(j) demonstrate two types of particles switched to the same side branch with two independent trajectories caused by their respective deflections at A1=5 V, A2=7 V, f=200 Hz, and a 5 nL/s flow rate. The two particle types can be directed into two individual branches simultaneously at A1=5 V, A2=5 V (Figure 7(g) and 7(i)), where the silica particles are deflected into the side branch and the polystyrene particles enter the central outlet. Therefore, the switching method introduced herein has been proved suitable for simultaneous switching of two types of particles into individual branches.

Figure 7 The demonstration of the feasibility for switching particles based on asymmetric ICEO flow. (a-e) The frequency applied on all the driving electrodes is 200 Hz and the phase difference between the two driving electrodes is 270° (Movie S4). The voltage applied on the left and the right driving electrodes are: (a) A1=5 V, A2=5 V; (b) A1=5 V, A2=10 V; (c) A=5 V; (d) A1=5 V, A2=5 V; (e) A1=5 V, A2=10 V. (f-j) Simultaneous switching of silica and polystyrene particles at β-α=0°. (f), (h) and (j): Two kinds of particles enter the side branch simultaneously. (g) and (i): Silica and polystyrene particles are switched into the side branch and central outlet respectively. CONCLUSIONS We developed a novel continuous particle-beam switching technique based on external-field-reconfigurable asymmetric ICEO-based microvortex pairs, which can be achieved by regulating the configurations of the external ac signals acting on the two driving electrode pairs. The nature of our device was tested by efficiently switching silica particles with a di-



ameter of 4 µm into three outlets. The incoming colloidal particles were well pre-focused into a thin beam with focusing efficiency exceeding 97% and focusing width ration below o.o7 by energizing the focusing driving electrode with an ac signal of A1=5 V and f=200 Hz an inlet flow rate of 5 nL/s. Further, the resulting particle beam could be perfectly switched into the side branches owing to an asymmetric ICEO-remolded stagnant line away from the floating electrode center in the deflecting region, when a deflecting voltage A2 excited on one of the electrodes E3 and E4 exceeds 5 V. We demonstrated that the lateral displacement can be flexibly controlled to achieve hundreds of micrometers by modulating the ac signals (~280 µm in our experiment at A2=5 V, A2=9 V, f=200 Hz and β−α=180°), which exhibits a long-range transient switching, thus enabling an increase in the number of outlets. Furthermore, fluorescent nanoparticle beam switching, yeast cell switching, five-outlet particle switching and simultaneous switching of two types of particles have been successfully accomplished to demonstrate the versatility of the switching mechanism. The microfluidic particle switching method with merits of portability, easy integration and low power consumption, provides a useful tool for flexible manipulation of colloidal particles in mod-ern micro total analytical systems.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Figure S1-S4 are referenced within the main text. Movies for each switching experiment as noted in the text.

AUTHOR INFORMATION Corresponding Author * Email: [email protected], [email protected]. Phone.: +86 451 86418028; Fax: +86 451 86402658.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported financially by the National Natural Science Foundation of China (Grant No. 11672095， NO. 11702075 and NO. 11702035), and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51521003).

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For TOC only



Figure 1 A schematic illustration of configuration of the experimental system. (a)-(c) Geometrical configuration of the experimental chip: (a) A three-dimensional schematic of the microfluidic device. (b) Top view of the structures in the focusing region and the deflecting region. (c) Side view of the main channel. (d)-(f) Working configuration and mechanism of the device. (d) Particles are pre-focused into a beam and then directed to the central outlet utilizing the standard ICEO. (e) Particle beam is deflected to enter the upper outlet as two ac signals (A2>A1) are respectively applied to the lower left and lower right driving electrodes with the same upper left ground electrode. (f) Particle beam is switched into the lower outlet when ac signals are changed to the opposite side. 159x71mm (300 x 300 DPI)


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Figure 2 Numerical simulation (β−α=0°) for understanding the switching mechanism of ICEO-based fluid motion. (a) Surface plot of the slip velocity in the y direction. (b) Comparison study between line-averaged zeta potential on various electrode surfaces: F-F-ζrepresents the line-averaged zeta potential (I–I line) on the floating electrode in the focusing region; D-F-ζ represents the line-averaged zeta potential (II–II line) on the floating electrode in the deflecting region; F-D-ζ represents the line-averaged zeta potential (I–I line) on the driving electrode in the focusing region; D-D-ζ represents the line-averaged zeta potential (II–II line) on the driving electrode in the deflecting region. (c)-(f) The left side shows the electric field lines outside the double layer are fully repelled and act on the induced surface charge to actuate the ICEO slip flow at steady state and the right side shows the corresponding streamline plots of ICEO-based microvortexes at selected positions when the particle beam is directed to the desired outlet: (c) Mechanism of asymmetric microvortexes formed above the floating electrode (I–I section) when the particle beam is directed to the upper outlet A; (d) Mechanism of asymmetric microvortexes formed on II–II section when the particle beam is deflected into the upper outlet A; (e) Symmetric ICEO flow emerges above the floating electrode (II–II section) to lead particles entering the central outlet B; (f) Particle beam is switched into the lower outlet C. 172x64mm (300 x 300 DPI)



Figure 3 Numerical simulation results of ICEO-based fluid motion when the particle beam is deflected to enter the upper outlet A. (a) Comparison study between the mean velocity of the particles induced by ICEO ACEO and DEP with various frequencies at selected positions (I–I line and II–II line). (b) The time-averaged slip velocity in the y direction on the central line (III–III) of 3 µm height above the floating electrode simulated with different phase difference. (c) The ICEO-based time-averaged slip velocity in the y direction on the I–I line in the focusing region. (d) The ICEO-based time-averaged slip velocity in the y direction on the II–II line in the deflecting region. (e) Time-averaged slip velocity profiles above the floating electrode surface at various frequencies when particles are focused into the central outlet. (f) Changes of electric field on I–I line at various phase difference. 178x91mm (300 x 300 DPI)


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Figure 4 Particle beam switching at A1=5 V, A2=10 V, f=200 Hz and an inlet flow rate of 5 nL/s (Movie S1). (a) A photograph of the device. (b) Diagram of particle beam transfer based on ICEO. (c) Particle beam is directed to the central outlet B with the ICEO-based symmetric microvortex pair. (d)-(g) Particle beam is flexibly controlled to enter the upper outlet A with the ICEO-based asymmetric microvortex pair varying with the phase difference(β-α) between the two driving electrodes. (h)-(k) Particle beam is flexibly switched to enter the lower outlet C. 119x92mm (300 x 300 DPI)



Figure 5 Characterization of particle beam behaviors based on ICEO. (a) Frequency dependence of the width ratio of silica at various focusing voltage when particle beam is directed to the central outlet B with an inlet flow rate 5 nL/s. (b) Trends in focusing efficiency and width ratio with flow rate at focusing voltage A=5 V, focusing frequency f=200 Hz when particle beam is focused to enter the central outlet B. (c) Voltage dependence of particle beam lateral displacement in the focusing region and deflecting region respectively with an inlet flow rate 5 nL/s when particle beam is deflected to enter the upper outlet A. (d) Deflection displacement of particle beam varying with flow rates at different phase difference. 127x91mm (300 x 300 DPI)


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Figure 6 Experimental results of sample diversity (a-c) 200 nm fluorescent polystyrene particles can also be focused into a beam and deflected to the desired outlet by modulating input voltage amplitude, frequency and phase difference (Taking A=10 V, A1=5 V, A2=10 V, f=200 Hz, β−α=0° as an example) (Movie S2). (d) Normalized fluorescent intensity profiles at a cross section IV–IV. (e-j) Experimental results of yeast switching at A=5 V (for the situation that yeast are focused into the central outlet), A1=5 V, A2=10 V, f=200 Hz, β−α=0° and an inlet flow rate of 3 nL/s. (e-g) The corresponding images captured using a 10× objective lens in the focusing region (Movie S3). 159x79mm (300 x 300 DPI)



Figure 7 The demonstration of the feasibility for switching parti-cles based on asymmetric ICEO flow. (a-e) The frequency applied on all the driving electrodes is 200 Hz and the phase difference between the two driving electrodes is 270° (Movie S4). The volt-age applied on the left and the right driving electrodes are: (a) A1=5 V, A2=5 V; (b) A1=5 V, A2=10 V; (c) A=5V; (d) A1=5 V, A2=5 V; (e) A1=5 V, A2=10 V. (f-j) Simultaneous switching of silica and polystyrene particles at β−α=0°. (f), (h) and (j): Two kinds of particles enter the side branch simultaneously. (g) and (i): Silica and polystyrene particles are switched into the side branch and central outlet respectively. 157x82mm (300 x 300 DPI)


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Flexible continuous particle-beam switching via external-field

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