DIELECTROPHORETIC ALIGNMENT AND SORTING OF
MICROPARTICLES IN MICROCHANNEL FLOWSUSING LADDER-TYPES ELECTRODES

Koki Kawano, Kazuya Tatsumi, Hiromichi Shintani, and Kazuyoshi Nakabe
Kyoto University, JapanABSTRACT
The present study describes a new micro-device which can accurately align and sort particles in a very
high rate using the dielectrophoretic (DEP) force. The rail- and ladder-type electrodes can precisely locate
the particles in certain spanwise and height positions, and moreover, control the distance between particles
and align them. In combination with the flip-type electrode, the particle could be sorted accurately.
KEYWORDS: Dielectrophoresis, Particle alignment and sorting, Ladder-type electrodes, Microchannel
INTRODUCTION
Although various means of manipulating and sorting particles have been presented in the past [1], theaccuracy and rate have not fully reached the level to satisfy the demands of the applications (e.g. >1,000particles/s). To increase the sorting rate, it is necessary to control the streamwise positions of theseparticles before guiding them to the on/off gate electrodes for manipulation. To tackle this problem, we
have developed a DEP force based device that can align the particles in the channel flow equally spacedin the streamwise direction using the “rail-type” [2] and “ladder-type” electrodes.
THEORY
Figure 1(a) shows the microchannel and the thee electrodes together with the principle how theparticle can be aligned and sorted. With the negative DEP force working on the particles, the three typesof electrodes can control the particle spanwise and height positions, streamwise distance between
particles, and sort the particles.
The particle alignment function of the ladder-type electrodes appears only when the voltage isfluctuated in a frequency synchronized with the particle streamwise velocity and the electrodes pitch.
Figure 2(b) shows the on/off characteristic of the voltage which is periodically changed in a square wave
form. High frequency fluctuation of 10MHz is applied when the voltage is on to generating DEP force.
As shown in Fig. 1 (b) and (c), when the phases of the voltage applied and the particle relative position tothe ladder-type electrode do not match, then FDEP, x and FDEP, z work to correct the position of the particle
to move toward the appropriate one.
By these effects, since the period when the particle is supplied from the ladder-type electrode issynchronized with the phase of the voltage on/off cycle, it is easy to adjust the timing for the on/off of theflip-type electrode used to sort the particles.
NUMERICAL
The particle motion was calculated by solving the translational and rotational equations of motion.
The hydrodynamic force working on particles was estimated employing the modified stokes law of force
on a particle considered the wall effect [3]. FDEP was obtained based on the Clausius-Mossotti function[4]using the electric field calculated by COMSOL Multiphysics Ver. 4.3a.
The A-C voltage applied to the electrodes was 20Vp-p with frequency of 10MHz. The on/off
frequency was fladder=125Hz. The channel mean velocity was um=12.5mm/s. The initial values of therelative positions of the particles at t=0s were xp,0=0 and 20μm, respectively.
EXPERIMENTAL
The microchannel is made of two glass substrates, which Pt electrodes are sputtered, with SU-8(Microchem) patterned with the microchannel. Flow rate and the um were 6.4μL/min and 11.6mm/s,978-0-9798064-7-6/µTAS 2014/$20©14CBMS-0001 1274

18th International Conference on Miniaturized
Systems for Chemistry and Life Sciences
October 26-30, 2014, San Antonio, Texas, USA

Figure 1: Schematics of (a) the microchannel, (b) FDEP, x on the x-y plane at z=6μm and (c) FDEP, z>0
on the y-z plane (A-A’ plane of (b)) and FDEP, z<0 on the y-z plane (B-B’ plane of (b)).

Figure 2: (a) The schematic of time series variation of particles’ positions, (b) the relative positionsof the particles initially located at xp,0=0 and 20μm at t=0s and the applied low frequency oscillation,and (c) the FDEP applied to the particles in these two cases and their streamwise velocities.
respectively. The 12μm polystyrene particles (Thermo fisher scientific, 4212A) suspended in PBS(phosphate buffered saline) solution was supplied by the center flow of the sheath flow.
High-speed camera was used to record the image and measure the particle motions. The particle
velocities and positions were analyzed using the motion analysis software (Library, Move-tr/2D ver7.90).
RESULTS AND DISCUSSION
Figure 2(b) shows the time series variation of the relative positions of particles, x’p(t)= xp(t) – xp,r(t),where xp,r(t) is of the particle whose initial position is xp(t)=0μm. xp(t)=0μm is the center of the pitch of
the ladder-type electrodes (see Fig 2(a)). xp,r(t) also presents the periodical trajectory of the particle afterconverging to a certain value by the FDEP. Figure 2(c) shows the FDEP,x and the particle streamwise
velocities up. The FDEP moves the particle to the relative positon where the positive and negative FDEP
balances during one periodical period.
Figure 3 shows the snapshots of the particles flowing in the microchannel. The particles are locatedat the same position relative to the ladder. They are spaced at multiples of ts, which is the distance of theparticle converted to time. The distance between particles will converge to the ladder pitch 50μm. And
the ts becomes 8ms=1/fladder.
Figure 4 shows the probability density distribution (PDD) of δ in the cases of the inlet and outlet ofthe ladder-type electrodes. δ is the deviation from the synchronized timing. At the inlet, the distribution isdispersed showing that the particle enters the region with random positions. At the outlet, however, apeak is observed around δ=0 with the deviation less than ±5% indicating that the particles are aligned in
the position synchronized with fladder.
Figure 5 shows the PDD of the particle spanwise positions, yp, measured at the outlets of the laddertype electrodes and the sorting region. The flip (gate) electrode is turned on and off at the same timingwith the ladder-type electrodes. In Fig. (a), yp measured at the outlet of the sorting region varies since theperiod when the flip electrode is turned on does not match with the one when the particles passes the flipelectrode.
1275Figure 3: Snapshots of the particles aligned equally spaced as they flow along the ladder-type electrodes.

Figure 4: Probability density distributions of the deviation of the particles δ from the synchronizedtime at the (a) inlet and (b) outlet, and (c) schematics of the measuring positions.

Figure 5: Probability density distributions of the particles spanwise positions y p at the outlets of theladder-type electrodes and the sorting region in the cases of (a) non-activated (only flip activated) and(b) activated (ladder and flip synchronized). (c) shows the measuring points of these two locations.
On the other hand, in the activated case, the flip-type electrode is activated at an appropriate timing
and the particle is successfully moved upward not to be trapped by the oblique-rail-type electrodes. The
yp at the outlets of the sorting region shows a peak in this case, and particle sorting of 125Hz is achieved.
CONCLUSION
It has been confirmed numerically and experimentally that the positions and interval of particles canbe controlled by using the ladder-type electrode. In combination with the flip-type electrode, the possibility of reaching the sorting rate higher than 125Hz was shown.
ACKNOWLEDGEMENTS
This work was financially supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan.
REFERENCES[1] Z. R. Gagnon, “Cellular Dielectrophoresis: Applications to the Characterization, Manipulation, Separation and Patterningof Cells,” Electrophoresis, 32, 2466-2487, 2011.
[2] K.Tatsumi, H. Shintani, Y. Katsumoto, and K. Nakabe, “Dielectrophoretic Sorting of Microparticles and Lymphocytes Using Rail-Type Electrodes,” micro-TAS2013, 2013.
[3] A. J. Goldman, R. G. Cox, and H. Brenner, “Slow viscous motion of a sphere parallel to a plane wall,” Chemical Engineering Science, 22, 637-660, 1967
[4] T. B. Jones, “Electromechanics of particles,” Cambridge University Press, 1995.

CONTACT* Kazuya Tatsumi; phone: +81-75-383-3606; tatsumi@me.kyoto-u.ac.jp1276