6th Micro and Nano Flows Conference
Atlanta,USA, 9-12 September 2018

Evaluation of Particle Motion Characteristics inMicrochannel Flow Alignment System with Ladder-type Electrodes
Atsushi NOMA, Kazuya TATSUMI*, Reiko KURIYAMA, Kazuyoshi NAKABE* Corresponding author: Tel.: +81-75-383-3606; Fax: +81-75-383-3608; Email: tatsumi@me.kyoto-u.ac.jp
Department of Mechanical Engineering and Science, Kyoto University Kyoto, Japan

Keywords: Microchannel flows, Particle motion control, Ladder-type electrodes, Dielectrophoretic force, Particle alignment
1. Introduction
40 Deceleration

Trapping and transporting

Even space
Constant velocity
𝐿pitch =50

Controlling the velocity, interval and timing of cells and particles
in microchannel flows is an important issue to develop a highthroughput sensing and sorting system in lab-on-a-chip and micrototal-analysis-systems[1]. We have developed a technique that cancontrol the particle velocity, interval between each particle and theexact timing of particles crossing a certain location in the channel byapplying dielectrophoretic (DEP) force to the particles periodicallywith time and space using ladder-type electrodes[2].
The motion characteristics of the particle in the ladder-type
electrode region and principle of the alignment mechanism, however,
have not yet been fully evaluated and discussed. In this paper,
position and velocity of the particles in the ladder-type electroderegion were measured and compared with the period when thevoltage (force) was applied to the particles to evaluate the phasedifference between the particle relative position and the time when
the force is exerted on the particles. We will then show how thephase difference contributes to the acceleration/deceleration ofparticles and convergence of the particle velocity, relative position
and timing to the equivalent state. Finally, accuracy of controllingthese particle characteristics are evaluated.

Figure 1. Schematic of microchannel and electrodes.
type electrode region. Aqueous solution with 0.1wt% sodium laurylsulfate (SLS) was used as working fluid. The cross-sectional averagevelocity of the flow was set as 𝑢m =9.26mm/s. The applied voltage
was 𝑉P−P =20V with frequency of 𝑓V =10MHz. Voltage was turnedon and off with frequency of 𝑓on−off =125Hz.
The position and velocity of the particles were measured using highspeed video camera. The starting time of the frame recording wassynchronized with the on / off phase of the applied voltage. Particle
motion tracking software Cosmos Move-tr/2D (Library Co.) wasused for particles motion analysis.

2. Experimental Methods and Conditions

3. Results and Discussion

Figure 1 shows the schematic of the electrodes and microchannel
used in this study. Rail-shaped electrodes and ladder-shaped
electrodes are attached to the bottom wall. Another electrode isattached to the top wall covering the overall area of the channel. AC
voltage is supplied to the electrodes generating the electric field andDEP force. In the rail-type electrode region, DEP force is exerted on
the particles in the height and spanwise directions and make themfocus at the centerline of the electrodes near the channel bottom wall.
In the ladder-type electrode region, area in which the DEP force
accelerate and decelerate the particles in the streamwise direction are
periodically located in the streamwise direction. Particles are alignedin the streamwise direction by periodically turning the supplied
voltage on and off. If the on/off frequency is defined as 𝑓on−off andthe pitch of the periodic area for particle acceleration anddeceleration as 𝐿pitch , then the average particle velocity inequivalent state will be 𝑢p,0 =𝐿pitch ∙ 𝑓on−off .
The microchannel is composed of the channel made of SU-8(MicroChem Co.) layer glass substrate on which Pt electrodes werepatterned.
The channel width and height were 70μm and 54μm, respectively.
Sheath-flow was generated by the three inlets of the microchannel.
Polystyrene micro-particles 12μm in diameter (Thermoscientific Co.,
4212A) were supplied from the center inlet and guided to the rail-Acceleration

1070
20

Rail-type

Sample flow
Rail-type electrodes
8000

Ladder-type

Side flow
Side flow Ladder-type electrodes
4000μm

3.1 Particle Motion and Alignment Characteristics
We measured the particle position and velocity from the leadingedge of the ladder-type electrodes and evaluated the particle motion
converging to the controlling value (equilibrium state). Figure 2 (a)shows the relationship between particle velocity 𝑢p and particle
position 𝑥p . Figure 2 (b) shows the phase difference 𝜃 of theparticle position from their equilibrium position. Figure 2 (c) shows𝑢̂p distribution in relation to the number of periodic region of theladder-type electrodes. 𝑢̂p is the particle velocity averaged in one
periodic region. The equilibrium position is where the total work
done on the particle by the exerted forces in one periodic region is
𝑊=0[2].
The horizontal line shown in the graphs of Fig. 2 represents thevalue when particle is in equilibrium position: 𝑢̂p = 𝐿pitch ∙𝑓on−off =6.25mm/s and 𝜃=0. In Fig.2 (a), gray area represents thelocation of electrodes and red hatching area shows the period inwhich DEP force is applied.
In Figs. 2 (a) and (c), 𝑢p and 𝑢̂p of different particles initiallyshow different value with each other and from the equilibrium one.
These velocities converges to an identical one at the same position
and time as the particles flow downstream. This shows that theparticles flow in the exact same trajectory with the same time profile.

6th Micro and Nano Flows Conference
Atlanta,USA, 9-12 September 2018

[m]

up[mm/s]Thus, the particles whose positions and velocities varies in the12upstream region will align in ladder-type electrodes.
10Fig. 2 (b) shows the phase difference 𝜃 for the particle depicted
8by the square symbols shown in Fig.2 (c). 𝜃 decreases as the6particle flow downstream. In the region of 𝑖=10 and 19, 𝜃 shifts to4
the next periodic phase. 𝑢̂p also decreases as the particle flow
2downstream. Thus, a positive correlation is obtained between(a) Particle velocity 𝑢𝑝 in each periodic unit. Gray area represents∂𝑢̂p /𝜕𝑖 and ∂𝜃/𝜕𝑖 which leads to 𝜃 converging to zero and 𝑢̂p
the location of electrodes and red hatching area shows the period into the velocity of equilibrium state. It should be noted that a sharp
which DEP force is applied.
increase of 𝑢̂p is observed in the region of 𝑖=11. This is due to the25
fact that SiO2 coating on the wiring area was partially damaged andremoved so that electric field and resulting DEP force was generated.

0P(up/up,o)P(up/up,o)[mm/s]

3.2 Evaluation of Alignment Performance
We measured the particle velocity and timing in the downstream-25region of ladder-type electrodes to evaluate the performance of(b)Phase difference 𝜃 in each periodic unit. The case corresponds
particle alignment. Timing represents the time when the particle
to the line with square symbols in Fig. (c).
passes through a certain position in the ladder-type electrode region.
7.5∆𝑢p represents the deviation from the velocity of equilibrium valueof 𝑢p,0 , and ∆𝑡 is the time deviation from the value of 𝑘𝑡s which
7.0
is integer multiple of 𝑡s which is the time when the particle passes
6.5
through one periodic area. Figs. 3 (a) and (b) show the probability
density distributions of ∆𝑢p /𝑢p,o at the inlet and outlet of the
6.0
ladder-type electrodes, respectively. Figs. 3 (c) and (d) show the5.5
probability density distributions of ∆𝑡/𝑡s . In the figures, ∆𝑢p /𝑢p,o0
2468 10 12 14 16 18 20and ∆𝑡/𝑡s distributes uniformly at the inlet. However, theiri[unit]
distributions show the maximum value with a value of zero. The
variation is approximately ±5%, which shows the accuracy of the(c) Average particle velocity in each periodic unit.
particles velocity and timing control of the present method.
Figure 2. Phase difference 𝜃, particle velocity 𝑢𝑝 , andaverage velocity in each periodic unit 𝑢̂𝑝 distributions in
4. Conclusions and Future Works
upstream region of ladder-type electrodes.
We studied the characteristics of controlling the velocity, interval
0.50.5and timing of particles in microchannel flows using the ladder-type
0.4
0.4
electrodes. A positive correlation between average velocity of each
0.30.3
periodic unit of ∂𝑢̂p /𝜕𝑖 and ∂𝜃/𝜕𝑖 was confirmed as 𝑢̂p
0.2
0.2
decreased with 𝜃 . Alignment performance and accuracy wasconfirmed by evaluating the deviation of velocity from equilibrium
0.1
0.1
state ∆𝑢p and time deviation from the time when the particle passes
0.0
0.0-0.50 -0.25
0.00
0.25
0.50
-0.50 -0.25
0.00
0.25
0.50
through a specific location ∆𝑡 in downstream region of ladder-typeup/up,oup/up,oelectrodes.
(a) Inlet distributions(b) Outlet distributions
As a future work, we need to increase the accuracy of distribution
0.50.5of dielectrophoretic force to enhance the particle alignment
0.4
0.4
performance.
0.3

P(t/ts)[1] P. Chen, X. Feng, W. Du, and B. Liu : “Microfluidic chips forcell sorting”, Frontiers in Bioscience, Vol.13, pp.2464-2483,
January 1, (2008)[2] K. Tatsumi, K. Kawano, H. Okui, H. Shintani, and K. Nakabe :“Analysis and measurement of dielectrophoretic manipulation ofparticles and lymphocytes using rail-type electrodes”, MedicalEngineering & Physics, Vol.38, No.1 pp.24-32 (2016)

P(t/ts)

0.3

References0.2

0.2

0.1

0.1
0.0
-0.50

-0.25

0.00

t/ts

0.25

(c) Inlet distributions

0.50

0.0
-0.50

-0.25

0.00

t/ts

0.25

(d) Outlet distributions

Figure 3. Probability density distributions of particles
velocity shown in (a) and (b) and timing shown in (c)and (d) at inlet and outlet of ladder-type electrodes.
PDF is for ∆𝑢𝑝 (particle velocity deviation from thevelocity of equilibrium value 𝑢𝑝,𝑜 ) and ∆𝑡 (timedeviation from the value 𝑡𝑠 ).

0.50