7th Micro and Nano Flows Conference
Imperial College London, UK, 24-26 May 2021

Toward 100% Yield of One-to-One Particle Encapsulation
Using Dielectrophoretic Particle Alignment TechniqueKazuya Tatsumi1,*, Kenichi Mabuchi1, Reiko Kuriyama1, Kazuyoshi Nakabe1
1 Department of Mechanical Engineering and Science, Kyoto University, Kyoto, Japan* Corresponding author: Tel.: +81 (0)75 3833606; Fax: +81 (0)75 3833608;
Email: tatsumi@me.kyoto-u.ac.jp

Abstract Particle encapsulation using microfluidics technology is effective for application of synthesis, single
cell analysis, and drug discovery. However, if the particles are supplied randomly to the region in which droplet
is generated, the number of particles in the droplet follows the Poisson distribution. This decreases the successrate of the encapsulation when a one-droplet-one-particle is required. We developed a technique using theboxcar-type electrodes that can control the streamwise interval and timing of the particles flowing in
microchannel by dielectrophoretic (DEP) forces, and applied it to the upstream region of the droplet-generation
section. The interval of the particles was adjusted to that of the droplet generation, and alignment performance
before they enter the droplet-generation region and the frequent distribution of the number of particles in thedroplet were measured. We achieved 100% success-rate of one-to-one particle encapsulation in the case ofaverage number of particles (number density) in a droplet equal to 0.40.
Keywords: Microfluidics, Dielectrophoretic Force, Alignment, Timing Control, Encapsulation, Droplets

1. Introduction

2. Methods

Controlling the position of the particles, theinterval (space) between the particles, and thetiming when they pass the specific location in
the microchannel is an important technique toperform the sensing, sorting, and encapsulation
accurately in microfluidic devices.
We developed a technique that can control
the timing, interval (spacing), and velocity ofthe particles in the microchannel by exerting
forces on the particles periodically over space
and time (see Fig. 1 (a)) [1-3]. Regions ofdielectrophoretic force accelerating anddecelerating the particles are formed
periodically in the streamwise direction byusing boxcar-type electrodes, and the force isactivated periodically over time. We showedthat the combination of these two schemes
could align the particles and Jurket cells withequal spacing and intervals [2,3]. In thispresentation, we will apply this technique to theparticle encapsulation system and evaluated thealignment of the particles, timing matchingwith the droplet generation, and the success rateof one-droplet-one-particle encapsulation.

Figs. 1 (b)-(d) show the schematic anddimensions of the microchannel and electrodes.
The boxcar-electrodes were patterned on thebottom wall, and the ground electrode covered
the top wall of the main channel. We used theflow-focusing type channel to generate droplets,
which was installed to the location downstreamof the boxcar-electrode region (see Fig. 1 (d)).
Polystyrene microparticles with diametersof 12 m (Thermoscientific, 4212A) were usedin the measurements. The particles were mixedin Milli-Q water at concentration of 0.2 %, andwas supplied to the main channel. Silicone oil(Shin-Etsu Chemical: KF-96-50CS) with
0.5wt% hydrophobic surfactant (Tokyo OhkaKagaku: Span80) was supplied to the side flowsof the flow-focusing type channel.
The flow rate of the center flow and side
flows were 0.5 and 1.85 L/min, respectively.
We applied an alternating current voltage to theelectrodes with voltage of Vp-p=8.5V, andfrequency of fV=10MHz. This voltage wasturned on and off periodically with a frequencyof fon-off=14 Hz, and with duty rate of 50%.
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Imperial College London, UK, 24-26 May 2021

0 2 4 6 8 10(a) Particle alignment and timing control

0.6
0.5
0.4
0.3
0.2
0.1
0.0

med=66ms
s=1.4ms
ns=490F(td/ton-off)F(Dtd)

Fig. 2: Snapshot of the flow-focusing region.

20 40 60 80 10050403020100avg=-7.91x10-4
s=0.016
ns=18-0.4 -0.2 0.0 0.2 0.4

tp/ton-off

Dtd [ms](a) Droplet generation(b) Particle supply

Fig. 3: Probability density function of time-intervalof the droplets and the particles.
1.0(b) Microchannel

Measurement
Poisson dist

P(n)

0.8
0.6
0.40.2(c) Rail-type and boxcar-type electrodes

0.00123n45Fig. 4: Frequency distribution of the number ofparticles in droplet (=0.4).

(d) Flow-focusing section

Fig. 1: Schematic images of (a) particle alignment byapplying periodic force over space and time using
dielectrophoretic force, (b) microchannel withparticle alignment and droplet generating regions, (c)rail-type and boxcar-type electrodes attached to thebottom wall, and (d) water in oil droplet generation
using flow-focusing channel (unit in m).

3. Results and Discussion
Fig. 2 shows the photograph of the aligned
particles, droplet generation, and particle
encapsulation at the flow-focusing section. Fig.
3 shows the probability density function (PDF)of the interval between the droplets, and theaccuracy of the interval between the particles atthe boxcar-electrode region. t is the deviationof the particle time interval from the on-off
period ton-off of the applied voltage. The droplets
are generated periodically at the flow-focusing
region, and the PDF gives a peak at 66ms withstandard deviation of 1.4ms. The PDF of tp

gives a peak at zero with reasonably small
variation showing accurate particle alignment.
Fig. 4 shows the frequency distribution of
the number of particles encapsulated in thegenerated droplets. The average number ofparticles in droplet is =0.4. By controlling theinterval of the particles using the present
method, all droplets which encapsulate theparticles have only one particle (n=1), anddroplets with more than one particle was notfound, representing 100% yield of one-to-one
particle encapsulation. If we compare it with thecase of particles flowing with random interval(Poisson distribution), the rate of n=1 increases
by 50%. This improvement of the encapsulation
performance becomes more significant as 
increases, and leads to higher throughput.
References
1. Tatsumi, K. et al., Medical Engineering andPhysics, (2016) 38, pp. 24-32.
2. Tatsumi, K. et al., Analytical Chemistry, (2019)91, pp. 6462-6470.
3. Tatsumi, K. et al., Microfluidics and Nanofluidics,(2021) 25:15.

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