PYKLINOPHORESIS: A NEW PARTICLE MIGRATION PRINCIPLE … · density (pyknotita), -klino-: incline (klino), -phoresis: migration), where a particle situated in a microfluidic interface

PYKLINOPHORESIS: A NEW PARTICLE MIGRATION PRINCIPLE DRIVEN BY DENSITY GRADIENT AND APPLICATION TO ANALYSIS

OF SOLUTION DENSITY IN A MICROFLUIDIC DEVICE

Joo H. Kang, Bumjun Kim and Je-Kyun Park Department of Bio and Brain Engineering, KAIST, KOREA

ABSTRACT

We report a novel particle migration principle, pyklinophoresis (Greek; pyk-: density (pyknotita), -klino-: incline (klino), -phoresis: migration), where a particle situated in a microfluidic interface with different fluid density moves toward the lower-density fluid which is driven by the pressure difference asymmetrically acting on the submerged particle. Moreover, we successfully applied this novel principle to analyzing the aqueous solution concentration which is related with the solution den-sity. KEYWORDS: Density gradient, Microfluidic, Microparticle, Separation

INTRODUCTION

Investigation of various forces acting on micro- and nanoparticles is important because they reveal ways to manipulate and separate particles in microfluidic envi-ronments. Several principles, such as magnetic, dielectric, hydrodynamic methods, have been devoted to handle and sort many kinds of particles.[1] Pyklinophoresis newly defined herein is physically different from the two-phase fractionation which is based on the difference of affinity between two aqueous phases.

THEORY

In general, buoyancy is the force acting on an immersed particle in a vertical di-rection, which is produced by the pressure difference between the top and bottom of the particle. However, it is based on the assumption that the submerged particle is

surrounded by the homogeneous fluid. If the particle is placed at the interface between two fluids with the different density, the pressure disparity produces an asymmetric force in perpendicular to the fluidic interface (Figures 1 and 2). The hydrostatic force on the submerged particle is the integral of the stress over the surface of the particle as follows [2].

∫∫ −=∇−=VV

p dVgdVpF ρ (1)

Figure 1. A scheme of pyklinophoretic movement of a particle in a microfluidic channel

978-0-9798064-1-4/µTAS2008/$20©2008CBMS 1468

Twelfth International Conference on Miniaturized Systems for Chemistry and Life SciencesOctober 12 - 16, 2008, San Diego, California, USA

EXPERIMENTAL The particle separation was conducted using poly(dimethylsiloxane) (PDMS)

microfluidic devices having three inlets and single outlet presented in Figure 3. The microfluidic channels were designed to have 20 μm in height and 100 μm in width, and the straightforward separation channel is about 11 mm in length. The flow rate at the inlet A and B is around 1.0 μL min-1 and 0.1 μL min-1 at the inlet C.

RESULTS AND DISCUSSION

Since the half of the particle is sub-mersed in each solution A and B in Fig-ure 2, the lateral force (FPKP) on the par-ticle can be presented as the force difference between the solution A and B (FA-FB). If a particle of 15.0 μm in diameter is at the interfacial region between 10% sucrose solution (ρ ≈ 1.041 g cm-3 at 25°C) and deionized water in half, FPKP driven by the pressure difference can be estimated 0.38 pN using eqn (1). FPKP is increased as the concentration of the sucrose solution becomes higher (F ≈ 2.02 pN, when 50% (ρ ≈ 1.234 g cm-3) sucrose solution). Because the particle size is related with FPKP and the lateral displacement of the particle (Figure 2(b)), our model can be proved by the

Outlet channel (1000 μm in width)

10 μm PS particle

15 μm PS particle

flow direction

Figure 4. The measured lateral position of polystyrene (PS) particles (10~15 μm) at the outlet channel which support the analytical model. Inset shows the particles (10 and 15 μm) flowing out at the extended outlet chan-nel.

Fbuo

Fgrav

symmetric force

asymmetric force

Density gradient across microchannel

Equal density across microchannel

O O’ρBρA

FPKP

ρA < ρB

PaPb

x

y

FB

FB

ρA = ρB

FA

FA = FB

FA < FB

FA

dp

ρB

x

yz

w

(Pyklinophoretic force)

(ρB = ρC) > (ρA)

ρC

ρA

FPKPO

O’(a)

(b)

B

C

A

Figure 2. The principle of pyklinophoresis in a two-phase laminar flow. (b) is the cross-sectional view of O-O’ in (a). A particle im-mersed in the center of the two-phase density gradient is pressured to move toward the lower-density solution.

100 μm-widemicrofluidic channel

Solution B(10% sucrose)

Solution C(10% sucrose + PS particles)

Solution A(DIW)

Interfacial linebetween two laminar flows with different density

PS particle

Figure 3. A picture shows an intersection of the microfluidic device.

1469

Twelfth International Conference on Miniaturized Systems for Chemistry and Life SciencesOctober 12 - 16, 2008, San Diego, California, USA

demonstration of the particle sorting depending on the size in the PDMS microflu-idic devices (Figure 4). As theoretically expected, the polystyrene particles were de-flected from their initially focused path by pyklinophoresis according to their size (Figure 4). Moreover, we increased the net flow rate to reduce the retention time of the particles in the microchannel, revealing the lateral velocity by pyklinophoresis. The particles rapidly migrate in the microchannel even at the considerably high flow rate of 16.8 μL min-1 (Figure 5). Finally, we applied pyklinophoresis to the analysis of the sucrose concentration varying the concentration of A in Figure 3 (Figure 6). This concentration analysis is based on the principle that the sucrose concentration is related with the density and that FPKP is consequently proportionate to the difference in the solution density.

CONCLUSIONS

Disclosing a new physical principle is significant not only for developing useful applications but also for understanding incomprehensible results we frequently meet in micro-scale environments. This new principle will bring a valuable contribution to the various multiphase microfluidic systems handling the microparticles.

ACKNOWLEDGEMENTS

This work was supported by the Korea Science and Engineering Foundation NRL Program grant funded by the Korea government (R0A-2008-000-20109-0). REFERENCES [1] J. H. Kang, S. Choi, W. Lee, J.-K. Park, Isomagnetophoresis to discriminate sub-

tle difference in magnetic susceptibility, J. Am. Chem. Soc., 130 (2), 396-397 (2008).

[2] G. A. Truskey, F. Yuan, and D. F. Katz, Transport Phenomena in Biological Sys-tems, Pearson Education, Inc., Upper Saddle River, New Jersey, (2004).

Figure 5. The lateral positions of 15 μm PS particles with variation of the net flow rate in the microfluidic channel. As the flow rate increases, the particles are less deflected from the centered line of the mi-crochannel.

Figure 6. An analysis of sucrose concen-tration using pyklinophoresis. Because FPKP is proportionate to the difference in density (concentration) between solution A and B, C, the lateral displacement of 15 μm PS particles is correlated with the concentration of the solution.

1470

Twelfth International Conference on Miniaturized Systems for Chemistry and Life SciencesOctober 12 - 16, 2008, San Diego, California, USA