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전기장에서 거대결정성 콜로이드 집합체의 제조
문준혁, 이기라, 양승만
한국과학기술원 화학공학과
Macrocrystalline colloidal assemblies in an electric field
Jun Hyuk Moon, Gi-Ra Yi, and Seung-Man Yang
Department of Chemical Engineering
Korea Advanced Institute of Science and Technology, Taejon
305-701
Introduction
Macrocrystalline colloidal assemblies show great promise as building block for advanced
materials. Closely packed crystals display a number of potentially applicable characteristics
such as photonic crystals [1], high packing density and surface-to-volume ratio, maximal
structural stability, and high catalytic/reactivity throughput. For practical application of these
structures, the crystals should be mounted or shaped into usable objects. a number of
methods have been exploited using colloidal crystallization in confined geometry. One of the
simplest approaches involves the use of suspension droplet as confined geometry. When the
suspension droplet is dried, the crystallization of submicron-sized colloidal particles occurs
inside the droplet leading to highly ordered colloidal assemblies [2]. Such structured
materials have an important application as chromatography and biocatalyst [3]. Complex
composite assemblies can be fabricated when the original drop contains a mixture of two or
more different particles. Such anisotropic particles can be manipulated by electric or
magnetic fields and can find application in electronic paper [4]. In the previous works, the
shapes of assemblies were controlled by means of thermodynamic properties, which led to
assemblies with several symmetric shapes such as spherical and disk [3]. However, in many
practical applications, asymmetric shape may be of considerable significance [4]. Therefore,
we designed the shape of the drop by use of an AC electric field.
Theory
Under an ac field, a liquid drop suspended in an immiscible liquid deform and oscillate,
which is due to the electric and the hydrodynamic stresses at the interface between the drop
and the medium. We defined the total deformation D in terms of the major and minor axis
lengths of deformed drop. We wrote this as the sum of a steady component D V and an
oscillatory component D T [5]:
(1)
D  DT  DV
First, the steady deformation stems from the balance of the sum of the steady electric and
hydrodynamic normal stresses at the interface and the interfacial tension because of changes
in surface curvature accompanying deformation of the drop.
2
9 K
Dv  0 2  v (E0b)
(2)
16
where  is the interfacial tension,  0 is the permittivity of free space and b is the ratio of
the drop.
R(11  14)  R2 [15(  1)  q(19  16)]  15a22 (1 )(1 2q)
(3)
5(  1)[(2R  1)2  a2 2 (1  2q)2 ]
where R, q ,  is the resistivity ratio, the dielectric constant ratio and the viscosity ratio,
respectively.
Second, the oscillation deformation results from the balance of the total oscillating normal
stress at the drop surface and interfacial forces through oscillation.
9 K Icos(2t  ) 2
(4)
DT  0 2
(E0b)
32
(1 k 222 )
 v  1
1

a2 2 (1 Rq)2 (19  16)[20q(1 )  (  4)]  2
I  2v 

25(1 )2 [(2R  1)2  a22 (q  2)2 ]2


where  is the frequency.
the steady value we consider the ratio:
DT Icos(2t  )

Dv  v 1 k 2 22
(5)
(6)
The dimensionless parameter k that characterize ( DT / DV ) represents the ratio of the
oscillatory hydrodynamic stress (  2 ) and the capillary pressure (  / b ) at the drop
interface. When (  2 )   and (  / b ) remains fixed, k   and ( DT / DV )  0 , the
drop surface being unable to respond to the oscillating stress. Therefore, we can obtain the
steady deformation without electrophoresis.
Experimental
In this study, we dispersed polystyrene latex particles inside the droplets, and applied an
AC electric field to the aqueous droplet floating on the partially fluorinated oil (Shin
Etsu: FL-100, viscosity  = 1 Pa s). In a DC field, a liquid drop suspended in an
immiscible liquid occurs the electrophoresis : the drop translated when it acquired a small net
electric charge. In order to eliminate the electrophoresis during the drying of the suspension
droplet, we use an AC field. During the evaporation of the drop-phase liquid, the aqueous
suspension droplet containing the polystyrene particles become macrocrystalline particles
with the deformation induced by the applied electric field.
Aqueous droplets of colloidal suspension were placed on the surface of the partially
fluorinated oil (Fig. 1). The suspension droplets, of initial volume of 40 micro-liters,
contained 10 % in volume of negatively charged polystyrene latex micro-spheres of 200-320
nm in diameter.
Top view
Suspension E
drop
dropdroplet
Side view
E
Oil Electrod
e
Figure 1. Schematic of the apparatus
2.2
2.1
Radius (mm)
2.0
1.9
1.8
1.7
1.6
1.5
1.4
0
50
100
150
200
250
300
Time (min)
Figure 2. Variation of the
drop size according to the
evaporation
A
The droplet shrank as a function of the
elapsed time (Fig. 2). After 6 hours, the
macrocrystalline
crystalline
structure
was extracted from the oil and dried out. In
the absence of the electric field, the
assembly process yielded smooth spherical
assemblies 1 to 2mm in diameter (Fig. 3A).
The surface of these balls was bright and
its color depended on the size of the PS
latex particles. The color is from light
diffraction by the colloidal arrays,
indicating long-range ordering of the PS
latex particles. In an electric field, we get a
spheroidal shape (Fig. 3B) and the
deformation of the particle increases with
electric field intensity.
B
1mm
1mm
Figure 3. Macrocrystalline particle without (A), and with the
action of an electric field (B).
Results
Cross-sectional images of scanning electron microscopy (Fig. 4) suggest that the
interstices of colloidal assemblies were filled with silicone oil. To use this particle as
structured porous materials, oil infiltration should be avoided, or even in the worst case,
infiltrated oil should be removed. The infiltration is mainly due to the high viscosity of the
oil (Fig. 4). We can infer that the oil diffused into the drop during the evaporation and flow
out while the particle being dried
A
B
C
Figure 4. (A) The oil infiltrates and remains inside the
particle. From the image of particle that has been in the
silicone oil of 0.01 Pa s (B) and 1 Pa s (C), we find the higher
viscosity keep the medium oil from infiltration.
We removed the infiltrated oil by solvent extraction. the partially fluorine-substituted
silicone oil was able to remove by hexane or diluted HF, which did not dissolve the colloidal
crystals of polystyrene (Fig. 5).
Figure 5. Image which is free from the infiltrated oil by
treating with the solvent.
Deformation of the particle reduced
during evaporation (Fig. 6). This is
because electrical property, size and
viscosity change as the time increasing.
Therefore, for obtaining the properly
deformed particle, we raised the electric
field intensity during evaporation. By the
equation (2) mentioned above, we could
0.12
0.10
D (=DV)
0.08
1.21 kV/
1.01 kV/
0.81 kV/
0.61 kV/
0.06
cm
cm
cm
cm
0.04
2
plot the deformation D to (E0b) .
There was a large deviation from the
equation, which is probably due to the
assumption of the equation (Fig. 7A, 7B).
0.02
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45

0.10
Figure
6.
Plot
of
the
deformation to the volume
fraction of the suspension
drop.
0.10
0.08
0.06
0.06
D
D
0.08
0.04
0.04
0.02
0.02
0.00
0.00
0
10
20
2
E2b (kV / cm)
30
40
0
5
10
15
20
25
2
E2b (kV / cm)
Figure
7.
Plot
for
the
volume fraction of 10% (A) and 40% (B). The deviation from
theory increases as the fraction increases.
References
1. J. E. G. J. Wijnhoven and W. L. Vos, Science 281, 802
(1998)
2. O. D. Velev, A. M. Lenhoff and E. W. Kaler, Science 287, 24
(2000)
3. F. Svec and J. M. J. Frechet, Science 273, 205 (1996)
4. B. Comiskey, J. D. Albert, H. Yoshizawa, J. Jacobson,
Nature 394, 253 (1998)
5. S. Torza, R. G. Cox and S. G. Mason, Proc. Roy. Soc. Lond.
269, 27 (1970)