We experimentally demonstrate an optical transportation and controllable positioning of polystyrene nanospheres using a 3 μm diameter microfiber. By placing the microfiber in a microfluidic channel and injecting a 980 nm laser light into the fiber, nanospheres suspended in the water were stably trapped to the microfiber and delivered along the direction of light propagation. Furthermore, by increasing the velocity of the fluid in the opposite direction of the laser light, it was found that, once the fluid velocity increased to 6 μm/s, spheres stopped their forward progress and halted on the microfiber, so the controllable positioning of spheres along the microfiber was realized.

Optical trapping capacitates noncontact manipulation of particles with high accuracy.1–5 Since the pioneering article concerning microparticle manipulation using evanescent wave was demonstrated in 1992 by a high refractive index prism,6 evanescent wave-based optical capture and transportation has been extensively applied to manipulate particles in the field of physics, chemistry, biology and medicine.6–9 For example, Schmidt group reported the propulsion of microparticles using a bending solid core waveguide which is directed opposite to the direction of the flow.8 Yang et al. utilized sub-wavelength slot waveguides to manipulate nanoparticles and DNA molecules.9 By comparison to the waveguides fabricates on substrates,10–12 optical nanofibers present some unique features, which include considerable power leakage of the fibers, extremely low optical insertion loss, high flexibility and so on, this make them more convenient and efficient to trap and transport particles.13–17 However, the preparation of nanofibers needs careful handing, which brings about many difficulties in the process of manipulation. In this paper, we demonstrate optical trapping and manipulation of nanospheres using an optical microfiber with the assistance of 980 nm laser light. The use of 980 nm laser is because this wavelength will be of weak absorption of most organisms,1 and the microfiber, made up of two small size fibers in a parallel and contact state, is easily fabricated from two twisting single-mode optical fibers through a flame-heated method, the evanescent wave of each small size fiber can be used to manipulate particles, and the overall size of the microfiber increases, which makes the experimental operation more easier. Therefore, this technique could find great potential for micro-/nano- particles trapping and manipulation.

The experimental apparatus is schematically shown in Fig. 1. A computer-interfaced charge-coupled device (CCD) camera is used to monitor experimental phenomena. A microfiber is positioned in a fluidic channel with a suspension of polystyrene spheres, the flow of the solution, prepared by diluting polystyrene spheres with a diameter of 700 nm in deionized water (volume ratio of spheres to water is 1:1,500), can be realized using a micropump (KDS LEGATO 270) with a push-pull configuration, the fluid velocity can be set accurately by micropump. In the experiment, the microfiber, fixed by two tunable microstages, was fabricated from two twisting single-mode optical fibers (SWF-28, Coring inc.) by means of “flame brushing” technique. The pigtails of two optical single-mode fibers are connected to a 980 nm laser source through a 1 × 2 fiber optical coupler (FOC).

FIG. 1.

Schematic of the experimental setup.

FIG. 1.

Schematic of the experimental setup.

Close modal

To show the trapping and delivery ability of the microfiber, the suspension was firstly kept stationary (i.e. the fluid velocity was 0 μm/s), and the optical power at the output of the laser source was obtained by an optical power meter. When the optical power of 980 nm laser increased to be 20 mW, it was observed that, polystyrene spheres near the microfiber were trapped to the microfiber by optical gradient force, and transported along the direction of light propagation owing to the scattering force originated from the evanescent field.18 When the laser was closed, the spheres immediately stopped their advance and suspended in the water. Fig. 2 shows three sequential pictures taken by the CCD in the optical power of 40 mW, and the interval is 1 s. It can been estimated that, the average velocities of spheres A, B, C, D, E, F, G and H were 8.8 μm/s, 9.4 μm/s, 10.3 μm/s, 6.4 μm/s, 9.7 μm/s, 5.2 μm/s, 6.1 μm/s, and 8.9 μm/s, respectively. Fig. 3 shows the relation of the propelling velocity of nanospheres and input optical power, it can be observed that, the delivery velocities increases with optical power with a high linearity, this is because the optical scattering force linearly increases with optical intensity (i.e. optical power).

FIG. 2.

Three sequential microscope images of polystyrene spheres A, B, C, D, E, F, G and H propelled along the microfiber at an optical power of 40 mW within an intervals of 1 s.

FIG. 2.

Three sequential microscope images of polystyrene spheres A, B, C, D, E, F, G and H propelled along the microfiber at an optical power of 40 mW within an intervals of 1 s.

Close modal
FIG. 3.

The relation between the measured velocities and input optical power. Error bars denote the standard deviation of sphere velocities. The inset is the scanning electron microscopy (SEM) image of 3 μm microfiber.

FIG. 3.

The relation between the measured velocities and input optical power. Error bars denote the standard deviation of sphere velocities. The inset is the scanning electron microscopy (SEM) image of 3 μm microfiber.

Close modal

Optical trapping and transportation of nanospheres can be easily realized by an optical microfiber. When the light is injected into the microfiber, optical force will be applied to the spheres. The optical force (FO) can be obtained by integrating the time-independent Maxwell srtess tensor (〈TM〉) along the enclosing surface of the sphere. 〈TM〉 is described by

T M = DE * + HB * 1 / 2 D E * + H B * I ,
(1)

where D is the electric displacement, H is the magnetic field, E is the electric field, B is the magnetic flux field, and Iis the isotropic tensor. The optical force FO can be obtained by

F o = s ( T M n ) d s ,
(2)

which include two orthogonal components, i.e. the gradient force (Fg) and the scattering force (Fs).

Moreover, when a flow suspension was pumped into the channel, and the direction of the fluid velocity is opposite to the laser light, spheres suspended in the water will exert a viscous drag force (Fd) induced by the fluid, as shown in Fig. 4. Fd act on the sphere can be acquired by integrating the the fluid tensor along the enclosing surface of the sphere, The fluid stress tensor TF is given by

T F = p I + μ ( v + v T ) ,
(3)

Fd is described by

F d = s ( T F n ) d s .
(4)
FIG. 4.

Schematic of force analysis.

FIG. 4.

Schematic of force analysis.

Close modal

From above analysis, it can be concluded that, if the Fd is equal to Fs acting in contrary direction, the spheres will be halted on the microfiber, and the controllable positioning can be realized. In the experiment, the optical power was set to be 30 mW. When the fluid velocity was increased from 0 to 6 μm/s, we observed that, the delivery velocity along the direction of laser light decreased with the increase of fluid velocity. By increasing the fluid velocity to be 6 μm/s, it can be seen that, spheres situated in the regions of A and B stopped their forward progress and halted on the fiber, as shown in Fig. 5. At t = 0 s, spheres in the regions A and B were halted and positioned on the microfiber (Fig. 5(a)). At t = 1 s, the spheres remained halted on the fiber (Fig. 5(b)). It should be pointed out that, spheres I and II was not captured to the microfiber but moved with the fluidic motion.

FIG. 5.

CCD captured images for controllable positioning of 700 nm polystyrene spheres at an optical power of 30 mW, and the flow velocity was 6 μm/s.

FIG. 5.

CCD captured images for controllable positioning of 700 nm polystyrene spheres at an optical power of 30 mW, and the flow velocity was 6 μm/s.

Close modal

In conclusion, we have experimentally demonstrated the stable optical delivery and controllable positioning of 700 nm polystyrene spheres using a 3 μm diameter microfiber in a microfluidic channel. When the 980 nm laser was injected into the microfiber, nanospheres suspended in the stationary suspension were trapped and transported along the fiber, and the measured velocity is linearly fitted with the input power. Moreover, the controllable positioning of spheres was also demonstrated when a flow opposite to the light propagation direction was applied. This optical method is expected to find applications in nanoparticles manipulation such as drug delivery in blood and tissue fluid.

This work was supported by the National Natural Science of Foundation of china (Grant 11404069).

1.
F. M.
Fazal
and
S. M.
Block
, “
Optical tweezers study life under tension
,”
Nat. Photonics
5
,
318
-
321
(
2011
).
2.
D.
Erickson
,
X.
Serey
, and
Y. F.
Chen
, “
Sudeep Mandal. Nanomanipulation using near field photonics
,”
Lab Chip
11
,
995
-
1009
(
2011
).
3.
D. G.
Grier
, “
A revolution in optical manipulation
,”
Nature
424
,
810
816
(
2003
).
4.
Y. Y.
Sun
,
X. C.
Yuan
,
L. S.
Ong
,
J.
Bu
,
S. W.
Zhu
, and
R.
Liu
, “
Large-scale optical traps on a chip for optical sorting
,”
Appl. Phys. Lett.
90
,
0311071–3
(
2007
).
5.
M. J.
Guffey
and
N. F.
Scherer
, “
All-optical patterning of Au nanoparticles on surfaces using optical traps
,”
Nano Lett.
10
,
4302
4308
(
2010
).
6.
S.
Kawata
and
T.
Sugiura
, “
Movement of micrometer-sized particles in the evanescent field of a laser beam
,”
Opt. Lett.
17
,
772
774
(
1992
).
7.
R. W.
Applegate
,
J.
Squier
,
T.
Vestad
,
J.
Oakey
,
D. W. M.
Marr
,
P.
Bado
,
M. A.
Dugan
, and
A. A.
Said
, “
Microfluidic sorting system based on optical waveguide integration and diode laser bar trapping
,”
Lab Chip
6
,
422
-
426
(
2006
).
8.
B. S.
Schmidt
,
A. H. J.
Yang
,
D.
Erickson
, and
M.
Lipson
, “
Optofluidic trapping and transport on solid core waveguides within a microfluidic device
,”
Opt. Express
15
,
14322
-
14334
(
2007
).
9.
A. H. J.
Yang
,
S. D.
Moore
,
B. S.
Schmidt
,
M.
Klug
,
M.
Lipson
, and
D.
Erickson
, “
Optical manipulation of nanoparticles and biomolecules in sub-wavelength slot waveguides
,”
Nature
457
,
71
-
75
(
2009
).
10.
S.
Gaugiran
,
S.
Getin
,
J. M.
Fedeli
,
G.
Colas
,
A.
Fuchs
,
F.
Chatelain
, and
J.
Derouard
, “
Optical manipulation of microparticles and cells on silicon nitride waveguides
,”
Opt. Express
13
,
6956
-
6963
(
2005
).
11.
B. S.
Ahluwalia
,
P.
Løvhaugen
, and
O. G.
Hellesø
, “
Waveguide trapping of hollow glass spheres
,”
Opt. Lett.
36
,
3347
-
3349
(
2011
).
12.
A. H. J.
Yang
and
D.
Erickson
, “
Stability analysis of optofluidic transport on solid-core waveguiding structures
,”
Nanotechnology
19
,
045704
(
2008
).
13.
G.
Brambilla
,
G. S.
Murugan
,
J. S.
Wilkinson
, and
D. J.
Richardson
, “
Optical manipulation of microspheres along a subwavelength optical wire
,”
Opt. Lett.
32
,
3041
-
3043
(
2007
).
14.
Y.
Li
and
Y. J.
Hu
, “
Optical manipulation of gold nanoparticles using an optical nanofiber
,”
Chin. Phys. B
22
,
034206
(
2013
).
15.
H. B.
Xin
and
B. J.
Li
, “
Targeted delivery and controllable release of nanoparticles using a defect-decorated optical nanofiber
,”
Opt. Express
19
,
13285
-
13290
(
2011
).
16.
H. B.
Xin
,
Chang
Cheng
, and
B. J.
Li
, “
Trapping and delivery of Escherichia coli in a microfluidic channel using an optical nanofiber
,”
Nanoscale
5
,
6720
(
2013
).
17.
C.
Xu
,
H. X.
Lei
,
Yao
Zhang
, and
B. J.
Li
, “
Backward transport of nanoparticles in fluidic flow
,”
Optics Express
20
,
1930
(
2012
).
18.
K. C.
Neuman
and
S. M.
Block
, “
Optical trapping
,”
Rev. Sci. Instrum.
75
,
2787
(
2004
).