optical separation of droplets on a microfluidic platform

RESEARCH PAPER

Optical separation of droplets on a microfluidic platform

Jin Ho Jung • Kyung Heon Lee • Kang Soo Lee •

Byung Hang Ha • Yong Suk Oh • Hyung Jin Sung

Received: 16 May 2013 / Accepted: 10 September 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract This paper describes the optical separation of

microdroplets according to their refractive indices. The

behavior of the droplets was characterized in terms of the

optical force and the hydrodynamic effects present upon

illumination of the droplets in a direction normal to the

flow direction in a rectangular microfluidic channel. The

optical forces acting on the droplets and the resultant

droplet trajectories were analyzed and compared with the

numerically predicted values. The relationship between the

drag force and optical force was examined to understand

the system performance properties in the context of

screening applications involving the removal of unwanted

droplets. Two species of droplets were compared for their

photophoretic displacements by varying the illumination

intensity. Because the optical forces exerted on the droplets

were functions of the refractive indices and sizes of the

droplets, a variety of chemical species could be separated

simultaneously.

Keywords Optical force �Droplet �Two-phase flow �Droplet migration � Passive separation � Optofluidics

1 Introduction

Screening large libraries of samples using conventional

techniques involving single cell analysis or chemical

reaction profiling is hindered by inherent time and cost

limitations. To overcome these limitations, lab-on-a-chip

technologies have been developed that take advantage of

continuous flow systems (Sia and Whitesides 2003; Squires

and Quake 2005; Song et al. 2006). The formation of

microdroplet emulsions in a flow cell provides one exam-

ple of a microfluidic technique and offers addressable

separated microreactors that are useful for isolating DNA,

cells, particles, or chemical reactions (Song et al. 2006;

Kelly et al. 2007; Teh et al. 2008; Tewhey et al. 2009; Um

et al. 2012). The carrier fluid in a microdroplet emulsion

system imposes a physical barrier between the isolated

samples, thereby preventing diffusion or cross-contamina-

tion. Several techniques have been developed to individu-

ally manipulate or store the micron-sized droplets of a

microfluidic platform (Fair 2007). The facility of individual

microdroplet manipulation is an essential function of

micrototal analysis systems (Theberge et al. 2010).

Microdroplets may be sorted using a variety of tech-

niques, including dielectrophoresis (Agresti et al. 2010),

magnetic force manipulation (Zhang et al. 2009), electro-

phoresis (Dittrich and Schwille 2003), surface acoustic

wave separation (Franke et al. 2010), pinched flow frac-

tionation (Maenaka et al. 2008), or deterministic lateral

displacement separation (Joensson et al. 2011). Methods

that enable label-free screening with passive separation of,

for example, chemical species or satellite droplets are

particularly in demand. Techniques based on optical forces

are appropriate for delicately controlling small objects

(Ashkin 1970; Grier 2003; Dholakia and Cizmar 2011). For

example, separation methods involving optical tweezers

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10404-013-1263-0) contains supplementarymaterial, which is available to authorized users.

J. H. Jung � K. H. Lee � K. S. Lee � B. H. Ha �Y. S. Oh � H. J. Sung (&)

Department of Mechanical Engineering, KAIST,

291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea

e-mail: [email protected]

123

Microfluid Nanofluid

DOI 10.1007/s10404-013-1263-0

http://dx.doi.org/10.1007/s10404-013-1263-0

can accurately control target samples (Imasaka et al. 1995;

MacDonald et al. 2003). Optical forces depend on target

objects’ physical properties, such as its size and the

refractive index contrast between the object and the sur-

rounding fluid media. Optical separation techniques may be

used to screen satellite droplets or droplets that yield false

positives. Optical tweezers have been integrated into

microfluidic passive separation platforms in which a col-

limated array of beam lines is directed into the flow car-

rying micron-sized dielectric objects (Ashkin 1997;

Gauthier and Wallace 1995; Grier 2003).

Some researchers have introduced optical forces into

microfluidic channels for optical chromatography applica-

tions (Hebert et al. 2011), optical lattice separation (Mac-

Donald et al. 2003), or cross-type optical particle

separation (Kim et al. 2008). The manipulation of particles

in a double emulsion using optical forces in a glass capil-

lary has been studied (Lee et al. 2012). Studies involving

the application of optical forces to small objects have

tended to neglect the hydrodynamic effects of the carrier

fluid; however, emulsion droplets are typically larger than

commercially available microparticles, and their motions

can be affected by the microfluidic channel shape and

geometry. Droplet behaviors under large drag forces, such

as those experienced in a confined rectangular microfluidic

channel, or under optical forces require additional study.

This paper describes an analysis of the behavior of

droplets in the presence of optical forces in a microfluidic

channel. A droplet screening application is demonstrated

using optical forces based on the refractive index mismatch

between the droplets and the carrier fluid. Droplets were

generated in a microfluidic channel and transported in the

bulk flow. The trajectories of the droplets in the micro-

fluidic channel were deflected by a focused light beam

directed normal to the fluid flow. The optical force on the

droplets was modeled using the photon stream method in

combination with the particle dynamics equations. Many

particle separation models have described particle motions

based on external forces and the Stokes’ drag force. Rel-

atively large droplets can experience additional drag forces

that must be considered. The additional drag force may be

defined in terms of the microfluidic channel and geometry

to more accurately model the optical manipulation of

droplets. This paper offers a theoretical model that

describes experimental droplet screening techniques in

terms of the intrinsic droplet properties, including the size,

refractive index, and behavior of the droplets.

2 Theory

Droplet behavior was observed using the microfluidic

device shown in Fig. 1. Two aqueous droplet species were

generated using a typical T-junction method. The aqueous

droplets were pinched off from the main stream into a

continuous fluid phase (the oil phase) and transported to the

test section (Fig. 1b, c). In the test section, the droplet

trajectories were observed in the presence of a focal vol-

ume introduced by illumination with a 1,064 nm laser

beam directed normal to the flow direction. The optical

forces and drag forces in the microfluidic channel with a

square cross-section were modeled using the particle

dynamics equations. The optical forces were modeled

using the ray optics method to describe the radiation force

Fig. 1 a Schematic illustration

of the PDMS microfluidic

device used to generate two

species of droplets. The device

successfully separated the

droplets at the outlets.

b Generation of droplets at the

T-junction. c Optical

manipulation of the droplets.

d The illumination conditions

induced the label-free droplet

separation at the y-shaped

bifurcation


123

that acted on the transparent objects. The drag force

became dominant for high ratios between the droplet size

and the channel dimensions (Thorsen et al. 2001). The

system performance was expressed in terms of the pho-

tophoretic displacement, which measures the lateral dis-

tance through which a particle trajectory had been deflected

from the main longitudinal streamlines due to the presence

of the optical force (Helmbrecht et al. 2007). The pho-

tophoretic displacement corresponds to the optical chro-

matography resolution in a cross-type optical particle

separation regime (Kim et al. 2008).

In addition to the optical radiation force, several

hydrodynamic forces, including inertial forces, wall

repulsion forces, drag forces, and buoyancy forces, influ-

ence particle migration in an optical microfluidic separa-

tion device (Hatch et al. 2013). In this study, the inertial

migration and wall lift forces were neglected because the

Reynolds number was not sufficiently high to affect the

overall particle movement. The buoyancy force was not

considered because the Bond number was low. The drag

and optical forces were modeled using the particle

dynamics equations (Kim et al. 2008),

md

dud

dtþ 6plrdðU � udÞ ¼ Fgrad; ð1Þ

md

dvd

dtþ 6plrdvd ¼ Fscatter: ð2Þ

where md is the droplet mass, ud is the x-directional droplet

velocity, l is the viscosity, rd is the droplet radius, U is the

velocity of the fluid flow, and vd is the y-directional droplet

velocity. Fgrad is the optical gradient force that pushes the

droplet toward the center of the light beam. Fscatter is the

optical scattering force that pushes the droplet along the

axial direction of the light beam.

Droplets with a size that is comparable to the micro

channel geometry experience a large flow-induced drag

force. Under these conditions, the Stokes’ drag force terms

6plrdðU � udÞ and 6plrdvdÞ must be modified. The drag

force acting on a particle or bubble is affected by the

confined channel geometry and may be modeled by

adjusting the effective viscosity and the drag coefficient as

(van der Sman 2010),

FD ¼ 6pl�rd k1u1 � k2udð Þ; ð3Þ

where u? is the velocity of the fluid at the streamline in the

absence of the particle and l* is the effective viscosity. The

effects of the co-flowing fluid inside the droplet were

addressed by considering the viscosity of the disperse

phase fluid. The effective viscosity may be expressed as

l� ¼ lc

1þ 2lc=3ld

1þ lc=ld

ð4Þ

where lc is the continuous phase flow viscosity and ld is

the disperse phase flow viscosity. k1 and k2 are the drag

coefficients and are functions of the droplets size, location,

and dimensions of the channel. k1 is calculated from a

stationary droplet in a bulk fluid flow, and k2 is estimated

from the motion of a droplet in a quiescent fluid. The

coefficients k1 and k2 may be estimated using the analytic

solutions described in previous studies of the drag forces

acting on a single plate. The solutions describing the drag

forces acting on a single plate may be linearly summed.

Geometrical diagrams showing the drag force acting on

a droplet in a rectangular channel are given in Fig. 2a, d. A

two-dimensional diagram is shown in Fig. 2b. Happel et al.

(1983) developed a model for the relationship (k2,1)

between the drag force and the geometric configuration of

the droplet and the adjacent plate,

k2; 1ðjÞ ¼Fdrag;1

Fstokes

¼ 1

1� 9=16jþ 1=8j3 � 45=256j4 � 1=16j5;

ð5Þ

where h is the distance from the center of the droplet to a

single plate, and j is the ratio between h and rd (j = rd/h).

The subnotation 1 in k2,1 refers to the relationship between

the drag force and the droplet motion adjacent to the single

plate. The drag force on a particle moving between two

parallel plates may be modeled (Fig. 2b) by applying

Eq. (5) to both plates using the Oseen superposition

approximation,

k2;2;x ¼ 1þ k2;1ðrd=hÞ � 1� �

þ k2;1ðrd=ðH � hÞÞ � 1� �

;

ð6Þ

where k2,2,x is the drag coefficient that acts in the x-direc-

tion on a particle positioned between two parallel plates.

The increase in the drag force due to the presence of the

side walls in the microfluidic channel may be calculated in

a similar manner using Eq. (6).

The geometrical configuration is illustrated in Fig. 2c.

Equation (6) then gives the drag force coefficient in the x-

direction, k2,x (the subnotation x in k2,x refers to the x-

directional coefficient),

k2;x ¼ 1þ k2;2ðrd; h;HÞ � 1� �

þ k2;2ðrd;w0;WÞ � 1

� �;

ð7Þ

where the Oseen superposition principle is extended to the

rectangular channel (van der Sman 2010). The coefficient

k1,x, which is the drag force coefficient for a stationary

droplet in a bulk fluid flow directed along the x-direction,

can then be obtained from the generalized Faxen theorem

(van der Sman 2010),


123

k1; x ¼ k2;x 1� 1

3gðaÞk2 � cðaÞk5

� �; a ¼ W

H; k ¼ 2rd

H;

gðaÞ ¼1 slit

1:7 squre

(

; cðaÞ ¼ 0:125gðaÞ:

ð8Þ

The function g(a) is approximated within the range 1–1.7,

depending on the cross-sectional shape of the channel. The

above numerical analysis yielded the drag coefficients k1,x

and k2,x, which could then be used to modify the particle

dynamics equations in the x-direction according to

md

dud

dtþ 6pl�rdðk1;xu1 � k2;xudÞ ¼ Fgrad; ð9Þ

where Fgrad is the optical gradient force. Fgrad will be

considered in the next chapter.

The drag force coefficient in the y-direction may be

calculated using the above procedure. The relevant force

balance diagram is shown in Fig. 2d. Figure 2e illustrates

the confinement effects due to the floor and ceiling of the

microfluidic channel. These effects can be modeled by

assuming that the droplet moves between two parallel

plates to which k2,2,x is applicable. The droplet experiences

scattering forces that push the particle toward the wall, as

shown in Fig. 2f. The relevant coefficient may then be

evaluated according to (Kim 2004)

k2;10 ¼Fdrag

Fstokes

¼ 1

1� 9=8j0 þ 1=2j03; j0 ¼ rd

w0; ð10Þ

which predicts that the particle moves toward a single plate.

The subnotation 10refers to the single plate. The drag force

coefficient for a parallel plate system may be described as

k2; 2; y ¼ 1þ k2; 10 ðrd=w0Þ � 1� �

þ k2; 10 ðrd=ðW � w0ÞÞ � 1� �

;

ð11Þ

where k2,2,y is the drag force coefficient of the droplet

moving along the normal direction between the two

parallel plates. The coefficient k2, which is the drag force

coefficient for a stationary droplet in a bulk fluid flow along

the y-direction, may be estimated as

k2;y ¼ 1þ k2;2;xðrd; h;HÞ � 1� �

þ k2;2;yðrd;w0;WÞ � 1

� �;

ð12Þ

where k1 represents the drag force correction parameter

that accounts for the fluid flow around a stationary droplet.

If the fluid velocity is characterized by an x-directional

component alone, then k1,y is not needed. The particle

dynamic motion in the y-direction may then be modified as

md

dvd

dtþ 6pl�rdk2;yvd ¼ Fscatter: ð13Þ

Once the drag force correction factors had been

calculated, the optical forces acting on the droplet could

Fig. 2 Diagrams showing the definitions of variables used for

calculating the forces acting on the droplets confined in a rectangular

channel. a The forces acting on a particle along the x-direction in the

presence of optical forces. b A droplet moving parallel to the wall

(side view). c A droplet moving parallel to the wall (top view). d The

optical forces directed along the y-direction, acting on a droplet. e A

droplet moving parallel to the wall (side view). f A droplet moving

toward the wall (top view)


123

be calculated in the x- and y-directions. Three different

approaches were used to model the dielectric objects,

depending on their size: Rayleigh scattering theory, Mie

scattering theory, and the ray optics approach. The ray

optics approach was used here because the droplet size was

much larger than the beam wavelength (rp [ 20 lm).

Fresnel’s law for deflections and reflections was used to

track the photon’s pathway and momentum. The photon

stream method (Kim et al. 2008) was used to calculate the

optical gradient force (radial) and the scattering force

(axial):

Fgrad ¼ �n0

2c

Z2p

0

Zp2

0

Iðqk; zÞ R sin 2h1 � T2 sinð2h1 � 2h2Þ þ R sin 2h1

1þ R2 þ 2R cos h2

� �

� r2p sin 2h1 cos udh1du; ð14Þ

Fscatter ¼ �n0

2c

Z2p

0

Zp2

0

Iðqk; zÞ R cos 2h1 � T2 cosð2h1 � 2h2Þ þ R cos h1

1þ R2 þ 2R cos 2h2

� �

� r2p sin 2h1 cos udh1du: ð15Þ

where c denotes the speed of light in free space and n0

is the refractive index of the continuous phase fluid. h1

and h2 are the incident and deflected angles of the

beam at the interface of the droplets, respectively.

R and T are the Fresnel reflectance and transmittance,

respectively. I(qk, z) is the beam intensity profile where

qk and z are the radial and axial displacements from

the center of the beam to the center of the droplets,

respectively.

3 Experimental

A schematic diagram of the experimental setup is shown

in Fig. 3. A CW Nd:YAG 1,064 nm laser (Advance Op-

towave) in the TEM00 mode was used as the light source

with a maximum power of 10 W. The beam was focused

using an objective lens (Olympus NA = 0.45, 209) and

aligned using an IR imaging card (Melles Griot, Inc.). A

laser power meter (OPHIR, nova display) was used to

measure the beam power profile. In this experiment, the

translational location and incident angle of the beam were

carefully controlled. A custom-made 5-axis adjusting

stage was used to control the xyz axis and two rotational

axes of the microchannel. LED illumination (Mightex,

Inc.) and a sCMOS camera (Neo sCMOS, Andor) were

used to obtain the images. To avoid sCMOS camera

damage and to achieve clear experimental data, an IR

filter (Edmund Optics, Inc.) was inserted into the

mounting cube (Navitar, Inc.) in front of the sCMOS

camera detection optics. A syringe pump (Nemesys

Centoni GmbH) was used to control the flow rate of the

working fluid.

A PDMS microfluidic channel was fabricated by regular

soft lithography processes using a negative photoresist (Su-

8 2075, Microchem). A schematic diagram of the micro-

fluidic channel used for droplet generation is shown in

Fig. 4a. The width of the T-junction was 30 lm, and the

mechanical filter was positioned after the inlet port to

prevent clogging. The main test section was 600 lm wide

to permit observation of the droplet migration motions. The

channel height was 40 lm at the T-junction and 140 lm in

the main test section. These heights were used to control

the droplet size. The bilayer structure reduced the drag

forces by generating smaller droplets. A bifurcation junc-

tion was positioned at the end of the channel to permit two-

channel passive sorting. Because outlet 2 displayed a lower

hydraulic resistance, the droplets aligned at the center

collected through outlet 2.

HFE-7500 (C7F15OC2H5, n = 1.29, 3 M) was used as a

continuous phase fluid and contained 10v/v% 1H,1H,

2H,2H-perfluoro-1-octanol (C8H5F13O, n = 1.313, Sigma

Aldrich). The light wavelength used in this study,

1,064 nm, overlapped significantly with a water optical

absorption band. Absorption effects were avoided by using

heavy water (D2O, Sigma Aldrich) in the aqueous fluid

phase, as D2O displays a negligible absorption cross-sec-

tion at this wavelength. The refractive index (n) of the

Fig. 3 The experimental setup used to optically manipulate the

droplets. A 1,064 nm CW laser was used as the light source. The

objective lenses positioned the focal point at a test section in the

PDMS device. A syringe pump (NEMESYS Corp.) was used to

prepare the fluid flow in the microfluidic channel. Because the

alignment between the droplet and the light can significantly affect

the results, the sample was carefully adjusted using a translational and

rotational stage. Experimental data were captured using a sCMOS

camera (Andor Corp.) through an IR filter that prevented the scattered

illumination light from damaging the detector array


123

heavy water was controlled by adjusting the concentration

of calcium chloride (CaCl2) present in solution. In this

experiment, a 6 M CaCl2 aqueous solution (n = 1.469) and

the heavy water solution without CaCl2 (n = 1.328) were

used to form the droplets (Lee et al. 2012). The aqueous

solution contained 1 wt% Tween 20 (TCI) as a stabilizer.

The water-in-oil droplets were stable only if the affinity

between the PDMS wall and the aqueous solution was

smaller than the affinity between the continuous phase fluid

and the PDMS wall. To ensure that this condition was met,

silane containing a fluorocarbon liquid (EGC-1720, 3 M)

was flowed through the channel prior to preparing the

emulsion to introduce hydrophobic surface characteristics.

4 Results and discussion

The motions of the relatively large droplets (compared to

the channel height) in the tightly confined microfluidic

channel were monitored by laser beam illumination of the

device. The droplet size was 40 lm, the channel height was

95 lm, the refractive index of the droplet was 1.469, and

the flow velocity was 1,567 lm/s. The beam power was

3.15 W, and the beam was focused using an objective lens

(Olympus, NA = 0.3, 109). Figure 5a shows a time

sequence image describing a single droplet’s trajectory in

the presence of an optical force. The droplet was trans-

ported to the right (from the left) by the continuous fluid

flow. The droplet was deflected normal to the flow direc-

tion by the optical force. Each droplet in the figure repre-

sents the droplet position per ten milliseconds (images

were collected at a frame rate of 100 Hz). The fourth and

fifth droplets from the left were in close proximity, unlike

the other particles, which were separated by a regular

distance.

The optical gradient force then pushed the droplets to

the center of the beam focus. Prior to entering the center of

the beam focus, the optical force pushed and accelerated

the droplets until the forces reached a balance. As the

droplet is passed through the beam focus, the optical gra-

dient force applied a restoring force that pushed the droplet

in the direction opposite to the direction of the motion, as

Fig. 4 a Schematic diagram showing a T-junction channel used to

generate the droplets. The continuous fluid (oil) flowed left to right,

whereas the fluid droplets (heavy water) formed from the flows

introduced from the bottom and top channels indicated here. A

mechanical filter was positioned after the inlet port to prevent channel

clogging. The ratio of the inlet width to the main channel width did

not exceed 2, thereby ensuring that the droplets were generated under

a squeezing scheme. b The bifurcation junction positioned at the end

of the device. An asymmetric channel shape contributed to an

imbalance in the fluid resistances at the Outlet 1 and Outlet 2 ports

Fig. 5 Optical manipulation of droplets confined in a rectangular

channel. The measured droplet size was about 40 lm, and the flow

velocity was 1,567 lm/s. a The time sequence of images was

captured to illustrate the trajectory of the droplet through the

illumination field. b The experimental droplet trajectory is compared

with the numerically calculated results


123

shown in Fig. 2a. The optical gradient force increased the

resident time of the droplet in the optical field and signif-

icantly deflected the droplet normal to the flow direction

(the photophoretic displacement). Figure 5b shows a

droplet trajectory predicted by numerical calculations. The

numerical predictions agreed well with the experimental

measurements of the photophoretic displacements and

trajectories. The small discrepancies between predicted and

experimental results may have arisen from the assumption

that the droplet did not deform. Under experimental con-

ditions, droplets generally undergo continuous shape

changes as a result of the high drag forces or optical forces

(Sibillo et al. 2006; Chang et al. 2012). For this reason, the

discrepancies between predicted and experimental droplet

trajectories may stem from the experimental droplet

deformation effects that seek to balance the surface ten-

sion, drag force, and optical force. The photophoretic dis-

placement was found to be 20 lm for this system, which

was quite low. The resolution of the device was not

sufficient to permit separation or manipulation, despite the

large optical force exerted on each droplet. We hypothe-

sized that the poor separation performance arose from the

high drag force in the confined geometry. The system

resolution may be enhanced by changing the channel size

or other physical properties.

Excessively large drag forces on the droplets may be

avoided by using smaller droplets in a larger channel. An

active droplet separation technique using an optical

switching function is illustrated in Fig. 6a, b. The measured

droplet size was 20 lm, the width of the test channel section

was 600 lm, and the height of the test channel section was

140 lm. The refractive index of the droplet was 1.469, and

the measured droplet velocity was 900 lm/s. The beam was

focused using a 209 magnification objective lens (Olym-

pus, NA = 0.45), and the beam power was 1.5 W after the

focusing lens. When laser was switched off, all droplets

flowed to the waste outlet (downward), as shown in Fig. 6a.

Under the light field, the droplets were deflected along the

Fig. 6 Optical switching was used to manipulate the droplets. The

droplets contained a 6 M CaCl2 deuterium oxide (D2O, Sigma

Aldrich) solution, and the continuous phase fluid was HFE-7500. The

velocity of the continuous fluid was 900 lm/s. a When the laser was

off, all droplets flowed to the waste branch (downward). b Under

illumination, the droplets were deflected and moved toward the upper

outlet. The laser beam was used to manipulate the droplet trajectories

from the left-hand side of the image, outside of the field of view

Fig. 7 The droplets displayed different behaviors, depending on the

refractive index contrast between oil and water phases, even under

identical conditions. Droplet A contained a 6 M CaCl2 deuterium

oxide (D2O, Sigma Aldrich) solution and droplet B contained

deuterium oxide (D2O, Sigma Aldrich). As the CaCl2 concentration

increased, the refractive index increased. Droplet A had a higher

refractive index contrast and was deflected by the illumination beam

to a larger extent than droplet B. As a result, droplet A was deflected

along the direction normal to the flow. The velocity of the continuous

fluid was 900 lm/s


123

direction normal to the flow and were transported to outlet 1

(upward). The trajectories were optically manipulated in the

left-hand region of the flow channel outside of the field of

view and away from the outlet branches (Online Resource

1) because the fluid flow at the branch affected the droplet

migration patterns. The photophoretic displacement was

sufficient to overcome the hydraulic resistance induced by

the asymmetric bifurcated channel design.

The photophoretic displacements of two different spe-

cies of droplet were measured to ensure that the device

performance was appropriate for passive optical separation

applications. The experimental procedure and conditions

were as described previously, except that the laser power

was set to 2 W. The droplet shown in Fig. 7a will be

referred to as ‘droplet A’ and features a high refractive

index, whereas the droplet shown in Fig. 7b will be

referred to as ‘droplet B’ and features a refractive index

similar to that of the fluid medium. Light passing through

droplet A experienced a larger degree of refraction than the

light passing through droplet B. As a result, the photoph-

oretic displacements of droplets A and B were 35.12 lm

and 2.53 lm, respectively. Additional data are provided in

the Online Resource 2. The photophoretic displacements of

both species of droplet are shown in Fig. 8 as a function of

the power. The flow velocity was adjusted to decrease the

hydrodynamic interaction effects due to the presence of

adjacent droplets by maintaining a minimal distance

between droplets. Figure 8 shows the measured photoph-

oretic displacement from experimental results and numer-

ical predictions. The error bars indicate the minimum and

maximum determined values. The discrepancies between

experimental and predicted results may have arisen from

the elastic motions of the droplets. A passive optical

screening application using droplets with a higher refrac-

tive index is shown in Fig. 9. Two droplets were present in

the fluid flow, and the laser illumination was turned on.

Droplet A was pushed along the direction of light propa-

gation (Fig. 9a). After passing through the illuminated

area, the droplets were pushed laterally along the flow

direction and transported by the bulk fluid flow without

displaying further drift in the lateral position (Fig. 9b);

however, droplet B showed no change in its lateral position

due to the droplet’s low photophoretic efficiency (Fig. 9b,

c). The effect of the droplet size on the photophoretic

displacement was shown to be small. Only high refractive

Fig. 8 Droplet behavior as a function of the refractive index contrasts

between oil and aqueous phase fluids under the same conditions.

Experimental data and numerical predictions are shown as, respec-

tively, lines and symbols. As the beam power increased, the system

performance improved. The error bars indicate minimum and

maximum data values

Fig. 9 Droplet B was screened using optical force separation

techniques. The carrier fluid flowed from left to right, and the

illumination beam was focused at the center of the microfluidic

channel in the z-direction. The velocity of the continuous fluid was

900 lm/s. a Droplet A entered the test section and was illuminated by

the laser beam. The bright spot of the droplet focused the light beam

due to the high refractive index of the droplet A. b Droplet B followed

along the same streamline and entered the illumination beam.

Because the refractive index of droplet B was similar to that of the

carrier fluid, the light did not deflect the path of droplet B. c The

trajectories of droplets A and B are shown. Only droplet A was

deflected laterally from its original trajectory within the streamline


123

index droplets were deflected normal to the direction of

flow and were split away from their former streamlines

(Online Resource 3).

5 Conclusions

This paper examined the optical and hydrodynamic char-

acteristics of droplets in a rectangular microfluidic channel

in an effort to separate the droplets based on their optical

properties. The droplets were generated using the T-junc-

tion method and were transported to the main test area in a

carrier fluid flow. The application of optical forces repelled

the droplets laterally away from their streamlines. The

lateral displacement distance in a cross-type optical particle

separation scheme is called the photophoretic displace-

ment. The droplet behaviors and photophoretic displace-

ments were characterized experimentally and predicted

theoretically using numerical calculations. The optical

forces were calculated using the photon stream method and

applied to the particle dynamics equations. Drag coeffi-

cients were introduced in place of the Stokes’ drag force as

an estimate for the drag force experienced by the droplets.

As the ratio of the droplet size to the channel cross-sec-

tional dimensions increased, the drag force also increased

and the system performance decreased. Thus, both the

channel geometry and the physical properties of the drop-

lets required optimization. To avoid introducing exces-

sively high drag forces on the droplets, the droplet size was

adjusted until a sufficiently high photophoretic displace-

ment could be obtained using the optical switching oper-

ation. The 6 M CaCl2 heavy water droplets overcame the

fluid resistance and were transported to outlet 1 or outlet 2

when the illumination was turned on. The beam intensity

could be adjusted to manipulate the droplets in the

microfluidic channel using the optical forces. The optical

responses of two droplets with different refractive indices

were compared in terms of their photophoretic displace-

ments. A beam power of 2 W in a given microfluidic

platform yielded photophoretic displacements of 35.12 or

2.53 lm, a difference that was sufficient to permit droplet

sorting. The optical characteristics of the droplets were

used for the label-free separation of chemical or biological

samples. Droplet separation was demonstrated by screen-

ing a particular droplet type at the outlet from an emulsion

flow containing different types of droplets. Unlike other

sample recognition techniques, the optical and hydrody-

namic characteristics of the device descried here could be

used to screen and separate the droplets in one step. The

alignment of droplets along a fixed lateral position was

important for separation. Once this condition had been

satisfied, the separation resolution was expected to be high.

These results may be applied toward the design of chemical

or biological sample screening and manipulating devices.

Acknowledgments This work was supported by the Creative

Research Initiatives program (No. 2013-003364) of the National

Research Foundation of Korea (MSIP).

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