Optical separation of droplets on a microfluidic platform

RESEARCH PAPER

Optical separation of droplets on a micro?uidic platform

Jin Ho Jung ?Kyung Heon Lee ?Kang Soo Lee ?Byung Hang Ha ?Yong Suk Oh ?Hyung Jin Sung

Received:16May 2013/Accepted:10September 2013/Published online:22September 2013óSpringer-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 ?ow direction in a rectangular micro?uidic 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 áDroplet áTwo-phase ?ow áDroplet migration áPassive separation áOpto?uidics

1Introduction

Screening large libraries of samples using conventional techniques involving single cell analysis or chemical reaction pro?ling 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 ?ow systems (Sia and Whitesides 2003;Squires and Quake 2005;Song et al.2006).The formation of microdroplet emulsions in a ?ow cell provides one exam-ple of a micro?uidic 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 ?uid 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 micro?uidic 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 ?ow 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 this article (doi:10.1007/s10404-013-1263-0)contains supplementary material,which is available to authorized users.J.H.Jung áK.H.Lee áK.S.Lee áB.H.Ha áY.S.Oh áH.J.Sung (&)

Department of Mechanical Engineering,KAIST,

291Daehak-ro,Yuseong-gu,Daejeon 305-701,Korea e-mail:hjsung@kaist.ac.kr

Micro?uid Nano?uid (2014)16:635–644DOI 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 ?uid media.Optical separation techniques may be used to screen satellite droplets or droplets that yield false positives.Optical tweezers have been integrated into micro?uidic passive separation platforms in which a col-limated array of beam lines is directed into the ?ow car-rying micron-sized dielectric objects (Ashkin 1997;Gauthier and Wallace 1995;Grier 2003).

Some researchers have introduced optical forces into micro?uidic 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 ?uid;however,emulsion droplets are typically larger than commercially available microparticles,and their motions can be affected by the micro?uidic channel shape and geometry.Droplet behaviors under large drag forces,such as those experienced in a con?ned rectangular micro?uidic 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 micro?uidic channel.A droplet screening application is demonstrated using optical forces based on the refractive index mismatch between the droplets and the carrier ?uid.Droplets were generated in a micro?uidic channel and transported in the

bulk ?ow.The trajectories of the droplets in the micro-?uidic channel were de?ected by a focused light beam directed normal to the ?uid ?ow.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 de?ned in terms of the micro?uidic 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.

2Theory

Droplet behavior was observed using the micro?uidic 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 ?uid 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 1,064nm laser beam directed normal to the ?ow direction.The optical forces and drag forces in the micro?uidic 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.1a Schematic illustration of the PDMS micro?uidic 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

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 de?ected 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,in?u-ence particle migration in an optical micro?uidic 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 suf?ciently 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),

m d d u d

d t

t6p l r deUàu dT?F grad;e1T

m d d v d

d t

t6p l r d v d?F scatter:e2T

where m d is the droplet mass,u d is the x-directional droplet velocity,l is the viscosity,r d is the droplet radius,U is the velocity of the?uid?ow,and v d is the y-directional droplet velocity.F grad is the optical gradient force that pushes the droplet toward the center of the light beam.F scatter 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?ow-induced drag force.Under these conditions,the Stokes’drag force terms 6pl r deUàu dTand6pl r d v dTmust be modi?ed.The drag force acting on a particle or bubble is affected by the con?ned channel geometry and may be modeled by adjusting the effective viscosity and the drag coef?cient as (van der Sman2010),

F D?6p l?r d k1u1àk2u d

eT;e3Twhere u?is the velocity of the?uid at the streamline in the absence of the particle and l*is the effective viscosity.The effects of the co-?owing?uid inside the droplet were addressed by considering the viscosity of the disperse phase?uid.The effective viscosity may be expressed as

l??l c 1t2l c=3l d

1tl c=l d

e4T

where l c is the continuous phase?ow viscosity and l d is

the disperse phase?ow viscosity.k1and k2are the drag

coef?cients and are functions of the droplets size,location,

and dimensions of the channel.k1is calculated from a

stationary droplet in a bulk?uid?ow,and k2is estimated

from the motion of a droplet in a quiescent?uid.The

coef?cients k1and k2may 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 con?guration of

the droplet and the adjacent plate,

k2;1ejT?

F drag;1

F stokes

?

1

1à9=16jt1=8j3à45=256j4à1=16j5

;

e5T

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

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

The subnotation1in k2,1refers 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?1tk2;1er d=hTà1

??

tk2;1er d=eHàhTTà1

??

;

e6T

where k2,2,x is the drag coef?cient 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 micro?uidic channel may be calculated in

a similar manner using Eq.(6).

The geometrical con?guration is illustrated in Fig.2c.

Equation(6)then gives the drag force coef?cient in the x-

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

directional coef?cient),

k2;x?1tk2;2er d;h;HTà1

??

tk2;2er d;w0;WTà1

??

;

e7T

where the Oseen superposition principle is extended to the

rectangular channel(van der Sman2010).The coef?cient

k1,x,which is the drag force coef?cient for a stationary

droplet in a bulk?uid?ow directed along the x-direction,

can then be obtained from the generalized Faxen theorem

(van der Sman2010),

k 1;x ?k 2;x

1à13

g ea Tk 2àc ea Tk 5

!

;

a ?

W H ;k ?2r d

H

;g ea T?1slit

1:7squre

(

;

c ea T?0:125g ea T:

e8T

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 coef?cients k 1,x and k 2,x ,which could then be used to modify the particle dynamics equations in the x -direction according to m d

d u d

d t

t6p l ?r d ek 1;x u 1àk 2;x u d T?F grad ;e9T

where F grad is the optical gradient force.F grad will be considered in the next chapter.

The drag force coef?cient 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 con?nement effects due to the ?oor and ceiling of the micro?uidic channel.These effects can be modeled by assuming that the droplet moves between two parallel plates to which k 2,2,x is applicable.The droplet experiences scattering forces that push the particle toward the wall,as shown in Fig.2f.The relevant coef?cient may then be evaluated according to (Kim 2004)

k 2;10?

F drag F stokes ?11à9=8j 0t1=2j 03;j 0

?r d w 0

;e10T

which predicts that the particle moves toward a single plate.

The subnotation 10

refers to the single plate.The drag force coef?cient for a parallel plate system may be described as

k 2;2;y ?1tk 2;10er d =w 0Tà1??tk 2;10er d =eW àw 0TTà1??

;

e11T

where k 2,2,y is the drag force coef?cient of the droplet moving along the normal direction between the two parallel plates.The coef?cient k 2,which is the drag force coef?cient for a stationary droplet in a bulk ?uid ?ow along the y -direction,may be estimated as

k 2;y ?1tk 2;2;x er d ;h ;H Tà1??tk 2;2;y er d ;w 0;W Tà1??

;

e12T

where k 1represents the drag force correction parameter that accounts for the ?uid ?ow around a stationary droplet.If the ?uid velocity is characterized by an x -directional component alone,then k 1,y is not needed.The particle dynamic motion in the y -direction may then be modi?ed as m d d v d d t t6p l ?r d k 2;y v d ?F scatter :e13T

Once the drag force correction factors had been calculated,the optical forces acting on the droplet

could

Fig.2Diagrams showing the de?nitions of variables used for calculating the forces acting on the droplets con?ned 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 )

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(r p[20l m). Fresnel’s law for de?ections and re?ections 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):

F grad?àn0

2c

Z2p

Z p2

Ieq k;zTR sin2h1àT2

sine2h1à2h2TtR sin2h1

1tR2t2R cos h2

!

?r2p sin2h1cos u d h1d u;e14T

F scatter?àn0

2c

Z2p

Z p2

Ieq k;zTR cos2h1àT2

cose2h1à2h2TtR cos h1

1tR2t2R cos2h2

!?r2p sin2h1cos u d h1d u:e15T

where c denotes the speed of light in free space and n0 is the refractive index of the continuous phase?uid.h1 and h2are the incident and de?ected angles of the beam at the interface of the droplets,respectively. R and T are the Fresnel re?ectance and transmittance, respectively.I(q k,z)is the beam intensity pro?le where q k and z are the radial and axial displacements from

the center of the beam to the center of the droplets, respectively.

3Experimental

A schematic diagram of the experimental setup is shown in Fig.3.A CW Nd:YAG1,064nm laser(Advance Op-towave)in the TEM00mode was used as the light source with a maximum power of10W.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 pro?le.In this experiment,the translational location and incident angle of the beam were carefully controlled.A custom-made5-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 ?lter(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?ow rate of the working?uid.

A PDMS micro?uidic channel was fabricated by regular soft lithography processes using a negative photoresist(Su-82075,Microchem).A schematic diagram of the micro-?uidic channel used for droplet generation is shown in Fig.4a.The width of the T-junction was30l m,and the mechanical?lter was positioned after the inlet port to prevent clogging.The main test section was600l m wide to permit observation of the droplet migration motions.The channel height was40l m at the T-junction and140l m 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 outlet2displayed a lower hydraulic resistance,the droplets aligned at the center collected through outlet2.

HFE-7500(C7F15OC2H5,n=1.29,3M)was used as a continuous phase?uid and contained10v/v%1H,1H, 2H,2H-per?uoro-1-octanol(C8H5F13O,n=1.313,Sigma Aldrich).The light wavelength used in this study, 1,064nm,overlapped signi?cantly with a water optical absorption band.Absorption effects were avoided by using heavy water(D2O,Sigma Aldrich)in the aqueous?uid phase,as D2O displays a negligible absorption cross-sec-tion at this wavelength.The refractive index(n)of

the Fig.3The experimental setup used to optically manipulate the droplets.A1,064nm 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?uid?ow in the micro?uidic channel.Because the alignment between the droplet and the light can signi?cantly 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?lter that prevented the scattered illumination light from damaging the detector array

heavy water was controlled by adjusting the concentration of calcium chloride (CaCl 2)present in solution.In this experiment,a 6M CaCl 2aqueous solution (n =1.469)and the heavy water solution without CaCl 2(n =1.328)were used to form the droplets (Lee et al.2012).The aqueous solution contained 1wt%Tween 20(TCI)as a stabilizer.The water-in-oil droplets were stable only if the af?nity between the PDMS wall and the aqueous solution was smaller than the af?nity between the continuous phase ?uid and the PDMS wall.To ensure that this condition was met,silane containing a ?uorocarbon liquid (EGC-1720,3M)was ?owed through the channel prior to preparing the emulsion to introduce hydrophobic surface characteristics.

4Results and discussion

The motions of the relatively large droplets (compared to the channel height)in the tightly con?ned micro?uidic channel were monitored by laser beam illumination of the device.The droplet size was 40l m,the channel height was

95l m,the refractive index of the droplet was 1.469,and the ?ow velocity was 1,567l m/s.The beam power was 3.15W,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 ?uid ?ow.The droplet was de?ected normal to the ?ow direc-tion by the optical force.Each droplet in the ?gure repre-sents the droplet position per ten milliseconds (images were collected at a frame rate of 100Hz).The fourth and ?fth 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.4a Schematic diagram showing a T-junction channel used to generate the droplets.The continuous ?uid (oil)?owed left to right ,whereas the ?uid droplets (heavy water)formed from the ?ows introduced from the bottom and top channels indicated here.A mechanical ?lter 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 ?uid resistances at the Outlet 1and Outlet 2

ports

Fig.5Optical manipulation of droplets con?ned in a rectangular channel.The measured droplet size was about 40l m,and the ?ow velocity was 1,567l m/s.a The time sequence of images was captured to illustrate the trajectory of the droplet through the illumination ?eld.b The experimental droplet trajectory is compared with the numerically calculated results

shown in Fig.2a.The optical gradient force increased the resident time of the droplet in the optical ?eld and signif-icantly de?ected the droplet normal to the ?ow 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 20l m for this system,which was quite low.The resolution of the device was not suf?cient 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 con?ned 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 20l m,the width of the test channel section was 600l m,and the height of the test channel section was 140l m.The refractive index of the droplet was 1.469,and the measured droplet velocity was 900l m/s.The beam was focused using a 209magni?cation objective lens (Olym-pus,NA =0.45),and the beam power was 1.5W after the focusing lens.When laser was switched off,all droplets ?owed to the waste outlet (downward),as shown in Fig.6a.Under the light ?eld,the droplets were de?ected along

the

Fig.6Optical switching was used to manipulate the droplets.The droplets contained a 6M CaCl 2deuterium oxide (D 2O,Sigma Aldrich)solution,and the continuous phase ?uid was HFE-7500.The velocity of the continuous ?uid was 900l m/s.a When the laser was off,all droplets ?owed to the waste branch (downward).b Under illumination,the droplets were de?ected 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 ?eld of

view

Fig.7The droplets displayed different behaviors,depending on the refractive index contrast between oil and water phases,even under identical conditions.Droplet A contained a 6M CaCl 2deuterium oxide (D 2O,Sigma Aldrich)solution and droplet B contained deuterium oxide (D 2O,Sigma Aldrich).As the CaCl 2concentration increased,the refractive index increased.Droplet A had a higher refractive index contrast and was de?ected by the illumination beam to a larger extent than droplet B.As a result,droplet A was de?ected along the direction normal to the ?ow.The velocity of the continuous ?uid was 900l m/s

direction normal to the ?ow and were transported to outlet 1(upward).The trajectories were optically manipulated in the left-hand region of the ?ow channel outside of the ?eld of view and away from the outlet branches (Online Resource 1)because the ?uid ?ow at the branch affected the droplet migration patterns.The photophoretic displacement was suf?cient 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 2W.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 ?uid 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 35.12l m and 2.53l m,respectively.Additional data are provided in the Online Resource 2.The photophoretic displacements of both species of droplet are shown in Fig.8as a function of the power.The ?ow velocity was adjusted to decrease the hydrodynamic interaction effects due to the presence of adjacent droplets by maintaining a minimal distance between droplets.Figure 8shows 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 ?uid ?ow,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 ?ow direction and transported by the bulk ?uid ?ow 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 ef?ciency (Fig.9b,c).The effect of the droplet size on the photophoretic displacement was shown to be small.Only high

refractive

Fig.8Droplet behavior as a function of the refractive index contrasts between oil and aqueous phase ?uids 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.9Droplet B was screened using optical force separation techniques.The carrier ?uid ?owed from left to right ,and the illumination beam was focused at the center of the micro?uidic channel in the z -direction.The velocity of the continuous ?uid was 900l m/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 ?uid,the light did not de?ect the path of droplet B .c The trajectories of droplets A and B are shown.Only droplet A was de?ected laterally from its original trajectory within the streamline

index droplets were de?ected normal to the direction of ?ow and were split away from their former streamlines (Online Resource3).

5Conclusions

This paper examined the optical and hydrodynamic char-acteristics of droplets in a rectangular micro?uidic 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?uid?ow.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 coef?-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 suf?ciently high photophoretic displace-ment could be obtained using the optical switching oper-ation.The6M CaCl2heavy water droplets overcame the ?uid resistance and were transported to outlet1or outlet2 when the illumination was turned on.The beam intensity could be adjusted to manipulate the droplets in the micro?uidic 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 of2W in a given micro?uidic platform yielded photophoretic displacements of35.12or 2.53l m,a difference that was suf?cient 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 ?ow 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?xed lateral position was important for separation.Once this condition had been satis?ed,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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