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Lab on a Chip c4lc00542b

Controlled splitting and focusing of a stream of

nanoparticles in a convergingdiverging

microchannel

Ravi Kumar Arun, Kaustav Chaudhury, Moumita Ghosh,

Gautam Biswas, Nripen Chanda and Suman Chakraborty*WeQ3 demonstrate a convergingdiverging channel design fornanoparticle focusing that facilitates the interaction ofAgNPs with H2O2at an interface.

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PAPER

Cite this: DOI: 10.1039/c4lc00542b

Received 8th May 2014,

Accepted 14th July 2014

DOI: 10.1039/c4lc00542b

www.rsc.org/loc

Q1 Controlled splitting and focusing of a stream ofnanoparticles in a convergingdivergingmicrochannel

Q2 Ravi Kumar Arun,a Kaustav Chaudhury,b Moumita Ghosh,a Gautam Biswas,cd

Nripen Chandaa and Suman Chakraborty*b

We demonstrate the potential of a convergingdiverging microchannel to split a stream of nanoparticles

towards the interfacial region of the dispersed and the carrier phases, introduced through the middle inlet

and through the remaining two inlets respectively, while maintaining a low Reynolds number limit (

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2 | Lab Chip, 2014,00, 19 This journal is The Royal Society of Chemistry 2014

systems (TAS), medical diagnosis and in bottom-up fabrica-

tion processes, where the primary requirement is to maintain

controlled reactions and thereby separate specific products.1416

It is important to mention in the present context that a lot

of attempts have been made to split streams of micro or

nano sized particles,13,1721 mainly by utilizing microchannels

with different geometries.2224 However, the end results are

mostly confined to the achievement of augmented mixing,

separation and reaction. On the other hand, the intrinsicmicroscale dimension of the channels can be potentially uti-

lized for focusing the nanoparticle stream over a particular

region, by allowing the controlled deposition of particles over

the channel surface in and around the targeted location.2527

This may eventually provide a useful means for the fabrica-

tion of microscale features with finer resolutions.28 Our study

focuses on utilizing the potential of both the channel geome-

try and the microscale dimensions, in tandem. Additionally,

our approach can enable one to maintain control over the

extent of splitting along the transverse direction.

Materials and methodsQ5 Problem description with schematic

Fig. 1 illustrates the schematic of the present converging

diverging microchannel, with three inlet systems.

We used the middl e inlet to introduce the dispersed

phases (i.e. the nanoparticle laden solution), and the other

two inlets were utilized to supply the carrier phase. We

maintained the channel height, (H) = 0.15 mm, and length,

(L) = 25 mm.29 The widths (W) of the converging and diverg-

ing zones were maintained at 3.3 and 0.8 mm, respectively.

Throughout the entire study, we maintained the flow rate of

the nanoparticle laden solution at 25 L min1, and the flow

rates of the carrier phase were varied as 25, 50, 100 and

250 L min1. We defined the Reynolds number for a particu-

lar entity as Re = Deqv/, whereDeqis the hydraulic diameter

of the flow cross section,v = Q/A(Ais the cross-sectional area

of the expansion region andQ is the flow rate), is dynamic

viscosity and is the density of the solution. With the various

possible combinations of the flow rates, as obtained from the

mentioned data, we maintained a low Reynolds number limit

(

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Lab Chip, 2014, 00, 19 | 3This journal is The Royal Society of Chemistry 2014

and used as the nanoparticle suspension for the present

study. The movements of the nanoparticles were monitored

under a LED fluorescence microscope (Leica DMI 4000B,

Germany) at a fixed wavelength of 470 nm and 530 nm, for

the 30 nm and 500 nm size particles, respectively. HereQ7 we

also observed the controlled divergence of the nanoparticle

stream, albeit to a further lateral extent than that of the for-

mer case. Subsequent quantitative colorimetric intensity anal-

ysis was used to demonstrate the controlled accumulation ofthe polystyrene beads over the interfacial regions of the two

fluids. With the quantitative evidence in support of this, we

then moved towards exploiting the potential of the present

setup in maintaining a controlled reaction and allowing

the separation of products towards a specific location, as

endeavoured in the subsequent experimentation with the

silver nanoparticles and hydrogenperoxide solution.

Experimentation with silver nanoparticles

Firstly, we prepared the silver-nanoparticle (AgNP) laden solu-

tion, following the reported chemical reduction method.

Briefly, 10 mg of AgNO3 was added to 1 ml of DI water andused as the stock solution. Subsequently, 20 mg of NaBH 4

was added to 1 ml of DI water and kept in a refrigerator to

avoid decomposition. Then, 20 mg of sodium dodecyl sul-

phate (SDS, Sigma Aldrich, Saint Louis, MO, USA) was mixed

with 20 ml of DI water and stirred for 1015 min. During stir-

ring, 100 l of AgNO3 from the stock solution was added to

the SDS solution; the SDS solution was used to stabilize the

silver nanoparticles. Immediately, 300 l of cooled NaBH4was added to the mixture and stirred for 10 min to achieve

the yellow colored AgNP solution. Finally, the nanoparticles

were characterized by UV-absorption spectroscopy and size

analysis (Fig. S1). The size of the AgNPs was measured usinga NANOSIGHT particle size analyzer (NS500), and was found

to be in the range of 45 nm. In this case, the movements of

the nanoparticles were also monitored under a LED fluores-

cence microscope (Lieca DMI 4000B, Germany). The AgNP

solution was introduced through the middle inlet, whereas

the H2O2 solution was introduced through the other two

inlets. Under the mentioned flow conditions, we observed the

accumulation of the reaction products in situover the interfa-

cial region of the two fluids. We also found that the microscale

dimension of the channel allowed controlled precipitation

of the reaction products over the bottom glass surface of

the channel. The microscale features were then observed

using scanning electron microscopy (SEM) operated at 5 kV

(Hitachi S-3000N).

Results

We began our observation with the coloured dye; the charac-

teristic features are shown in Fig. 2. This behaviour was

observed under the flow of the carrier phase at 25, 50, 100

and 250 L min1 and the dispersed phase at 25 L min1.

The first noteworthy fact is that we observed a divergence of

the dye stream; the distribution is earmarked by the

distribution of the coloured dye with the expansioncontraction

zone, as shown inFig. 2. Under the mentioned flow rates, the

Reynolds numbers for the carrier and the dispersed phases

are found to be 0.585.8 and 1.123.64 respectively. When we

progressively increased the flow rates of the carrier phase

from 25 (ReDI = 0.58) to 250 L min1 (Re DI = 5.80), we

observed a similar behaviour in the dye distribution, as

shown in Fig. 3. From the figure it is evident that with an

increase in the flow rate of the carrier phase, the width of the

dye distribution decreases. From the mentioned observations,it is indicative that the present setup could be utilized to

focus the stream of particles by controlling the expansion

contraction of the dye distribution.

However, it was imperative to investigate whether the

mentioned behaviours are associated with the channel geometry

or something else. To investigate this, we also conducted

experiments with three inlet straight microchannels, with the

same width as that of the expanded section of the present

convergingdiverging channel. The results are shown in

Fig. S2. With the various combinations of the flow rates of

the carrier and the dispersed phases, we found that none of

the combinations yield the expansioncontraction type dye

distribution. Thus, by comparing Fig. 2 and S2, we can

proclaim that the typical expansioncontraction type dye

distribution in the convergingdiverging channel is inherent

to the intrinsic geometry of the channel. However, with the

change in the combinations of the flow rates, the width of

the dye distribution changed within the straight channel as

Fig. 2 Superimposed microscope image of a convergingdiverging

PDMS microchannel showing the characteristic feature of dye

distribution in DI water.

Fig. 3 Dye solution distribution under the flow rates of the carrier

phase at 25, 50, 100, 250 L min1, keeping the central dye flow rate at

25 L min1.

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well. Thus, the controllability of the system seems to stem,

mainly, from the flow rate control.

An important inference that can be drawn from the above

observations is that the convergingdiverging microchannel

can cause expansioncontraction of the dye stream. However,

the size of the dye molecules is typically of the order of the

size of the water molecules, acting as the carrier phase. Thus,

it is difficult to analyse the ability of the present setup to split

the stream of particles. Therefore, to investigate this abilitywe have conducted experiments with the dispersed phase,

composed of an aqueous solution of polystyrene beads; the

results are shown inFig. 4for the dispersed phase containing

the nanoparticles with dimensions of the order of 30 nm. In

this case we also maintained the flow rate of the dispersed

phase at 25 L min1, while the flow rates of the carrier

phase varied from 25 to 250 L min1 (alongFig. 4ad). For

the sake of quantitative estimation, we also conducted a

colorimetric intensity analysis by measuring the fluorescence

intensity of the particles, as shown in the graphs along with

the corresponding images. From these graphs we can identify

that the intensities show a deepening tendency around thechannel centreline (specifically see Fig. 4f and g), while

showing peaks near the interfacial region of the two fluids.

This estimation suggests the localized distribution of the sub-

merged nanoparticles, specifically near the interfacial region,

in contrast to the homogenous distribution all over the dis-

persed phase region. However, fromFig. 4h, the fluorescence

intensity based estimation does not confirm the preferential

splitting of the transported nanoparticles; this may be attrib-

uted to issues with the resolution.

For the sake of further verification, we also conductedexperiments with nanoparticles with dimensions of the order

of 500 nm; the essential features are shown in Fig. 5. We

obtained approximately similar interfacial distances for the

500 nm particles with better focusing as compared to the

30 nm fluorescent particles (compareFig. 4and5). Thus, the

results confirm that the present setup can be potentially

utilized for the splitting of streams of nanoparticles and to

focus them towards the interfacial region of the two fluids.

We performed similar experiments with the straight micro-

channel (Fig. S3). We can clearly see that the focusing of the

nanoparticles is almost absent at all of the flow rate ratios.

Thus, with the experimental results in hand, it is suggestivethat the inherent geometry of the channel allows such typical

splitting of the streams of particles and makes them settle

over the interfacial region.

After gaining knowledge about the splitting and focusing

ability of the present setup, we explored the possibility of

achieving a controlled reaction and the subsequent separa-

tion of products, as it is one of the primary requirements in

micro-total-analysis systems. We explored the distribution of

metallic silver nanoparticles, 45 4 nm in size, upon chemi-

cal reaction with H2O2 in the present microchannel; shown

in theFig. 6. We introduced dilute H2O2(0.9 M) as the carrier

fluid which reacts with the AgNPs at the interfaces to form

silver microstructures along the interfacial planes. Being an

oxidizing agent, H2O2oxidizes the AgNPs to silver ions, at an

alkaline pH (pH ~ 8.5) of the medium, that are spontaneously

precipitated as micron-size structures at the interfaces. The

interaction performance of the AgNPs and H2O2in the micro-

channel, as shown inFig. 6, was recorded under an inverted

microscope for 30 min. We determined this 30 min reaction

time by monitoring the amount of reaction precipitate at dif-

ferent time intervals (Fig. S4). At the beginning, there was

no precipitate at the interfaces, indicating the absence of the

reaction products from the interaction between the AgNPs

and H2O2. However, as the flow of the fluids continued, the

AgNPs started focusing at the interfaces and the formation of

Fig. 4 Hydrodynamic focusing of fluorescent nanoparticles (30 nm) in

the convergingdiverging microchannel. (ad) Microscopic

fluorescence images at 470 nm wavelength at different flow ratios of

the nanoparticle solution and DI water (25: 25, 25 : 50, 25: 100 and 25 : 250

respectively). (eh) Plots of the fluorescence intensity versusdistance along

the width of the channel showing particle distribution at the interfaces.

Note, that the maximum concentrations of the nanoparticles are observed

at two interfacial zones with a separation width of 100 m at the flow ratio

of 25:50.

Fig. 5 Hydrodynamic focusing of fluorescent nanoparticles (500 nm)

in a convergingdiverging microchannel. (a) Fluorescent nanoparticles

are systematically focused at both of the interfaces while flowing

through the channel at a flow rate ratio of nanoparticles: DI of 25 : 50.

(b) The particle distribution in terms of the fluorescence intensity

versusdistance along the width of the channel.

Fig. 6 Superimposed microscope image demonstrating the interaction

of the AgNPs and H2O2 throughout the patterned micro-channel. A

gray coloured precipitate can be seen at the interfaces indicating the

in situformation of silver microstructures as a product of the reaction.

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the products in the presence of co-flowing H2O2was seen at

10 min and gradually increased to a visible level at 30 min.

From the outset, we stated that the microscale dimension

of the channel can allow controlled deposition over the

bottom surface of the channel. Fig. 7shows the precipitation

of the silver ions over the channel surface, after reaction; this

was confirmed by introducing aqueous NH3 to the channel

after the reaction (Fig. S5). Aqueous NH3 dissolves the

precipitate by forming a soluble diaminesilver complex([Ag(NH3)2]

+) and confirms the existence of the Ag+ ions.30,31

The gray coloured silver ion precipitate remains at the inter-

facial planes; this can be attributed to the strong absorption

of the silver ions onto the PDMS and glass substrate. Upon

producing silver ions, the oxygen atoms of both of the

substrates are bound with Ag+ ions in such way that the

surface-bound Ag+ ions can subsequently function as seeds

for the growth of silver microstructures on glass and PDMS

substrates.

In order to investigate the growth mechanism of the silver

ion precipitate at the interfaces, we used SEM and EDS to

observe the material at different flow rates.Fig. 7(ad)showsthe SEM images of the silver ion precipitate formed at the

interface due to the interaction between the AgNP and H 2O2solutions at different flow rates after 30 min. The precipitate

appears as three-dimensional (3-D) agglomerates or irregu-

larly shaped micron-size structures. At a higher magnifica-

tion, the topography of these structures was revealed to be

formed by flower-like silver ion precipitates arranged at the

micrometre scale (Fig. 8a). The EDS spectrum shows the pres-

ence of elemental Ag in the precipitate (Fig. 8b). The peaks of

the Si and O elements are due to the Si substrate.

The reaction of AgNPs with H2O2 was performed by

varying the flow rates of the carrier fluid (QH2O2 = 25, 50, 100

and 250 L min1) while keeping the nanoparticle solution

flow rate, QAgNPs, constant at 25 L min1. The applied

flow rates and corresponding Reynolds numbers (Re) are

shown inTable 1. At low ReAgNP(3.72), the precipitate started forming after

5 min, but the amount was less at the interface compared to

all lower ReAgNP (1.15, 1.88 and 2.35). This indicates thatReAgNP in between 1.152.35 is the optimum for the nano-

particles to be distributed in a controlled manner at the

interfaces and to increase the contact area for accelerating

the interaction process with H2O2 in a controlled manner

throughout the channel, as further confirmed by the SEM

and EDS studies (Fig. 7(ac)and8).

From the experimental results we can see that the con-

vergingdiverging microchannel is capable of splitting and

subsequently focusing a stream of nanoparticles. Further

cross examination reveals that it is the very geometry of the

channel that makes this splitting possible. Therefore, it is

now imperative to analyze the inherent physics leading

towards these characteristic features. We discuss this issue in

the perspective of our numerical simulation studies.

Discussions

In order to unveil the underlying physics leading to particle

focusing, we conducted 3D numerical simulations for a

convergingdiverging channel with the same dimensions as

those used in the experiments (see the ESI for details).

Essentially, the velocity field, assuming incompressible flow,

is divergence free and is obtained by solving the Navier

Stokes equation. Subsequently, the particle distributions were

obtained from Lagrangian tracking. Owing to the periodicrepetition of the channel geometry, we conducted a simula-

tion over a domain which was periodic along the direction of

the mean flow. Then the imposed flow rate was maintained

Fig. 7 Scanning electron microscopy images of the Ag ion precipitate

formed at the interfaces due to the interaction between the AgNP and

H2O2solutions at different flow rates of the carrier phase at (a) 25 L min1,

(b) 50L min1, (c) 100 L min1 and (d) 250 L min1 for 30 min. The AgNP

solution flow rate was kept constant at 25 L min1. The insets show

the magnified images of the silver ion precipitate.

Fig. 8 (a) Magnified SEM image of the silver ion precipitates which

appear as flower-like structures. (b) The EDS spectrum of Ag ion

precipitates showing the presence of Ag. The peaks of the Si and Oelements are due to the Si substrate.

Table 1 Flow ratio and corresponding Reynolds number of the AgNP

and H2O2solutions

Flow ratio Q= 25:25 Q= 25:50 Q= 25: 100 Q= 25: 250

ReH2O2 0.58 1.16 2.32 5.79ReAgNP 1.15 1.88 2.35 3.72

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as the sum of the flow rates of the carrier and the dispersed

phases. InFig. 9, we show the 3D simulation results of the

distributions of the 45 nm particles, with 10 500 kg m3

density (resembling the density of silver), at a flow rate of

25 L min1 for both the carrier and the dispersed phases.

The figure shows that the observation from the present simu-

lation is in close agreement with the experimental results

(compared withFig. 7a).

Initially, the particles were distributed in and around thecentral axis of the channel. In response to the flow field, the

distribution of the particles was then obtained by solving the

kinetics equation for each particle

d

dand

d

d

p

D g M S

p

p

uf f f f

xu

t t (1)

where xp and up represent, respectively, the position and

velocity of the particle (in vector form), measured with

respect to a fixed-to-the-lab Cartesian reference frame, as

indicated inFig. 9. The forces (per unit mass of the particle),

as shown in eqn (1), are the hydrodynamic drag force ( fD),

force due to gravity (fg), virtual mass force (fM) and the

Saffman lift force (fS). They are estimated as3235

f u u

f g

f u u

D

p p

D p

p

g

p

M

p

p

Re

d

d

18

24

1

1

2

2

d

C

t

,

,

,

.

:

.

and

S

p p

pf D

D D

u u5 1881 2

1 4

d

(2)

Here g denotes the acceleration due to gravity (acting

along the z direction),CDbeing the drag coefficient, and ,

and are the density, dynamic viscosity and kinematic

viscosity of the carrier phase (aqueous medium). Accordingly,

pand dpdenote the density and the diameter of the particle.

In eqn (2), the velocity vectors for the carrier phase and the

particle are denoted byu and up. Accordingly, the strain rate

tensor is defined as D = u+ (u)T. Note that here the parti-

cle based Reynolds number is defined as Re p = dp|u u p|/.It is worth mentioning that for the simulation purpose, we

considered the contributions from all of the mentioned

forces.

During the tracking of particles, we considered the particle

particle and particlewall interactions to be elastic type. To the

present level of approximation, we did not take into account

the implication of any electrostatic interaction between the par-

ticles. In fact, any implication due to its accounting can be con-

sidered to be significant where the length scale over which the

characteristic changes in the particle distribution can be

observed, approaches the nanoscale regime.36 Pertaining to the

present situation, the characteristic dimension of the zone ofthe particles'divergence is well above this limit. However, in

our model we consider the two way coupling between the parti-

cles and the continuous phase. Specifically, that the gain (loss)

in momentum of a particle is accommodated as the loss (gain)

in momentum of the continuous phase.

It is now prudent to estimate the contribution of each of

the contributing forces in deciding the particle distribution

observed in mutually agreeing simulations and experiments.

First we note that studies have shown the influence of the

Saffman lift force on nanoparticle distribution.25,26,36,37

However, their magnitudes are generally very small (~1014 N

to 1016 N, as estimated for the present experiments) and

there seems to be only a marginal change in the distribution

of the particles due to the accounting of this force. However,

for the sake of verification, we also conducted simulations

with and without the Saffman lift consideration in the model;

the comparison is shown inFig. 10. From the figure, it is evi-

dent that the Saffman lift force does not induce any consider-

able change in particle distribution, at least for the present

setup. Subsequently, by comparing the order of magnitude

Fig. 9 3D simulation results of the distributions of 45 nm particles in a

converging diverging channel at a flow rate of 25 L min1, for both

the carrier and the dispersed phases. The flow was imposed along the

xdirection and the particles were initially distributed axially around the

central axis of the channel.

Fig. 10 Splitting of the nanoparticle stream withdp = 45 nm and p=

10500 kg m3, with and without considering the Saffman lift force in

the convergingdiverging channel at a flow rate of 25 L min1, for

both the carrier and the dispersed phases.

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contributions of the other forces, we found that fg and fScould be neglected as compared to fD. Thus, it seems that the

particles are basically dragged along the flow field. Addition-

ally from our simulation we found thatu ~ u p. Thus, we can

consider that the particles move in the flow field as passive

tracer particles. Therefore, it is the inherent feature of the

intrinsic flow field that determines the distribution of the

particles. The dimension and the subsequent mass of each of

the nanoparticles are indicative of the fact that the particlescan be considered as passive tracer particles that are likely to

follow the streamlines closely at the steady state.13,38

In search of the intrinsic flow features for the present

setup, inFig. 11awe present the distribution of the stream-

lines, on an xyplane passing through the central axis. From

the figure it is evident that the streamlines are compressed

and expanded in the converging and diverging sections,

respectively. The distribution of particles, therefore, follows a

similar pattern. For further verification, we performed simu-

lations in a straight channel with similar dimensions as

those used in the experiment; the particle distribution and

the streamlines are shown inFig. 11b. The simulations werealso conducted with p = 10 500 kg m

3 and dp = 45 nm and

all the above mentioned forces were retained for Lagrangian

tracking of the particles. FromFig. 11bit is evident that the

particles with the given size closely follow the streamlines at

the given flow rate. Therefore, by comparingFig. 11a and b

we can proclaim that the typical splitting of the nanoparticle

stream is a characteristic of the convergingdiverging

channel geometry as considered in our work.

It is worth mentioning at this point that studies have

shown that even for flows with low Reynolds number,

secondary flows may exists specifically near the zones of

spatially varying curvatures of the channel geometry, and can

lead to some interesting features.3942 It is to be noted that

the cross section of the present device is also spatially vary-

ing. It is, therefore, imperative to unveil the possibility of the

influence of any such flow features. For this purpose, we

conducted simulations with different flow rates (Q). The flow

rate as maintained for presenting the results in Fig. 9, is con-

sidered as the base flow rate (Qbase). Starting from Q/Qbase =

1, we carried out simulations up to Q/Qbase = 10; note that

this consideration is in tune with the present experimental

setup. As per our choice of reference frame, the central axis

was defined by the locus y = 0 and z = h/2, as shown in

Fig. 12a. In all the simulations, we then monitored the veloci-

ties at offsets y = (keepingz= h/2) as indicated inFig. 12a.In the left column ofFig. 12bwe demonstrate the flow veloci-

tiesux, uyand uz, along the x,y and zdirections respectively,

as obtained from the data monitored at = 5 102 mm at

different x/Lref; here Lref was chosen as the width of the

expansion section. It needs to be emphasized that we

Fig. 11 (a) Distribution of the streamlines on an xyplane passing

through the central axis, for (a) the convergingdiverging channel and

(b) a straight channel. In both cases, the flow rates of both the carrier

and the dispersed phases were maintained at 25 L min1.

Fig. 12 (a) The description of the central axis and the offset. (b) The

variation of ux, uyand uz, (left column), measured along the x, yand z

directions respectively, and the corresponding ux/Uref, uy/Uref and

uz/Uref (right column), with x/Lrefas obtained from the data monitored

at = 5 102 mm.

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specifically focused our attention over the region around the

central axis where the particles are distributed. Specifically,

we endeavoured to scrutinize the hydrodynamic features

(namely the flow velocities) over this region so as to unveil

the essential physics of interest. Thus, we considered the

location at the mentioned offset around the central axis. The

gross flow feature at different zones within the channel is

already shown in Fig. 11a, through the distribution of the

streamlines. For distributions ofuxand uy, here we only showvariations over a characteristic extent ofx/Lref. The distribu-

tion over the entire extent ofx/Lrefis just the periodic repeti-

tion of those characteristic features. From the distributions

ofux in Fig. 12b, it is noteworthy thatux attains maxima at

the contraction zone. This is in tune with the continuity of

the flow rates across different cross sections. From the distri-

butions ofuzin Fig. 12b, one can appreciate that at the con-

traction zone, it assumes magnitudes that are close to zero.

However,Q8 near the contraction zone, peaks of uy can be

observed at just upstream and downstream sides. Signifi-

cance of those optima were delineatedQ9 post priory. Neverthe-

less, if we observe the variation ofux/Uref, uy/Urefand uz/Uref,whereUref= Q/A(A is the cross-section area of the expansion

section), the corresponding curves collapse on each other, as

shown in the right column ofFig. 12b. The collapse indicates

that ux, uy and uzchange proportionally with the imposed

flow rate. From this collapsing, we can also infer an impor-

tant fact: with the choice of a normalization scheme (using

the chosenLrefandUref) it is possible to provide a generalized

depiction of the inherent hydrodynamics, so long as the

Reynolds number is kept low.

Among the ux/Uref, uy/Uref and uz/Uref distributions, as

shown in Fig. 12b, we found that ux/Uref and uy/Uref play

dominant roles in determining the inherent dynamics of the

present device, as can be appreciated from the negligible

order of magnitude ofuz/Uref, in comparison to that ofux/Urefand uy/Uref.Q10 It is worth mentioning that the distribution of

both uz (Fig. 12b left column) and uz/Uref (Fig. 12b right

column) appear as noises. However, the ranges of the abso-

lute values within which their variations occur implicate that

the apparent fluctuations in uzand uz/Urefare not likely to

make any significant contribution in comparison to the veloc-

ity components along the other directions. Here the varia-

tions of uz and uz/Urefare presented to highlight the issue

that uz ux and uz uy. Thus, the essential physics is

primarily decided byux and uy (subsequently byux/Urefand

uy/Urefrespectively).InFig. 13we have compared the distributions ofux/Uref

anduy/Ureffor = 5 102 mm. From the figure it is evident

that in both cases, ux/Uref remains same. However, from

the distribution ofuy/Urefone can appreciate that at a given

x/Lref, the uy/Urefdistributions are the same in magnitude but

with a different sign. When the particles are approaching

the contraction zone, they experience symmetrically opposite

uy/Uref that endorses the squeezing of the distribution zone

of the particles. Subsequently, after coming out of the con-

traction zone, the uy/Urefdistribution aides the expansion of

the distribution zone. The symmetrically opposite features

explain the symmetric distribution of the particles around

the central axis. With the above mentioned considerations

and the contrasting observations concerning the particle dis-tributions in straight channels and convergingdiverging

channels, we infer that the present splitting behaviour of

particles, in the present setup, is primarily due to the inher-

ent flow field, as realized from the geometrical features of

the confining domain. The very geometry of the channel

endorses periodic divergence and expansion of the stream-

lines, thereby causing the nanoparticles to trail along.

This type of distribution of nano-sized particles can be

beneficial to fabricate various microstructures inside the con-

fined channels for numerous applications. In the present

study, for example, the focused silver nanoparticles formed a

precipitate after reacting with H2O2 at the interfaces thus

containing highly active Ag+ ions and can directly be involved

in specific chemical or biological reactions for catalytic

purposes.43 In the reported literature, silver is deposited at

the interfaces of microfluidic channels to encounter many

practical settings.28,44 However, their fabrication processes

follow expensive and multi-step methods including multi-

stage photolithography, etching, accurate alignment and so

on. Moreover, there is little information known about the

morphological features of the precipitate formed inside

microchannels. In the present study, we have utilized a pro-

cess for the controlled synthesis of silver microstructures

throughout its interfacial length in a wavy fashion which will

provide a large surface area as compared to the straightmicrochannel. The micro-structured silver ion precipitate

may have potential use in microelectronics and catalysis and

we may utiliz e this precipitate as microcatalysts for various

catalytic reactions in microchannels.29

Conclusions

Our observations reveal that in a convergingdiverging micro-

channel, with a smoothly varying cross-section, a stream of

nanoparticles can be split into two halves, and can be made

Fig. 13 The distributions of ux/Uref and uy/Uref at = 5 102 mm to

show the particle distribution at the near contraction and near

expansion zones.

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Lab Chip 2014 00 19 | 9This journal is The Royal Society of Chemistry 2014

to remain focused over a particular region, specifically over

the interfacial region of the dispersed and carrier phases.

Moving a step ahead, one can conduct a controlled reaction,

and thereby separate the reaction-product over the targeted

location, using the present setup. Moreover the reaction

product is found to be deposited over the bottom surface of

the channel along a track parallel to the interfacial region of

the two fluids. The splitting and subsequent focusing ability

of the present setup primarily stems from the geometryassisted convergence and divergence of the streamlines, and

thereby induces the dispersed particles to trail along. The

controlled deposition, on the other hand, endorses a simple

strategy for bottom-up fabrication of microscale features.

Acknowledgements

The authors are thankful to the CSIR-CMERI, India, for the

financial support through the projects OLP-190312 and OLP-

101512. The authors also acknowledge the help of the insti-

tute CRF facility for conducting SEM studies.

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