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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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Lab on a Chip
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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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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8 | Lab Chip, 2014,00, 19 This journal is The Royal Society of Chemistry 2014
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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