elastic effects of dilute polymer solution on bubble

© 2017 The Korean Society of Rheology and Springer 147

Korea-Australia Rheology Journal, 29(2), 147-153 (May 2017)DOI: 10.1007/s13367-017-0016-0

www.springer.com/13367

pISSN 1226-119X eISSN 2093-7660

Elastic effects of dilute polymer solution on bubble generation in a microfluidic

flow-focusing channel

Dong Young Kim1, Tae Soup Shim

1,2 and Ju Min Kim

1,2,*1Department of Energy Systems Research, Ajou University, Suwon 16499, Republic of Korea

2Department of Chemical Engineering, Ajou University, Suwon 16499, Republic of Korea

(Received February 19, 2017; final revision received March 29, 2017; accepted April 1, 2017)

Recently, two-phase flow in microfluidics has attracted much attention because of its importance in gen-erating droplets or bubbles that can be used as building blocks for material synthesis and biological appli-cations. However, there are many unresolved issues in understanding droplet and bubble generationprocesses, especially when complex fluids are involved. In this study, we investigated elastic effects on bub-ble generation processes in a flow-focusing geometry and the shapes of the produced bubbles flowingthrough a microchannel. We used dilute polymer solutions with nearly constant shear viscosities so that theshear-thinning effects on bubble generation could be precluded. We observed that a very small amount ofpolymer (poly(ethylene oxide) at ~O(10) ppm) significantly affects bubble generation. When the polymerwas added to a Newtonian fluid, the fluctuation in bubble size increased notably, which was attributed tothe chaotic flow dynamics in the flow-focusing region. In addition, it was demonstrated that the bubbleswere thinner along the minor axis in the viscoelastic fluid than they were in the Newtonian fluid. We expectthat the current results will contribute to understanding the dynamics of two-phase flow in microchannelsand the design and operation of the microfluidic devices to generate microbubbles.

Keywords: bubble generation, viscoelastic flow, multiphase flow, microchannel, flow focusing

1. Introduction

Microfluidic technologies are often employed because

material synthesis and analysis can be precisely controlled

using very small sample volumes (Squires and Quake,

2005). Microfluidic platforms have been also harnessed to

determine the rheological properties of complex fluids,

which are difficult to measure with conventional rheom-

eters (Del Giudice et al., 2015; Pipe and McKinley, 2009;

Zilz et al., 2014). In addition, many researchers have used

microfluidic techniques to study the fundamentals of fluid

mechanics and their applications (Anna, 2016; Di Carlo,

2009; D'Avino et al., 2017). The generation of droplets (or

bubbles) in a microchannel is an important and exten-

sively studied subject (Anna, 2016; Kim et al., 2014a).

Moreover, droplet or bubble production has also practical

applications in targeted drug delivery (Kim et al., 2014a),

cell encapsulation (Niepa et al., 2016), optical sensors

(Zhao et al., 2008), colored pigments (Kim et al., 2014b),

microreactors (Shum et al., 2009), novel material synthe-

sis, and biomedical engineering (Teh et al., 2008). It is dif-

ficult to precisely control the size and shape of droplets or

bubbles with conventional techniques, such as in a

homogenizer (Sahin et al., 2016). However, microfluidics

enables the accurate modulation of the droplet (or bubble)

size and shape by adjusting the ratio of flow rates and the

channel geometry (Teh et al., 2008).

On the other hand, polymers such as alginate are often

used during processes for the microfluidic production of

droplets (or bubbles) (Skurtys et al., 2008). In this case, it

is important to understand the elastic effects of the poly-

mer solution on droplet (or bubble) generation. It has been

demonstrated that elasticity significantly affects the break-

up process during droplet generation in T-junction geom-

etry (Husny and Cooper-White, 2006). In previous work,

it was also observed that bubble generation in a viscoelas-

tic fluid with shear-thinning viscosity was significantly

destabilized (when compared to that for a Newtonian

fluid) in a flow-focusing geometry (Fu et al., 2011). How-

ever, it is not yet well understood how elasticity destabi-

lizes the bubble generation process. Addressing this issue

requires sophisticated experimental designs to preclude

shear-thinning effects because the elastic effects on drop-

let (or bubble) generation can be confused with those of

shear-thinning viscosity.

A droplet (or bubble) confined by side walls is elon-

gated, but it is not clear how elasticity of the polymer

solution affects the droplet (or bubble) shape. Chung et al.

(2008) numerically predicted that a droplet is significantly

more elongated in a viscoelastic fluid than in a Newtonian

fluid, and that this difference is caused by the first normal

stress difference developed between the droplet and chan-

nel wall, squeezing the droplet more in a viscous fluid

than in a Newtonian one. Fu et al. (2011) demonstrated

that the bubbles in a viscoelastic fluid with shear-thinning

viscosity have a more slender shape than those in a New-*Corresponding author; E-mail: [email protected]

Dong Young Kim, Tae Soup Shim and Ju Min Kim

148 Korea-Australia Rheology J., 29(2), 2017

tonian fluid. They proposed shear-thinning viscosity as the

origin of this phenomenon.

In this work, we investigated bubble generation in a

flow-focusing geometry, paying particular attention to

elastic effects on the stability of bubble generation. The

polymer solution concentrations used in this work were

much lower than those of the overlapping polymers, ensur-

ing that the shear viscosities of the polymer solutions

would be nearly constant, irrespective of shear rate. There-

fore, the elastic effects on bubble generation could be iso-

lated from the shear-thinning effects. We analyzed the

flow instability caused by the polymer addition using flu-

orescent microscopy, and demonstrated that irregular bub-

ble generation in viscoelastic fluids can be attributed to

unstable flow dynamics near the flow-focusing geometry.

In addition, we observed the shape of bubbles flowing

through a microchannel in a viscoelastic fluid with nearly

constant shear viscosity, further demonstrating that elas-

ticity plays a critical role in determining bubble shape in

confined channels.

2. Theoretical Background

We investigated two-phase flows in a microfluidic flow-

focusing geometry, paying particular attention to the elas-

tic effects of dilute polymer solutions on bubble genera-

tion and the bubble shapes formed in a confined channel.

The bubbles were generated in a Newtonian fluid or a vis-

coelastic polymer solution. Bubble generation and the

resulting shapes are affected by the interfacial tension

between the gas and liquid phases, as well as on the vis-

coelasticity of the bubble suspending liquid. In the two-

phase flows of bubbles within a suspending liquid, the

capillary number (Ca) is a dominant dimensionless num-

ber used for characterizing the dynamics (Tabeling, 2005).

Ca is the relative ratio of the viscous force to the inter-

facial tension and is typically defined as Ca = ,

where μ is the shear viscosity of the suspending liquid, U

is the characteristic velocity, and σ denotes the interfacial

tension between the bubble and its suspending liquid. In

this work, the characteristic velocity is defined as ql/hw,

where ql is the volumetric flow rate of the liquid phase,

and h and w denote channel height and width, respec-

tively.

The liquid phase of a dilute polymer solution (visco-

elastic fluid) is typically modeled using the Oldroyd-B

model when the shear viscosity is constant (Bird et al.,

1987). The total extra stress (τ = τs + τp) of a polymer

solution can be expressed as the combination of the New-

tonian solvent stress (τs = 2μsd) and the polymer stress (τp

+ λτp(1) = 2μpd) in the Oldroyd-B model, where d is the

deformation rate tensor ( ) and L is the veloc-

ity gradient tensor ( ). The solution viscosity (μ) is

the sum of the solvent (μs) and polymer (μp) viscosities.

The variable τp(1) denotes the upper convective derivative

of τp ( ). When the viscoelas-

tic flow of dilute polymer solution is modeled using the

Oldroyd-B model, three dimensionless groups—the Weis-

senberg (Wi) and Reynolds (Re) numbers, and the solvent

contribution to solution viscosity, ( )—can

be used to analyze the viscoelastic flow of a polymer solu-

tion (Kim et al., 2005). Wi represents the relative ratio of

elastic to viscous properties (Bird et al., 1987) and is

defined as Wi = , where λ is the relaxation time of the

polymer solution and is the characteristic shear rate.

The value is defined in a microchannel by =

(where q is the volumetric flow rate, h is the channel

height, and w is the channel width) (Rodd et al., 2005). Re

denotes the relative ratio of inertial to viscous forces and

it is defined as Re = LUρ/μ = 2qlρ/μ(h + w) where L is the

characteristic length ( ), and ρ and μ are the

fluid density and viscosity, respectively.

In this work, the polymer concentrations (c) were all

below the overlapping concentration (c*), which indicates

that the relaxation times of the polymer solutions are

nearly equal to the Zimm relaxation time (Liu et al., 2009).

It has previously been observed that the relaxation time of

the polymer solution, which was measured using the small

amplitude oscillatory shear (SAOS) test, did not signifi-

cantly deviate from the Zimm relaxation time when c < c*

(Liu et al., 2009). On the other hand, the viscosity of the

polymer solution can be expressed by

(Tirtaatmadja et al., 2006), where [μ] is the intrinsic

viscosity of the polymer solution, and has the relationship

of (Graessley, 1982). Therefore, β can be

denoted as .

In the literature, bubble generation in flow-focusing

microfluidic geometry has been extensively studied for

bubbles in Newtonian liquids. The bubble volume (Vb)

generated in flow-focusing geometry can be predicted

using the following relationship (Lu et al., 2014):

(1)

where Qg/Ql is the ratio of the gas and liquid phase flow

rates. The above relationship was obtained by fitting

experimental data gathered for viscous suspending liquids

(μ≥ 5.7 mPa·s; 0.00065 < Ca < 0.2; 0.01 < Re < 3.2) (Lu

et al., 2014). However, it is not well understood how the

elasticity of the polymer solution affects bubble genera-

tion in flow-focusing geometry. In particular, studies on

the elastic effects on bubble generation stability are very

limited.

On the other hand, if the size of the droplets formed in

μU/σ

d L LT

+( )/2≡L ∇u≡

τp 1( )

∂τp∂t

-------≡ u+ ∇τp⋅ L– τp⋅ τp– LT⋅

β μs

μ-----≡ 1=

μp

μ-----–

λγ·c

γ·c

γ·c γ·c2ql

h2w

---------

L 2hw/ h w+( )≡

μ μs 1 c μ[ ]+( )≅

μ[ ] = 0.77/c*

β = 1/ 10.77c

c*-------------+⎝ ⎠

⎛ ⎞

Vb

w3

------ Qg/Ql( )0.52Ca0.29–∼

Elastic effects of dilute polymer solution on bubble generation in a microfluidic flow-focusing channel

Korea-Australia Rheology J., 29(2), 2017 149

the flow-focusing geometry is greater than the channel

width, the shape of the droplets is elongated by the con-

finement of the channel walls. In previous theoretical

studies, it was predicted that a droplet flowing in a vis-

coelastic fluid would have a longer shape than one flow-

ing in a Newtonian fluid because the first normal stress

difference existing between the droplet and the channel

wall squeezes the droplet (Chung et al., 2008). The first

normal stress difference in polymer solution has positive

value in shear flows. Therefore, the gap between the chan-

nel wall and the droplet is enlarged by the first normal

stress difference developed between the channel walls

(Chung et al., 2008). The more elongated shape of drop-

lets flowing in a microchannel was experimentally observed

in the viscoelastic fluid, as compared with those flowing

in the Newtonian fluid (Fu et al., 2011). However, only

shear-thinning fluids have been investigated as suspending

liquids, and the origin of this phenomenon was attributed

to the shear-thinning viscosity of the polymer solution (Fu

et al., 2011). Here, we investigate the shape of bubbles

flowing in dilute polymer solutions—viscoelastic fluids

with nearly constant shear viscosity called Boger fluids—

to reveal the origin of this elongated bubble shape.

3. Experimental

3.1. Microfluidics and materialsSchematic diagrams for the flow-focusing geometry

used to generate bubbles are presented in Fig. 1. The

microfluidic device used to generate N2 bubbles was com-

posed of two inlets and one outlet, and was fabricated with

polydimethylsiloxane (PDMS) following the conventional

soft lithography technique (Xia and Whitesides, 1998).

Specific conditions for the channel fabrication are pre-

sented in our previous work (Yang et al., 2011). The chan-

nel height and width were constant at 50 μm. The N2

bubbles were generated in the flow focusing region (cross-

slot), as shown in Fig. 1. The distance between the focus-

ing region and the outlet was 3 cm, and the distance

between the solution (liquid phase) inlet and the focusing

region was 1.2 cm. The N2 gas inlet was located 0.3 cm

upstream from the focusing region. The flow rate of liquid

solution was controlled with a syringe pump (KDS100,

KD Scientific) and the N2 stream was modulated with a

compressed gas flow system (Bong et al., 2011). The spe-

cific conditions of our compressed gas flow system were

presented in our previous work (Kim et al., 2012). The

flow rate of the N2 stream was estimated from the volume

of the bubbles flowing through the channel per unit time.

In this work, we used poly(ethylene oxide) solutions

(M.W. = 8,000,000 g/mol (8 M); Sigma-Aldrich) in deion-

ized (DI) water as the viscoelastic media. The tested poly-

mer concentrations were 5, 10, 20, 30, 40, and 50 ppm. DI

water was used as the Newtonian fluid. We added 1 wt%

sodium dodecyl sulfate (SDS) (Sigma-Aldrich) to all solu-

tions to ensure a constant interfacial tension (37 ± 1 mN/

m). The shear viscosities of the solutions were measured

at 20°C using a rotational rheometer (1° cone-plate geom-

etry (60 mm diameter); AR-G2, TA Instruments). The sur-

face tension was measured by the pendant-drop method

using contact angle meter (Attension Theta, Biolin Scien-

tific). The overlapping concentration of 8 M PEO was

estimated at 300 ppm (Graessley, 1980). It is well known

that SDS forms a complex with PEO, resulting in a

change to the PEO conformation and rheological proper-

ties (Brackman, 1991). Conversely, it was observed that

the relaxation time of the SDS-PEO complex solution did

not significantly deviate from that of a pure PEO solution

when the relaxation time was measured using the small

amplitude oscillatory shear (SAOS) test (Miller and Coo-

per-White, 2009). The relaxation time measured using a

capillary breakup extensional rheometer (CaBER) test

increased slightly with the addition of SDS to the pure

PEO solution. However, at the largest SDS concentration,

the increase was less than that for a 30% solution (Miller

and Cooper-White, 2009). Therefore, we adopted the

Zimm relaxation time (λzimm = 3.4 ms) of the pure PEO

solution as the relaxation times for all of the polymer solu-

tions, since the relaxation time measurement for a low vis-

cosity polymer solution is still challenging. The Zimm

relaxation time was estimated following previous studies

(Rodd et al., 2005; Yang et al., 2011). In addition, we

assumed that the overlapping concentration of SDS-PEO

was equal to that of pure PEO.

Fig. 1. (Color online) Schematic diagram of the microfluidic

device used for generating bubbles—The device has two inlets

and one outlet: the central stream (N2 gas) and the outer streams

(liquid phase) meet at the cross-slot region (flow-focusing zone)

where bubbles are continuously generated by a bubble pinch-off

process. The bubble shape and size are observed 0.25 cm down-

stream from the flow-focusing zone. Two representative images

for the bubble generation processes in Newtonian (deionized

water) and viscoelastic (50 ppm 8 M poly(ethylene oxide)(PEO)

solution) fluids are presented in the figure. The flow rate of the

liquid phase was 1.5 mL/h and the pressure of N2 gas was set to

15 psi (volumetric flow rate ≅ 0.33 mL/h).



3.2. Flow visualizing techniquesBright-field optical microscope images were acquired

0.25 cm downstream from the focusing region with a

high-speed camera (MC2, Photron) installed on an upright

optical microscope (BX60, Olympus) (refer to Fig. 1).

Exposure time was set to 1/54000 s and 8000 images were

obtained at approximately 60 or 2000 frames per second

(fps). The images were analyzed with ImageJ software

(NIH). The lengths of the major and minor axes of each

bubble were measured using the ‘Analyze particle func-

tion’ option in the ImageJ software.

The vortex dynamics in the focusing zone were visual-

ized with 0.5 μm fluorescent microspheres (FluoSpheres

F8812, Invitrogen) at a concentration of 500 ppm. The

channel walls were pre-treated with a 0.01 wt% Tween 20

(Sigma-Aldrich) aqueous solution to prevent adhesion of

the fluorescent microspheres to the walls. The images for

the vortex dynamics were acquired with a charge-coupled

device (CCD) camera (DMK 21AF04, ImagingSource)

installed on an inverted optical microscope (IX71, Olym-

pus). A metal-halide lamp (Lumen 200, PRIOR) was used

to illuminate the fluorescent microspheres. The images

were recorded at 30 fps. The brightness and contrast of the

snap-shot images were enhanced using the ImageJ soft-

ware.

4. Results and Discussion

In this work, the microfluidic device for bubble gener-

ation had two inlets and one outlet (Fig. 1). The N2 gas

formed a central stream that was regulated by a com-

pressed gas flow system (Bong et al., 2011; Kim et al.,

2012), while the outer liquid phase streams were con-

trolled by a syringe pump. The gas and liquid streams

meet at the cross-junction (flow-focusing region), where

the pinch-off of N2 gas occurs, generating bubbles peri-

odically. We observed that both the bubble generation pro-

cess and shape changed significantly, even when a very

small amount of polymer (≈ 20 ppm) was added to the

Newtonian fluid. The representative images for the bub-

bles generated in the Newtonian (Wi = 0; Re = 8.8; Ca =

4.5×10−4; = 1.0) and viscoelastic (Wi = 22.4; Re

= 4.1; Ca = 9.1×10−4; β = 0.49) fluids are presented in Fig.

1 (upper-right). In this image, μs denotes the solvent vis-

cosity (1.06 cP). The zero shear viscosity (μ0) was

obtained from the Carreau model (note that the Reynolds

numbers were also calculated based on the zero-shear vis-

cosity). The images demonstrate that the elasticity of the

liquid phase notably impacts bubble generation and shape,

though the polymer solution is a very weakly elastic fluid

( ). As shown in Fig. 2, the viscosity at the

highest concentration of 50 ppm is very slightly shear-

thinning (power-law index (n) = 0.93) and the zero shear

viscosity for the 50 ppm PEO solution was 2.15 cP. The

shear viscosities for all of the polymer solutions ( )

in this work can therefore be considered nearly constant,

irrespective of shear rate (see Fig. 2). Therefore, all of the

viscoelastic fluids used in this study can be considered

Boger fluids.

In our study, we first observed the size distribution for

the generated bubbles at the location 0.25 cm downstream

from the flow focusing region, with a liquid flow rate of

1.5 mL/h and the N2 gas pressure set to 15 psi. The gas

phase flow rate at this pressure was estimated to be 0.33

mL/h. Changes in the lengths along the major and minor

axes of bubbles, depending on the concentration of poly-

mer solution, are presented in Fig. 3A, and the major and

minor axes are defined in Fig. 3B. Lengths were normal-

ized with the channel width. The lengths along the major

and minor axes of the bubbles were quite uniform in the

Newtonian fluid. The coefficient of variance (CV = stan-

dard deviation/average value) of the length along the

major axis was 20%. The length change along the minor

axis was negligibly small, since the bubble is confined by

the channel side walls (CV 2%) (refer to Fig. 3(B)(upper)).

However, the fluctuation in the lengths became notably

larger with increasing polymer concentration ( .

Therefore, we can conclude that the addition of the poly-

mer in the liquid phase causes instability in the bubble

generation process.

It has previously been observed that bubble generation

becomes unstable when the liquid phase is a shear-thin-

ning fluid (Fu et al., 2011). The current results demon-

strate that the instability of bubble generation occurs in the

viscoelastic fluid with substantially constant shear viscos-

ity. Therefore, the elasticity of a polymer solution is pos-

sibly the main reason for instability during bubble generation.

On the other hand, when , the bubble length

along the minor axis becomes significantly smaller than

the channel width (normalized length < 1.0), while the

β μs

μ0

-----≡⎝ ⎠⎛ ⎞

λzimm 3.4 ms≅

c 50 ppm≤

≅

c 10 ppm≥

c 30 ppm≥

Fig. 2. (Color online) Shear viscosities of viscoelastic (50 ppm

8 M poly(ethylene oxide)(PEO) solution) and Newtonian (deion-

ized water) fluids at 20°C.



length along the major axis is larger than the channel

width. It was previously predicted that a droplet, flowing

through a viscoelastic fluid and confined by side walls,

would experience a squeezing force by the first normal

stress difference developed between the droplet and side

wall (Chung et al., 2008). Therefore, the bubble shrinkage

observed in the channel width direction can be determined

in the same way as the numerical work that explains the

droplet squeezing by the elasticity of the suspending

medium (refer to Fig. 3B(lower)).

We performed additional experiments to reveal the ori-

gin of instability occurring during the bubble generation

process with the addition of polymer to the solvent. Flu-

orescent micospheres were added to the polymer solution

for visualization of the flow fields, and the flow rate of the

liquid phase and the pressure condition of the gas phase

were the same as those used in Fig. 3. The streaklines at

different polymer concentrations are presented in Fig. 4.

The flow stream is stable when the liquid phase is a New-

tonian fluid (water; c = 0 ppm), but a chaotic flow field is

observed near the reentrance corner when viscoelastic flu-

ids are used (refer to the yellow arrows in Fig. 4). The

chaotic motion of the flow field near the reentrance corner

becomes increasingly severe as the polymer concentration

increases. In a previous work, chaotic flow fields were

observed in a dilute polymer solution near the reentrant

corner of a micro-contraction geometry (Rodd et al., 2005).

Chaotic flow dynamics were also observed near the corner

in the current flow-focusing geometry, and we speculate

that chaotic flow destabilizes the bubble generation pro-

cess. We observed different bubble generation processes

for the Newtonian and viscoelastic (at 40 ppm) fluids, as

shown in Fig. 5. The bubble pinch-off always occurred at

nearly the same place in the Newtonian fluid, and the

shape of the interface between the liquid and gas phases

before the pinch-off point was well maintained. However,

an irregular forward and backward motion of the interface

between the liquid and gas phases was observed in the vis-

coelastic fluid (40 ppm PEO solution). We speculate that

the chaotic flow field near the reentrant corner that is gen-

erated in the viscoelastic fluid is possibly one reason for

the difference in the bubble generation processes between

the Newtonian and viscoelastic fluids.

Finally, we studied the bubble shape depending on the

solution flow rate (Ql) and the gas pressure (Pg) at a poly-

mer concentration of 20 ppm (n = 0.97) (Fig. 6). In the

figure, ‘×’ denotes that bubbles could not be generated in

Fig. 3. (Color online) (A) Major and minor axes of the bubble

lengths at different 8 M poly(ethylene oxide)(PEO) concentra-

tions—The lengths are normalized by the channel width. The

flow rate of the solution was constant at 1.5 mL/h and the pres-

sure of N2 gas was also constant at 15 psi (a volumetric flow rate

≅ 0.33 mL/h). (B) Representative images for the bubbles in a

Newtonian (deionized water) or viscoelastic (40 ppm 8 M

poly(ethylene oxide) aqueous solution)—The major and minor

axes are indicated using a bubble in the Newtonian fluid. The

mechanism of bubble shrinkage along the minor axis in the vis-

coelastic fluid is explained by the elastic force (the first normal

stress difference) between the bubble and the channel wall.

Fig. 4. (Color online) Time-lapse images of streaklines in the

flow-focusing region, where the flow fields were visualized with

fluorescent microspheres—The time interval between two suc-

cessive images is 33 ms and the polymer concentration changed

from 0 to 50 ppm. The arrows in the time lapse images denote

the irregularity of the flow field.

Fig. 5. (Color online) Time-lapse images for the bubble gener-

ation in a Newtonian (deionized water) or viscoelastic (40 ppm

8 M poly(ethylene oxide) aqueous solution) fluid. The time lapse

from the first image is displayed on each image.



the imposed Ql and Pg conditions. We categorized the gen-

erated bubble shapes into three different types: Bullet-

shaped (A), squashed (B), and ball-shaped (C) bubbles.

The bubble shape generated by the flow-focusing geom-

etry notably depends on Ql and Pg. The bubble shape is

similar to that in a Newtonian fluid (type A) when Ql is

low, since the elastic behavior of the polymer solution (20

ppm PEO solution) is weak. However, irregularity in the

bubble shape increases as the flow rate of the polymer

solution increases, causing a mixture of types A, B, and C.

This irregularity in the bubble shape possibly originates

from the chaotic flow motion near the reentrant corner, as

was explained previously. Squashed long bubbles (type B)

were also observed in the 20 ppm PEO solution, and orig-

inated from the first normal stress difference developed

between the bubble and the channel wall. Because local

shear rate is higher in the position near the channel wall

than bubble and it makes stronger normal stress difference

in that position. This pressure difference squeezes bubble.

(Chung et al., 2008). The current observations demonstrate

that the bubble generation process significantly depends

upon the operating conditions.

5. Conclusion

We investigated the effects of polymer addition on bub-

ble generation in a flow-focusing geometry and the shapes

of produced bubbles confined by side walls. The dilute

polymer solutions used in this work can be considered

Boger fluids because the shear viscosities of the solutions

are nearly constant. We clearly demonstrated how the elas-

ticity of the polymer solutions affects bubble generation

and shape, precluding shear-thinning effects. We observed

that a very small amount of polymer can destabilize the

flow field in the flow focusing geometry, which conse-

quently results in severe fluctuations in the bubble size. In

addition, the bubble shape in a viscoelastic fluid was sig-

nificantly different from that in a Newtonian fluid, and

was caused by the first normal stress difference between

the bubble and channel walls. We can conclude that the

very tiny elasticity of the liquid phase affects bubble gen-

eration and shape. This study will be useful for the design

and operation of microfluidic devices used to generate

bubbles.

Acknowledgment

This research was supported by the Research Program

through the National Research Foundation of Korea (NRF)

(No. NRF-2016R1A2B4012328) and partially supported

by the Ajou University Research Fund. We are thankful to

Prof. Chang Soo Lee Group at Chungnam National Uni-

versity for measuring surface tension.

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Fig. 6. (Color online) Phase diagram for bubble types at different

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gorized into three types: (A) Bullet-shaped, (B) squashed, and

(C) ball-shaped bubbles.



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