electrohydrodynamic quenching in polymer melt electrospinning
TRANSCRIPT
Electrohydrodynamic quenching in polymer melt electrospinningEduard Zhmayev, Daehwan Cho, and Yong Lak Joo Citation: Phys. Fluids 23, 073102 (2011); doi: 10.1063/1.3614560 View online: http://dx.doi.org/10.1063/1.3614560 View Table of Contents: http://pof.aip.org/resource/1/PHFLE6/v23/i7 Published by the American Institute of Physics. Related ArticlesDynamics of magnetic chains in a shear flow under the influence of a uniform magnetic field Phys. Fluids 24, 042001 (2012) Travelling waves in a cylindrical magnetohydrodynamically forced flow Phys. Fluids 24, 044101 (2012) Two-dimensional numerical analysis of electroconvection in a dielectric liquid subjected to strong unipolarinjection Phys. Fluids 24, 037102 (2012) Properties of bubbled gases transportation in a bromothymol blue aqueous solution under gradient magneticfields J. Appl. Phys. 111, 07B326 (2012) Magnetohydrodynamic flow of a binary electrolyte in a concentric annulus Phys. Fluids 24, 037101 (2012) Additional information on Phys. FluidsJournal Homepage: http://pof.aip.org/ Journal Information: http://pof.aip.org/about/about_the_journal Top downloads: http://pof.aip.org/features/most_downloaded Information for Authors: http://pof.aip.org/authors
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Electrohydrodynamic quenching in polymer melt electrospinning
Eduard Zhmayev, Daehwan Cho, and Yong Lak Jooa)
School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853, USA
(Received 29 September 2010; accepted 29 June 2011; published online 26 July 2011)
Infrared thermal measurements on polymer melt jets in electrospinning have revealed rapid
quenching by ambient air, an order of magnitude faster than predicted by the classical Kase and
Matsuo correlation. This drastic heat transfer enhancement can be linked to electrohydrodynamic
(EHD) effects. Analysis of EHD-driven air flow was performed and included into a comprehensive
model for polymer melt electrospinning. The analysis was validated by excellent agreement of
both predicted jet radius and temperature profiles with experimental results for electrospinning of
Nylon-6 (N6), polypropylene (PP), and polylactic acid (PLA) melts. Based on this analysis, several
methods that can be used to inhibit or enhance the quenching are described. VC 2011 AmericanInstitute of Physics. [doi:10.1063/1.3614560]
I. INTRODUCTION
Electrospinning utilizes applied electric field to form
and drastically accelerate charged fluid jets. It is an effective
technology to produce nanofibers for applications requiring
high surface area to mass ratios. Nanofibers have already
been successfully applied to high efficiency filtration and
will find numerous uses in medical, energy, textile, and
chemical sensing fields.1–3 To date, polymer solution electro-
spinning has been used almost exclusively due to its simplic-
ity, but it has been recently shown that submicron scale
fibers can also be electrospun from polymer melts,4–7 which
is commercially attractive due to environmental and eco-
nomic benefits of the solvent-free process. However, careful
process optimization and control is required to obtain sub-
micron fibers. In melt systems, this is accomplished by
management of extensional viscosity and solidification via
the thermal environment.
Numerous studies on heat transfer in conventional melt
spinning8–12 have served as a basis for thermal management
in melt electrospinning. However, significant differences
between the two processes, especially in the jet thinning rate,
warranted a dedicated study. In this paper, we will discuss
the physics governing heat transfer from the polymer jet to
the quenching ambient air in melt electrospinning, and then
support the arguments by comparison of experimental
thermal and radius profiles for various polymer melts to the
theoretical model predictions.
II. EXPERIMENTAL
The melt electrospinning experimental setup, Figure 1,
and procedure have been described in detail in our previous
publications.7,13–15 A number of temperature control features
have been incorporated, with special attention dedicated to
the melt reservoir and nozzle heaters. Ability to separately
control the spinning region ambient temperature and the
collector has been included, although these features were
intentionally disabled in this study to limit interference with
thermal measurements and to suppress the lateral motion of
the jet. The air-quenched jet temperature profiles were cap-
tured using non-contact thermal imaging with a FLIR Ther-
maCAM EX320 infrared (IR) camera. The jet diameter
profiles were obtained using a Redlake MotionPro HS-3 high
speed camera, as described in literature.13–15 Electrospinning
studies were carried out using 24-gauge needles (0.292 mm
ID) for spinnerets, 29 kV applied potential, and 0.09 m dis-
tance between the spinneret and the collector. The power
supply by Gamma High Voltage Research, Inc., ES30P-
20W, is rated for 0 to 30 kV, with maximum output current
of 660 lA.
Three different polymer melts Nylon-6 (N6), polypro-
pylene (PP), and polylactic acid (PLA) have been studied.
The N6 pellets were provided by Hyosung, Inc., Korea, and
have a melt flow index (MFI) of 3. The 129 kDa isotactic PP
melt was supplied by Clarcor Corporation. The 186 kDa
PLA pellets were purchased from NatureWorks LLC. The
nozzle temperatures and the corresponding flow rates for
various experiments are presented in Table I. The material
properties have been determined in previous studies and are
summarized in Table II. IR thermal measurements are
strongly dependent on the material emissivity, which is a
function of material, temperature, and jet thickness. Obtain-
ing the correct values for emissivity is challenging and is
typically accomplished by equipment calibration at known
conditions. In this work, emissivity for each polymer was
obtained by heating the polymer pellets on a hot plate to a
temperature representative of the electrospinning process
and then calibrating the thermal camera reading by varying
the emissivity. The resulting emissivities and calibration
temperatures are included in Table II.
Due to equipment limitations, neither the coronating
electrode current nor the resulting air velocities could be
measured accurately in experiments. The corona output cur-
rent needed for our model was estimated by extrapolation of
numerical calculations17 for a similar needle-plate geometry
and was found to be �221 lA at 29 kV, Figure 2, which is
within the power supply specifications and is in line with the
experimentally estimated power output. The air velocities
along the polymer jet could not be measured due to strong
a)Author to whom correspondence should be addressed. Electronic mail:
1070-6631/2011/23(7)/073102/8/$30.00 VC 2011 American Institute of Physics23, 073102-1
PHYSICS OF FLUIDS 23, 073102 (2011)
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electric field interference. The air velocities away from the
spinline axis decay rapidly, as noted in the numerical
study,17 and in experiments could not be distinguished from
the usual laboratory ambient air circulation. Therefore, no
direct validation for the calculated air velocities could be
presented. Instead, the resulting polymer jet temperature and
radius profiles were used for in-direct validation.
III. THEORY AND MODEL
A. Electrospinning heat transfer model
Electrospinning is a convoluted problem, in which heat
transfer has to be treated in conjunction with momentum and
mass conservation, electric field equations, viscoelastic
effects, and, in some cases, in-flight crystallization. These
equations have been incorporated in our comprehensive melt
electrospinning model, which is utilized as the framework
for present investigations. For the details on derivation and
validation of the electrospinning model, interested readers
are referred to publications.13–15 Here, we will focus on the
heat transfer aspects.
The conservation of energy equation was obtained by
the steady state shell balance on a fluid element in the
Lagrangian frame. Axial conduction was neglected, since it
is dominated by the axial convective heat transport.
Ultimately, the change in internal energy of the fluid element
is balanced by viscous heating, heat released due to crystalli-
zation, and radial convection to the surrounding cooling air:
qCpVdT
dz¼ szz � srrð Þ dV
dz� qDHf V
dhdz� 2h T � T1ð Þ
R; (1)
where q, Cp, and DHf are the polymer melt density, heat
capacity, and heat of fusion; V, T, R, and h are the average
cross-sectional jet velocity, temperature, radius, and degree
of crystallinity; z is the axial jet coordinate; szz and srr are
the axial and radial normal stresses in the jet; and finally, hand T1 are the convective heat transfer coefficient and the
bulk cooling air temperature, respectively.
The various terms in the energy equation have been dis-
cussed in detail previously,13–15 but the radial convection
(last term on the RHS) is of primary interest to this work. It
stems from the jet surface heat flux boundary condition and
enters the conservation equation due to thin filament approx-
imation used in one-dimensionalization of the problem. The
heat transfer coefficient is typically obtained from empirical
correlations, relating the system geometry and fluid flow
characteristics to the thermal boundary layer formation. Our
previous model included a correlation proposed by Kase and
Matsuo (KM),18,19 which is widely used in conventional
melt spinning literature and produces good agreement with
experimentally observed thermal profiles.8–12 The KM corre-
lation is based on the empirical formula for longitudinal
external flow along a cylinder:
NuD ¼ 0:42 Re1=3; (2)
where NuD is the Nusselt number (hDjet=kair) and Re is the
Reynolds number (VairDjet=�air). Djet is the polymer jet di-
ameter, Vair is the axial air velocity, kair is the air thermal
conductivity, and �air is the air kinematic viscosity. Substi-
tuting, rearranging, and accounting for the jet axial velocity,
V, one obtains:
h ¼ 0:388 kairVair � Vj jvairAjet
� �1=3
; (3)
where Ajet is the jet cross-sectional area. Kase and Matsuo
introduced an additional correction factor to account for
transverse air flow, Vc,air, to obtain their final correlation:
h ¼ 0:388 kairVair � Vj jvairAjet
� �1=3
1þ 8Vc;air
Vair
� �2( )1=6
: (4)
The air properties are evaluated at film temperature, Tfilm,
defined as the arithmetic average of the jet and bulk air tem-
peratures. For dry air, the following correlations18 (in SIunits) can be used in the range 300 K<Tfilm< 650 K, with
an expected error of less than 2%:
kair ¼ 1:8786� 10�4T0:866film ; (5)
�air ¼ 4:29059� 10�9T
5=2film
Tfilm þ 120: (6)
B. Conventional heat transfer enhancementmechanisms
Thermal images collected in the present study have
revealed that the quenching heat transfer rate in electrospin-
ning is significantly higher, �10-fold, than predicted by KM
correlation. Therefore, we consider a number of alternative
heat transfer mechanisms and enhancements.
FIG. 1. (Color online) Melt electrospinning setup schematic.
TABLE I. Experimental conditions.
Case Polymer
Nozzle temperature,
Tnozzle (K)
Flow rate,
Q� 10�10 (m3=s)
1 N6 543 1.17
2 N6 563 2.5
3 N6 583 5.0
4 PP 513 5.0
5 PLA 483 1.67
073102-2 Zhmayev, Cho, and Joo Phys. Fluids 23, 073102 (2011)
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1. Natural convection effects
Natural convection is typically neglected in mechanical
melt spinning in favor of forced convection due to relatively
high jet velocities. In the case of melt electrospinning, partic-
ularly near the spinneret, Re for air surrounding the jet can
often be on the order of 1 or less, and thus, natural convec-
tion may be more important than assumed by KM. The fol-
lowing correlation can be used to estimate natural
convection in horizontal cylinder geometry:20
NuD ¼hD
kair¼ 0:36þ 0:518Ra
1=4D
0:559= Prairð Þ9=16þ1� �4=9
; (7)
RaD ¼gD3
vairaair
T � T1T1
� �; (8)
where RaD is the Rayleigh number, g is acceleration due to
gravity, and Prair and aair are the Prandtl number and thermal
diffusivity of air, evaluated at film temperature. The air prop-
erties can be calculated using correlations (5) and (6), with
heat capacity being essentially constant at 1009 J=kg-K.
2. Radiation effects
Radiation heat transfer is typically negligible below
�600 K compared to convection. Even though most of the
jet is at lower temperatures, radiation close to the spinneret,
especially in electrospinning of polymers with a high melting
point, such as N6, may be significant. Radiation can be esti-
mated using gray body formulation:21
Qrad ¼ h T � T1ð Þ ¼ reR T4 � T41
� �; (9)
where Qrad is the radiative heat flux, r is the Stefan-
Boltzmann constant, and eR is the polymer jet emissivity.
3. Humidity effects
It has been observed experimentally that environmental
humidity can have significant effects on spinnability. Hygro-
scopic polymers, e.g., N6, are especially sensitive. They
have to be thoroughly dried and carefully handled prior to
spinning, and environmental humidity has to be controlled.
Even for non-hygroscopic polymers, e.g., PLA, it was
observed that spinning conditions have to be adjusted
depending on air humidity. Literature suggests that air hu-
midity can result in as much as 20% heat transfer enhance-
ment.22,23 This enhancement was modeled as a constant
multiplier to the KM correlation.
C. Electrohydrodynamic (EHD) heat transferenhancement
The conventional heat transfer mechanisms discussed in
Sec. III B are not expected to produce the experimentally
evident 10-fold heat transfer enhancement relative to the
KM correlation. The largest enhancement mechanism
was found in the heat exchange and electronic circuit litera-
ture24–33 and is due to electrohydrodynamic (EHD) effects. It
has been documented that strong electric fields can result in
significant corona currents, which disturb the thermal bound-
ary layer and can enhance the heat transfer rate as much as
60-fold in some applications. When air is used as the
quenching fluid, an order of magnitude heat transfer
enhancement is typical.24–27,32
1. Mechanism of the EHD-driven air flow
EHD heat transfer enhancement has not been previously
documented in electrospinning, but a recent publication34
and our experimental observations did note stable corona
TABLE II. Polymer material properties used in modeling.
Properties=Parameters N6 (Ref. 15) PP (Ref. 16) PLA (Ref. 13)
Melt density, q (kg=m3) 1000 770 1240
Heat capacity, Cp (J=kg K) 2553 1925 1800
Thermal conductivity, kT (W=m K) 0.245 0.12 0.2
Dielectric constant ratio, e=e0 3.2 2.25 3.1
Surface tension, c (N=m) 0.030 0.0221 0.0435
Stress relaxation time, k (s) 0.025 @ 523 K 0.035 @ 453 K 0.1 @ 453 K
Giesekus mobility factor, a 0.04 0.4 0.015
Activation energy of flow, DH=Rig (K) 6600 2350 9060
Zero-shear-rate viscosity, g0 (Pa s) 193 @ 523 K 905 @ 453 K 1320 @ 453 K
Ratio of solvent to zero-shear-rate viscosity, b 0.3 0.01 0.35
Emissivity, eR 0.56 @ 445 K 0.68 @ 443 K 0.68 @ 425 K
FIG. 2. Coronating electrode current versus applied voltage.
073102-3 Electrohydrodynamic quenching Phys. Fluids 23, 073102 (2011)
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formation. In contrast with electrically conducting polymer
solutions commonly studied in electrospinning literature,
polymer melts are highly insulating, and when a high voltage
is applied to a needle-plate geometry separated by dielectrics
(in this case air and polymer melt), a corona field is often
produced. This condition is obtained when the applied poten-
tial exceeds the corona onset, but is below the air gap break-
down voltage. During the corona discharge, the local electric
field near the needle tip exceeds the breakdown electric field
strength in air, E0¼ 3.2� 106 V=m, and as a result, air mole-
cules become ionized. This region is referred to as the ioni-
zation zone, and it contains ions of both polarities. Further
away from the needle, where the local electric field decreases
below E0, no further ionization occurs. However, in this
drifting zone, also known as the space charge plume, ions of
the same polarity as the needle are transported from the nee-
dle to the plate due to action of the electrostatic forces.
Along their flight path, the ions can exchange momentum
with and accelerate neutral air molecules. In the present
case, high positive voltage is applied to the needle, so a posi-
tive corona is formed, and thus, positive air ions are observed
in the drifting zone. Schematic of this process is shown in
Figure 3 and further detailed discussions can be found in
literature.17,24–33,35
The set of governing equations for this process has been
reported previously17,25 and is as follows:
Conservation of momentum :
qairVair � rVair ¼ �rpþ lairr2Vair þ qE; (10)
Mass continuity : r � Vair ¼ 0; (11)
Ionic current density : J ¼ lEEqþ Vairq� DErq; (12)
Charge conservation : r � J ¼ 0; (13)
Poisson0s equation : r2U ¼ �q=e; (14)
Electric Field : E ¼ �rU; (15)
where qair and lair are the air density and dynamic viscosity,
p is the pressure, q is the space charge density, E is the elec-
tric field, J is the electric current density, lE is the air ion
mobility (2.0� 10�4 m2=V-s for positive air ions31), DE is
the ion diffusivity coefficient, U is the electric potential, and
e is the permittivity of air. The last term in the momentum
equation, Eq. (10), is the Coulombic force exerted on the air
by the action of space charge in the electric field.
Solution of this set of coupled equations requires
specialized numerical techniques discussed in the litera-
ture.17,25,27,29,31 In this work, we utilize the findings from
various numerical studies to simplify the system and obtain
an approximate analytical expression for the air velocity
along the polymer melt jet and consequently determine the
EHD heat transfer enhancement.
2. Axial velocity approximate solution
In this study, we are particularly concerned with the
axial air velocity along the spinline axis, which disturbs the
thermal boundary layer in the region from the needle tip up
to �35 needle radii (R0) from the needle. To obtain an ap-
proximate solution, we utilize the findings of a numerical
investigation for a similar geometry.17
First let us consider the conservation of momentum, Eq.
(10). In the numerical study, the air flow streamlines in the
vicinity of the spinline axis were found to be nearly parallel
to the axis. Furthermore, as discussed elsewhere,35 in the
boundary layer close to the polymer jet, we can neglect the
pressure perturbation and the viscous term due to large air
Re (expected to be on the order of 1000). The momentum
balance is then reduced to:
qairVz;airdVz;air
dz¼ qEz: (16)
To determine the Coulombic force term, we turn to the cur-
rent and charge conservation Eqs. (12) and (13). From the
numerical investigation, the space charge plume in air takes
on a nearly conical shape, with the space charge density
decreasing with distance from the needle due to Coulomb
repulsion. As a first approximation, it could be assumed that
the charge density is inversely proportional to the cross-sec-
tional area of the plume cone, which in turn would be
approximated as p(R0þz)2. Since current transport is limited
to this conical plume region, the axial current flux averaged
over the transverse direction, Jz,avg, is then:
Jz;avg zð Þ ¼ I
p R0 þ zð Þ2; (17)
where I is the total current emitted from the coronating nee-
dle tip and R0
is the needle radius. Using this average flux to
determine the Coulombic acceleration would be an underes-
timation. From experimental measurements,36 it is evident
that the charge density and, therefore, the current flux are at
their maximum along the spinline axis and decay rapidly in
the transverse direction. Based on the experimental data, we
propose the following radial current flux, Jr, profile:
Jr r; hð Þ ¼ Ccos2 h
r2; (18)
where h is the inclination angle with respect to the spinline
axis and C is the normalization constant, which can be deter-
mined by spherical averaging of 18 and comparing to 17 toFIG. 3. (Color online) Schematic of EHD-driven air flow in melt
electrospinning.
073102-4 Zhmayev, Cho, and Joo Phys. Fluids 23, 073102 (2011)
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find C¼ 3 I = p. We then obtain a better approximation for
the axial current flux along the spinline:
Jz zð Þ ¼ 3I
p R0 þ zð Þ2: (19)
Finally, based on previous studies in the literature31,35 and
our estimations on the magnitudes of various terms in
Eq. (12), the conduction term (lEEq) dominates the convec-
tive and diffusive fluxes (last two terms) in the region near
the needle, which is of interest to this study. Further from the
needle, the air velocity can increase sufficiently to make the
convection term comparable to conduction. However, at this
point, the space charge density would have diminished sig-
nificantly, thus, causing the current flux and consequently
the Coulombic acceleration term in the momentum balance
to be low.
With the above assumptions and approximations, we
arrive at the axial momentum balance near the spinline axis:
qairVz;airdVz;air
dz¼ 3I
lEp R0 þ zð Þ2; (20)
and after applying the boundary condition Vz,air(0)¼ 0, we
get:
Vz;airðzÞ ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi6I
qairlEpR0
1� 1
1þ z=R0
� �s: (21)
This approach was found to provide the correct qualitative
behavior of the air velocity along the spinline, but some of
the simplifying assumptions may limit its accuracy. Further
refinements to this model are discussed in Sec. III C 3.
3. EHD model refinements
In the preceding discussion, the transverse components
of current and velocity, as well as the pressure perturbations
were eliminated. These restrictions may limit the accuracy of
the calculated axial air velocity and also conceal the rapid
decay of the velocity further from the spinline. These
restrictions can be removed by solving the conservation of
momentum in the stream function form. However, to obtain
a non-trivial solution, the current flux must be known and it
must have both r- and h- components. No analytical solution
for the current flux in this problem has been reported to date,
and we have not been able to find a suitable approximation
that satisfies all of the governing Eqs. (13)–(15). Therefore,
at present, we must rely on numerical methods or crude
approximations to elucidate the EHD effects.
IV. RESULTS AND DISCUSSION
The calculated EHD-driven air velocity profiles for N6
are compared to the jet velocities in Figure 4(a). Due to co-
rona discharge in the vicinity of the needle, the air in the
drifting zone is rapidly accelerated. It is apparent that the
resulting air velocities are three orders of magnitude higher
than the jet velocities. Subsequently to the short initial tran-
sient (z=R0 < 3), the driving electric field rapidly decreases
and the air velocity approaches its asymptotic value. We
should note that eventually the velocity should decrease due
to viscous forces, but we have assumed that these effects are
not significant in the region of interest, z=R0 < 35.
FIG. 4. (Color online) Calculations for N6 met electrospinning: (a) EHD-
driven air velocity profiles compared to the jet velocities, (b) KM local heat
transfer coefficient profiles, and (c) heat transfer enhancement due to natural
convection, radiation, and EHD effects.
073102-5 Electrohydrodynamic quenching Phys. Fluids 23, 073102 (2011)
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The KM heat transfer coefficient profiles for N6 are
shown in Figure 4(b). The rapid initial increase in heat trans-
fer is due to high jet attenuation, and the asymptotic values
are reached when the jet profiles become nearly cylindrical.
The higher polymer flow rates for higher nozzle temperature
cases produce higher jet velocities and thus higher heat trans-
fer coefficients.
Observing the various heat transfer enhancement ratios,
Figure 4(c), it can be concluded that radiation effects are in
fact negligible, and natural convection provides only a slight
enhancement very near the spinneret. Interestingly, the natu-
ral convection heat transfer coefficient in its asymptotic
region is close to the KM correlation predictions. The 20%
enhancement due to air humidity (not shown) is clearly not
sufficient to match experimental observations. Finally, the
EHD effects produced the expected pronounced enhance-
ment, consistent with experimental observations. While only
N6 cases are shown, similar results were obtained for PP and
PLA.
As discussed above, the KM model predicts significantly
slower jet cooling than observed experimentally, illustrated
for the case of N6 at nozzle temperature of 543 K in Figure
5. Incorporation of the proposed EHD mechanism allowed
for the correction of the thermal profile prediction, while
other possible heat transfer enhancement mechanisms failed
to match experimental observations. Since the emission cur-
rent for the present case (221 lA) was obtained by extrapola-
tion of numerical data, in Figure 5 we also show results for
other currents to demonstrate that the exact value of the
emission current is not required for these calculations to pro-
vide suitable agreement with experiments. For clarity, exper-
imental error bars are not included in the figure, but based on
camera resolution, accuracy of emissivity estimation, and
image variability, the associated experimental error varies
from 62% at the nozzle to 610% as room temperature is
reached. The major reasons for the increased error at lower
temperature are the influence of the background temperature
and the emissivity estimation based on the high temperature
thick jet condition.
Experimental jet radius and thermal profiles along the
spinning axis and the model results with EHD enhancement
for a number of polymer melts are presented in Figure 6.
Using the present model, excellent agreement with all of the
FIG. 5. (Color online) Experimental jet temperature profile for N6 (at noz-
zle temperature 543 K) compared to KM predictions, radiation and natural
convection enhancements, and the present EHD model.
FIG. 6. (Color online) Experimental jet radius and temperature profiles for
(a) N6, (b) PP, and (c) PLA compared to model predictions. Notes: for N6,
the images shown in the insets are for 563 K nozzle temperature; in all ther-
mal images, the needle edges have been outlined in post-processing.
073102-6 Zhmayev, Cho, and Joo Phys. Fluids 23, 073102 (2011)
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investigated cases was achieved. The discrepancy in the ther-
mal profiles for the N6 583 K case should be attributed to
low thermal image quality in the thin jet region (most likely
due to the jet veering away from the camera focal plane,
which is commonly observed at higher temperatures). While
only the results with EHD enhancement are shown in Figure
6, the other discussed heat transfer modes were also calcu-
lated. As demonstrated above, the other mechanisms result
in much lower heat transfer coefficients, and thus signifi-
cantly different thermal profiles. The agreement of the pres-
ent model with the experimental data indicates that the
proposed EHD heat transfer enhancement mechanism in
electrospinning is plausible, and most of the assumptions
used in the model derivation are reasonable in the region of
importance. Further investigations on the assumptions of the
uniform current flux and negligible pressure perturbation are
warranted.
Based on the performed analysis and discussions, the
undesirable EHD quenching can be minimized=eliminated
using the following techniques. First, forced convection of
heated ambient air would reduce the jet cooling by not only
minimizing the temperature differential between the jet and
the air, but also by perturbing the EHD quenching mecha-
nism.33 Second, applying high potential to the collector,
rather than the spinneret, would allow for elimination of co-
rona discharge, while preserving the electric field profile.
These modifications have been implemented in a different
study37 and were shown to be effective at preventing the
undesirable quenching, and consequently resulting in pro-
duction of smaller fibers from polymer melt electrospinning.
V. CONCLUSIONS
In summary, IR thermal measurements have revealed
enhanced air quenching of polymer melt jets in electrospin-
ning. We have postulated that EHD effects are the root cause
of this quenching and developed a correlation to account for
this enhancement. The correlation was implemented in a
comprehensive model for polymer melt electrospinning and
validated by excellent agreement with experimental observa-
tions. Based on this analysis, we concluded that addition of
forced hot air convection and application of high potential to
the collector rather than needle can be used to minimize/
eliminate the undesirable jet quenching, which will in turn
allow for production of smaller fibers from polymer melt
electrospinning.
ACKNOWLEDGMENTS
The authors would like to thank the National Science
Foundation for funding this work through CAREER Award,
Grant No. CTS-0448270, and E.I. du Pont de Nemours and
Company for funding through DuPont Young Professor
Grant to YLJ.
1H. Fong and D. H. Reneker, “Electrospinning and the formation of nano-
fibers,” in Structure Formation in Polymeric Fibers, edited by D. R. Salem
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