Journal of Electrostatics 69 (2011) 540e546
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Journal of Electrostatics
journal homepage: www.elsevier.com/locate/elstat
A comparative study of jet formation and nanofiber alignment in electrospinning
and electrocentrifugal spinning systems
F. Dabirian a, S.A. Hosseini Ravandi a, *, A.R. Pishevar b, R.A. Abuzade a
a
b
Nanotechnology & Advanced Materials Institute, Department of Textile Engineering, Isfahan University of Technology, 84156-83111 Isfahan, Iran
Department of Mechanical Engineering, Isfahan University of Technology, 84156-83111 Isfahan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 March 2011
Received in revised form
11 June 2011
Accepted 21 July 2011
Available online 18 August 2011
Electrocentrifugal spinning is a recently developed spinning system whose performance is still under
investigation by researchers. In this study the process of jet formation in electrocentrifugal spinning is
explored and compared to the same process in electrospinning and centrifuge spinning. The results show
that the incorporation of the electrical and the centrifugal forces in the electrocentrifugal spinning
system leads to the formation of a more stable jet at lower viscosities. It is also shown that the
electrocentrifugal spinning method is an efficient technique for the production of aligned nanofiber
bundles with enhancement in the mechanical properties.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Electrocentrifugal spinning
Jet formation
Nanofiber
1. Introduction
Polymeric nanofibers enjoy excellent physical and mechanical
characteristics embedded in their nature of decreased diameters
down to submicron. Very large surface area to volume ratio of
nanofibers offers different significant applications for them
including nanocatalysis, tissue engineering scaffolds, protective
clothing, filters, nanoelectronics, high performance fibrous instruments, nanobiosensors, drug delivery, wound dressing, composites,
etc [1]. Different techniques have already been proposed to produce
nanofibers such as drawing, template synthesis, gas jet spinning,
island-in-the-sea, electrospinning and recently electrocentrifugal
spinning. Among these methods, electrospinning seems to be the
most versatile, applicable, high-potential and simplest technique.
The formation of nanofiber via electrospinning is based on the
continuous stretching of a viscoelastic jet derived from a polymer
solution or melt by the electrostatic forces. The electrospinning
technique may be considered as a variant of the electrospray
process. Both of these techniques involve the use of a high voltage
supply to induce the formation of a liquid jet. In electrospray small
droplets or particles are formed as a result of the breakup of an
electrified jet that is often produced from a low viscosity solution.
* Corresponding author. Research Center of Science and Fiber Technology,
Department of Textile Engineering, Isfahan University of Technology, 84156-83111
Isfahan, Iran. Tel.: þ98 311 3915034; fax: þ98 311 3912444.
E-mail address: hoseinir@cc.iut.ac.ir (S.A. Hosseini Ravandi).
0304-3886/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.elstat.2011.07.006
In electrospinning a solid fiber is generated as the electrified jet is
continuously elongated due to the electrostatic repulsions between
the surface charges and the evaporation of the solvent [2]. Due to
the unique features and applications, the process has recently been
reviewed in a number of publications [3e11], but here we only
intend to review the most recent activities in the area of alignment
of nanofiber by this technique. The associated bending instability of
the spinning jet causes the produced nanofibers to be deposited
on the surface of the collector as randomly oriented nonwoven
mat. However, in many applications, aligning the deposition of
nanofibers in a specific direction is a requirement. For example, in
the fabrication of electronic and photonic devices, well aligned and
highly ordered architectures are often required. Even for applications as simple as fiber based reinforcement, it is also critical to
control the alignment of fibers.
In recent years a number of approaches have been demonstrated to directly collect electrospun nanofibers as uniaxial
aligned arrays [2]. These approaches include electrospinning which
uses a collector consisting of two pieces of electrical conductors
separated by a gap [12e17], collecting spun nanofibers on a rotating
thin wheel with sharp edge [18], fabricating aligned yarn of
nanofibers by rapidly oscillating a grounded frame within the jet
[19,20], using a metal frame as a collector to generate parallel arrays
of nanofibres [21,22], using magnetic field to produce aligned
nanofibrous arrays [23,24], and using a rotating drum at a very high
speed up to thousands of rpm [25,26]. In another approach, Deitzel
et al. [27] used a multiple field technique which can straighten the
F. Dabirian et al. / Journal of Electrostatics 69 (2011) 540e546
polymer jet to some extent. Subsequently, Dabirian et al., employed
two connected needles to the positive and negative voltages to
collect high bulk nanofiber on a slowly rotating drum [28e30].
The first significant work on the free surface flow of liquid jets is
that of Lord Rayleigh [31]. Rayleigh analyzed the linear stability of
a liquid jet. He showed that the unstable nature of liquid jets is
caused by the surface tension force acting on the jet. It is generally
accepted that Rayleigh’s inviscid linear model describes the
beginning of liquid jet breakup. Weber [32] introduced viscosity into
the stability analysis. A significant review of the early work on liquid
jets is given by Bogy [33] and a more up-to-date and extensive
review is given by Eggers [34]. Other important related works
include Middleman [35], Hilbing and Heister [36], Partidge et al. [37],
Parau et al. [38], Keller et al. [39], Tuck [40], Vanden-Broeck and
Keller [41], Entov and Yarin [42], Dias and Vanden-Broeck [43], Yarin
[44] and Cummings and Howell [45].
Recently, a new method of producing aligned nanofiber called
electrocentrifugal spinning was introduced [46]. In order to apply
a high rate of tension to a polymer solution or melt, methods
which can apply uniform distribution of stress are required. When
centrifugal force acts on a solution, it affects all parts of the
solution and applies uniform distribution of stresses. Thus, this
force can be used to apply high rates of tension on a polymer
solution. During the tension process, if the polymer solution has
sufficient viscosity, it is stretched as a string and transformed to
a polymeric fiber after drying. Weitz and coworkers used centrifuge force to produce nanofiber without nozzle; it involves the
application of drops of a polymer solution onto a standard spin
coater [47]. The viscosity of the polymer solution arises from the
frictional forces between polymer chains in the solution. Since the
nature of frictional forces is dependent on the speed of the applied
exterior forces, with an increase in this rapidity, the frictional
forces between polymer chains increase. Consequently, by
increasing the rate of the applied exterior forces, the polymer
solutions with lower concentrations can be spun. The simultaneous use of electrostatic and centrifugal forces provides new
possibilities to manufacture nanofibers [48].
In continuation, jet formation in this new method will be
investigated by high-speed photography. The jet formation in
541
electrospinning and electrocentrifugal spinning will be compared.
Finally, the mechanical properties of both electrospun and electrocentrifugal spun aligned nanofiber bundle will be considered.
2. Experimental
2.1. Materials
Laboratorial distilled water was produced manually. Commercial polyacrylonitrile (PAN) polymer powder with Mw ¼
100,000 g/mol and Mn ¼ 70,000 g/mol was supplied by Poly Acryl,
Iran. The solvent used was dimethyl formamide (DMF) from Merck
Company, Germany. The polymer solution of PAN in DMF with the
concentration of 15% wt was prepared.
2.2. Set-up
2.2.1. Centrifuge spinning
As shown in Fig. 1 a needle of 18 mm in length with interior and
exterior diameter of 160 and 300 mm, respectively was attached to
a container with 4.6 mm interior diameter to form a nozzle for the
purpose of supplying polymer solution. The nozzle was mounted
on a circular plate. The polymer solution with proper viscosity runs
through the nozzle and forms a jet once the circular plate starts to
rotate and centrifugal force is applied. The jet is dried on its way out
of the needle and nanofibers are formed. The flow of surrounding
air near the nozzle can dry up the ejecting solution at the needle tip
and as a result, causes obstruction in the solution flow. To overcome
this problem, the centrifuge set-up was modified and as shown in
Fig. 1, a cylinder was used to enclose the nozzle and the container as
only 2 mm of nozzle tip was left uncovered. Due to this modification, the surrounding air in the vicinity of the nozzle tip achieves
the same velocity and therefore, the convective heat and mass
transfer are reduced considerably. Consequently, the solvent
vaporization rate is reduced and the obstruction problem is
avoided. The procedure called centrifuge spinning method, will
work continually. The trajectory of the jet is captured by means of
a high-speed digital camera (Kodak Motion Corder 3000, with
a capture rate of up to 3000 frames per second).
Fig. 1. Schematic representation of centrifuge (without high voltage) and electrocentrifuge (with high voltage) spinning set-up, (A) axle of rotation, (B) polymer solution container,
(C) nozzle tip, (D) encircling cylinder, (E) collector and (F) polymeric jet.
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F. Dabirian et al. / Journal of Electrostatics 69 (2011) 540e546
2.2.2. Electrocentrifuge spinning
With the purpose of applying electrostatic and centrifugal
forces simultaneously, a DC high voltage power supply was used.
A metallic cylinder with diameter of 26.6 cm and height of 10 cm
was used as the collector. It was attached to the negative electrode of power supply while the nozzle was connected to the
positive electrode, as can be seen in Fig. 1.
It was observed that the distance between the center of rotation,
the needle tip and the collector surface should be set at optimum
values. For values below the optimum point, fibers reach the
collector surface before they can get dried whereas for values above
the optimum point fibers suspend within the cylinders space and
never reach the collector surface. The set-up was assembled
together in a way that the distances between the needle tip to the
center of rotation and the collector to the center of rotation were set
to 5.3 and 13.3 cm, respectively. Fig. 2 shows a photograph taken by
the camera with a special micro-lens from the set-up while rotating
at 6630 rpm. The image illustrates a part of the cylindrical cover, the
nozzle tip and the formation of the fiber clearly.
To describe the differences between the mechanisms of jet
formation in electrospinning and electrocentrifugal spinning, first
the experiment is performed on water and the drift of the jet and
the formation of droplets in these two methods are compared using
a camera to record sequential snapshots of the process in equal
intervals. Then, the role that viscosity plays in the process is
considered by replacing water with a polymeric solution. At the
end, the mechanical properties of aligned polyacrylonitrile (PAN)
nanofiber produced by both methods are compared and discussed.
3. Results and discussion
Jet formation process is quite different in electrospinning,
centrifuge spinning and electrocentrifugal spinning. In the
following section, the formation of jet by these methods is
compared for two different cases of a water jet and a polymeric jet
to stress the effect of viscosity. Aligned nanofiber produced by the
electrospinning and electrocentrifugal spinning will be discussed
and the mechanical properties of nanofibers produced by these
methods will be compared in the last section.
3.1. Comparison of water behavior in jet formation process
To provide an experimental comparison of water behavior in
three fiber spinning techniques, centrifugal spinning, electrospinning and their combination; all these processes run at the same
flow rate. The flow rate in electrospinning could be easily controlled
by the rate of syringe pump; therefore we calculate the flow rate in
centrifuge spinning and then set the same flow rate for electrospinning. It is logical to expect that, the amount of solution in the
container influences the flow rate in centrifuge spinning. To calculate the flow rate we chose an average value of 0.3 ml and filled the
container with solution to 0.3 þ X ml and run the process until the
amount of solution becomes 0.3 X ml and measured the consumed
time. For further precision we used weight measurements instead
of volume measurement. The calculated flow rate was 135 ml/h for
the centrifuge spinning at 2358 rpm, and the same flow rate was
applied for electrospinning and electrocentrifuge spinning as well.
3.1.1. Water behavior in electrospinning
In electrospinning a droplet of polymer solution is formed at the
needle tip as a result of the pump pressure. The droplets will be
intensely charged when the high voltage is applied. In such a case
the droplet will be affected by two types of electrostatic forces:
repulsive forces between same charges distributed on the surface of
droplet and the electric field force generated between the nozzle and
the collector. As a result of electrostatic forces, the droplet will be
deformed and a Taylor cone is formed at the needle tip. The electrical
forces will then overcome the surface tension of the droplet and
a stable fluid jet will be ejected from the apex of the Taylor cone. The
jet will speed up toward the collector while it exhibits bending
instabilities due to repulsive forces between charges accumulated on
its surface. This makes the jet trajectory a bent spiral path. Thus, the
jet travels a long distance in the short gap between needle tip and
collector and will be drawn thousand times to become very fine in
thickness. Ultimately, the solvent evaporates, or the melt solidifies
and nanofibers are obtained on the collector in diameters down to
submicron or even few nanometers. For the solution with low
viscosity or low molecular weight, the jet will break up into droplets
of very small size. In such a case, the process is called electrospraying
and has significant applications in different industries for the
purpose of producing submicron sized droplets.
Because of the low viscosity of water, applying high voltage to
water droplet leads to electrospraying instead of electrospinning.
As can be seen in Fig. 3, the process starts with a very thin fluid
thread, then turns to a short jet, and due to its low viscosity
becomes transformed to droplets soon. However, the resistant
viscous force, a requirement for the formation of a jet, is not high
enough to withstand the inertial force. As a result, the fluid thread
is ruptured and the droplets are ejected from it.
Fig. 2. Image of PAN nanofiber formation in electrocentrifugal spinning at rotational
speed of 6630 rpm and polymer solution of 15% wt.
3.1.2. Water droplet behavior in centrifuge spinning
In this section the effect of centrifuge force on expulsion of water
droplet and formation of a water jet will be discussed. In this
experiment, the centrifuge set-up (explained in Section 2.2.1) is filled
F. Dabirian et al. / Journal of Electrostatics 69 (2011) 540e546
543
with water and rotated with various rotational speeds. The centrifuge force pumps the fluid into the needle and, therefore, the flow
rate is dependent on the rotational speed. Based on rotational speed,
two different modes can be developed in the experiment. At lower
rotational speed water droplets will be formed and expelled out
from the nozzle whereas a jet of fluid is generated at higher
rotational speeds as can be seen in Fig. 4. The breakup of the jet into
smaller drops is also clearly observed in these images.
Fig. 3. Images of water behaviors in electrospinning; a short unstable jet formed and
turned into droplets soon.
3.1.3. Water droplet behavior in electrocentrifugal spinning
In electrocentrifugal spinning the formation of the jet is solely
due to the centrifuge force which also drives the fluid into the
needle and a jet is formed without the development of the Taylor
cone. The exit jet is then deformed by the electrostatic forces and is
highly stretched as shown in Fig. 6. This image was taken for the
operating conditions of 2385 rpm, 15 kV and 8 cm spinning gap
between nozzle and collector. Again, due to the low viscosity of
water, the stretching process cannot continue and as Fig. 5 clearly
shows, the growth of instability waves ultimately leads to the
breakup of the jet into small drops.
Comparing Figs. 3e5 reveals that by conjugate use of
electrostatic and centrifuge forces, it is possible to achieve a stable
jet at higher flow rates even for a low viscosity fluid as water. This is
a significant factor in fabricating nanofibers by this method because
it can increase the production rate considerably.
3.2. Comparison of polymer solution behavior
Fig. 4. Images of water droplet formation, water jet ejection, and breaking up to
droplets in centrifuge spinning at rotational speed of 2385 rpm.
Fig. 5. Image of electrocentrifugal spinning of water (2385 rpm,15 kV, 8 cm spinning gap).
As explained in the previous section, the mechanism of jet
formation and nanofiber production is different in electrospinning
and electrocentrifugal spinning. In this section, the effect of the
viscoelastic forces on the formation of the jet by these two methods
is compared using a PAN solution instead of water.
3.2.1. Polymeric jet in electrospinning
The PAN solution can form a droplet stabilized by its surface
tension at the end of the needle tip. Strong electrostatic field is
applied to the droplet. The droplet will deform into a Taylor cone.
When the applied voltage exceeds a critical value at which the
electrostatic force overcomes the surface tension and viscoelastic
forces, a stable jet of polymer solution can be ejected from the
droplet. In contrast to previous experiment, due to the resistance
viscoelastic forces the jet is not broken up to the droplets. Instead,
the ejected polymeric jet travels a short distance following
a straight path and then bending instability is developed in the jet
due to the mutually repulsive forces resulting from the electric
charges of the jet. Fig. 6 shows these three stages schematically and
graphically for electrospinning of 15% wt PAN solution.
Fig. 6. (a) Schematic illustration of jet path in electrospinning, and (b) Taylor cone, straight section and spiral path in electrospinning of 15% wt PAN solution.
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F. Dabirian et al. / Journal of Electrostatics 69 (2011) 540e546
Fig. 7. Electrocentrifugal spinning of 15% wt PAN in DMF at rotational speed of
6630 rpm.
3.2.2. Polymeric jet in electrocentrifugal spinning
In this method the polymeric jet is formed only by the
centrifugal force and undergoes elongation by the simultaneous
action of centrifugal and electrostatic forces until it lies down on
collector. Fig. 7 shows an image of the process using 15% wt PAN
solution at the rotational speed of 6360 rpm. The ejected jet from
the nozzle encounters bending instabilities after a short distance
traveling in a curved path. As it can be seen in this image, due to
the high flow rate, the Taylor cone has not appeared and the
polymeric fluid conforms to a jet as a consequence of the applied
centrifugal force.
In comparison with electrospinning, bending instabilities are
observed with less frequencies and amplitude in electrocentrifugal
spinning. This can be explained in this way: by increasing the
rotational speed, the relative velocity between the jet and the
surrounding air, which tends to dry up the solution, is increased.
As a result, the growth of instability wave is suppressed due to the
evaporation of solvent. On the other hand, by increasing the
polymer concentration, the polymeric bending instability of the jet
is reduced and the jet travels in a straight path toward the
collector. This provides the possibility of collecting highly aligned
nanofibers on the outer cylinder. Fig. 8 shows images of electrocentrifugal spinning for 16% wt PAN at speed of 6360 rpm where
the bending instabilities have considerably vanished and as
a result, the polymeric jet takes on a completely straight and
aligned form.
This method can be employed to produce highly aligned
nanofibers from solutions with high concentration. Fig. 9 shows
a scanning electron microscopy of nanofibers produced by electrocentrifugal spinning of 15% and 16% wt PAN in DMF at rotational
speed of 6360 rpm and applied voltage of 15 kV.
The morphology of nanofibers was studied and explained in
previous work [46]. In brief, as shown in Fig. 10 the results indicated
that the production of nanofibers with drop sprinkling was possible,
and at low rotational speed and concentrations, the nanofibers
contain drops and beads in centrifuge spinning. When using electrocentrifuge spinning, the nanofiber diameters have been changed
compared to centrifuge spinning. Results showed drastic reduction
in fiber diameter which is the effect of adding electrical force to
centrifuge forces.
3.2.3. Comparison of mechanical properties
The mechanical properties of aligned nanofiber bundles were
measured by Zwick 1446e60. Zwick was set for constant rate of
elongation. To obtain load elongation curves, the sample length was
Fig. 8. Electrocentrifugal spinning of 16% wt PAN in DMF at rotational speed of 6360 rpm where the bending instabilities are considerably reduced.
Fig. 9. Scanning electron microscopy of nanofibers produced by electrocentrifugal spinning of (a) 15% and (b) 16% wt PAN in DMF at rotational speed of 6360 rpm and applied
voltage of 15 kV.
F. Dabirian et al. / Journal of Electrostatics 69 (2011) 540e546
545
Fig. 10. Typical SEM images of nanofibers for a polymer solution of 15% wt and a rotational speed of 6615 rpm; (a) Nanofibers with drop and bead produced by centrifuge spinning
[48], (b) Un-uniformity in nanofiber diameter in centrifuge spinning [48], (c) Nanofibers without drop or bead produced by electrocentrifuge spinning at voltage of 10 kV, and
(d) Uniformity in nanofiber diameter.
10 cm with cross head speed of 60 mm/min. All samples were
produced at room temperature (25 C) and dried at approximately
70 C in an oven for 2 h. Before the experiment, samples were in
standard conditions (20 2 C and 65% RH) for 24 h. The results of
measurement are listed in Table 1.
The strength of electrospun PAN aligned nanofiber bundle was
53 MPa and its modulus was 1.99 GPa whereas these values were
112 MPa and 2.33 GPa for the electrocentrifugal spun PAN aligned
nanofiber, respectively. The results show an increase in the strength
and modulus of the aligned nanofiber bundle produced by electrocentrifugal spinning and a decrease in the strain at the brake point.
However, no significant change occurs in work up to break parameter. The enhancements of mechanical properties can be accounted
for by the considerable mechanical contact drag forces between the
jet and the surrounding air which lead to a better alignment of
fibers. Work up to break is a quantity which considers both strength
and strain simultaneously. An increase in work up to break is
expected due to increase of strength, but it should be noted that due
to the strain reduction, the variation of work up to break is not
considerable.
Table 1
The results of mechanical properties measurement of aligned nanofiber bundles by
Zwick.
Parameters
Electrospinninga
Electrocentrifugalb
Number of samples
Nanofiber diameter (nm)
Stress at F_max (MPa)
Strain at break (%)
Work up to break (N mm/tex)
E-modulus (GPa)
30
410
53
75.9
3.16
1.99
30
(9.5)
(17.1)
(17.3)
(24.2)
(19.8)
440
112.42
60
3.06
2.33
(11.3)
(8.3)
(16.3)
(22.2)
(24.6)
a
16% wt PAN in DMF, 15 kV, 0.8 ml/h, 12 cm spinning distance, 2.5 cm gap
distance on collector.
b
16% wt PAN in DMF, 6360 rpm, 15 kV, distances explained in Section 2.2.
4. Conclusion
The procedures of water jet formation and polymeric jet
formation in three kinds of spinning systems involving electrospinning, centrifuge spinning and electrocentrifugal spinning were
considered in this study.
Electrocentrifugal spinning is a powerful technique for spinning solution with extremely low viscosity. In addition, electrocentrifugal spinning is a more suited technique for production of
aligned nanofibers than electrospinning, since the bending instability of jet could be reduced via centrifugal force through the effect
of surrounding air contacts.
This can partially improve the mechanical properties of
produced nanofiber.
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