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Gao D, et al.
pulsating jet deposition can achieve higher sensitivity and downstream . Melcher and Taylor also pointed out that
[18]
generate the customized volume of droplets by changing for an interface to be subjected to shear stress, the surface
pulse duration. In general, pulsation frequency depends must simultaneously support the surface charge and be
on flow rate, properties of liquid and nozzle diameter. subjected to a tangential electric field [53,54] . In addition,
Several models were proposed to express the relationship Melcher et al. mentioned two limiting cases and the first
between frequency and parameters, and these models will one is the perfectly conducting interface . In this case,
[53]
be discussed in section 3.2.3. electrical surface forces always act perpendicularly to the
When the applied potential difference is further surface and thus interface supports no tangential electric
increased, continuous emission of liquid through cone stress . The second limiting case is a perfectly insulated
[53]
jet is developed. This mode is known as axial mode surface with no free charge density while the polarization
III . In addition, a concavely shape cone observed in force density is operative at the interface. Therefore,
[20]
the two pulsation mode is changed to a nearly straight the electric surface force density acts in the direction
cone in the continuous spray mode . Marginean et al. of permittivity gradient (−∇ε), which is perpendicular
[20]
revealed that a complex transition into an astable regime to an interface . Thus, the liquid must not be perfectly
[53]
may exist between pulsating and continuous cone-jet conducting or insulating , and for a leaky dielectric
[18]
regime . In Figure 2A a complex jetting behavior, like liquid, the tangential component of the electric field can
[19]
tilted jets, is shown at higher electric field strength, but develop a shear force along the conical surface.
these phenomena are not considered to be controllable When a steady flow rate is issuing from the cone’s tip
in a precision deposition. When the applied voltage is in the form of a microjet, an electric field toward liquid
slightly lower than the voltage required to obtain a single surface must appear in the cone to supply the electric
permanent jet, the jet may be emitted only intermittently, current, which is emitted by the microjet in the form of
which means the apex of meniscus alternately takes on a charged droplets . The tangential electrical stress pulls
[55]
pointed or rounded form, and emission phases may occur the charged surface toward the tip, and tangential electrical
at perfectly regular time intervals . This intermittent or stress provokes sufficient axial momentum to transfer
[18]
pulsed cone-jet is similar to the axial mode I and II. The onto the liquid to begin a progressive deformation of the
diameter of the jet varies during these emission phases, cone into a jet . Hayati et al. showed an axisymmetric
[55]
and thus the distribution of droplet size is never narrow . circulation pattern where the particles travel downward
[18]
These studies suggest that pulsating and continuous along the surface of the cone and reverse upward along
cone-jet can be obtained by adjusting the voltage and its axis in the conical base . The electric field drives
[54]
flow rate. However, there are noteworthy exceptions, free charges to the surface of the cone; hence, the surface
and polycaprolactone is one of them. A stream of is charged and is subjected to a normal electric field E .
n
pulsating droplets has not been observed since it is hard A potential difference will exist between the base of the
to accumulate enough charges to overcome the surface cone at the end of the capillary and the tip of the cone
tension and high viscous force . For paraffin wax, due to the semiconducting nature of the liquid . This
[54]
[49]
pulsating cone-jet was only formed , and this result may potential difference ensures that the interface is subjected
[50]
attribute to its low viscosity and electrical conductivity. to a tangential electric field E in the direction of flow .
[54]
In one word, generation of pulsating or continuous cone- Barrero et al. observed a similar recirculating meridional
jet is controlled not only by changes of voltage and flow motion and an additional vigorous swirl coupled to the
rate but also material properties, such as surface tension, meridional motion, and they attributed the appearance of
electrical conductivity, and viscosity. different motion to a distinct value of liquid conductivity
Zeleny first investigated the continuous production of and viscosity . The meridional motion can be considered
[56]
drops by the breakup of a permanent jet extending from as a proof of the existence of tangential electrical stress
a meniscus in the conical form [13,51] , and then Vonnegut on the cone surface. De la Mora considered the domain
and Neubauer acquired the same mode during their of cone as hydrostatic since recirculating flow is slow and
experiments on liquids . Cloupeau and Prunet used the have little influence on jet formation .
[52]
[45]
compound word “cone-jet” to describe the phenomenon Even though the phenomenon of cone-jet transition
that a permanent jet extending from an axisymmetric has been known for over 200 years, the actual process
conical volume of liquid and jet is stretched along the is not understood completely. It is complex to analyze
capillary axis . The creation of a jet requires penetration theoretical multi-physical free surface flow by
[18]
of the field lines into the liquid with a dielectric liquid, mathematical methods, and there are also limitations in
and only penetration will allow the appearance of a simplifying assumptions in models . In addition, based
[5]
component of the electric field tangent to the surface; on the complexity of EHD phenomenon controlled
this acts on the surface charges and creates a force by multi-parameters, it is not always possible to know
that drives the liquid and an acceleration of the jet beforehand the mode obtained when the values of main

International Journal of Bioprinting (2019)–Volume 5, Issue 1 5