Biomedical Engineering Reference
In-Depth Information
Taylor's cone
Polymer jet
(a)
(b)
FIGURE 4.84
Photographs of viscous polymer solution suspended at a capillary tip with (a)
no applied potential and (b) just above the critical voltage.
curiosity because it was overshadowed by the important technological development
of synthetic textile fibers (diameter > 5
m). In electrospinning of polymers, the
polymer solution (melts or solution) is fed through a tube (glass) with a capillary
opening. By applying an electric field between the capillary and the counterelectrode
(collector), a pendant drop at the tip of the capillary is shaped into a conical
protrusion often called Taylor cone. At the critical voltage (~0.5 kV/cm), the elec-
trostatic force exerting on this cone overcomes the existing surface tension of the
drop, ejecting the jet form of the cone toward the counterelectrode. When the electric
field reaches a critical value, the charge overcomes the surface tension of the
deformed drop, and a jet is produced. In general, the long chain molecules are
oriented and entangled in the jet as the fiber solidifies. The electrically charged jet
undergoes a series of electrically induced bending instabilities during its passage to
the collection screen that results in hyperstretching of the jet stream.
Important process information and physical properties are
µ
polymer density (
)
polymer viscosity (
ρ
)
polymer surface tension (
η
)
polymer electric conductivity (
σ
σ
e
)
polymer dielectric permittivity (
ε
=
ε
r
ε
o
)
capillary radius (
R
)
applied electric voltage (
E
)
capillary-to-target distance (
H
)
current density (current/area) (
I
)
volumetric flow rate (
Q
)
According to Senador and coworkers (2001), a general relationship between the
critical voltage and other variables during jet formation can be written as
3
ε
σ
η
ρσ
σ
ε
ρ
σ
r
h
r
e
e
(4.4)
E
F
=
,
,
r
r
e
where
F
[ ] is an undetermined function relating to the dimensionless groups. Also,
the known expressions for the critical voltage are
