Biomedical Engineering Reference
In-Depth Information
on the droplet surface will overcome the surface tension and break up into many charged jets having
a controllable size within a narrow distribution [11,12]. This kind of electrohydrodynamic process
is called electrospraying.
The atomization modes of electrohydrodynamic processing can be divided into two groups
[13]. The fi rst group includes modes in which only fragments of liquid are ejected from the nozzle.
This group comprises the dripping mode, microdripping mode, spindle mode, multispindle mode,
and ramifi ed-meniscus mode. The second group includes modes in which the liquid ejects from the
nozzle in the form of a long continuous jet, which disintegrates into droplets only at some distance
from the tip of the nozzle. This group contains cone-jet mode, oscillating-jet mode, procession
mode, multijet mode, and ramifi ed-jet mode. The modes of spraying are classifi ed and characterized
in terms of the jet and droplet formations. These atomization modes are complicated functions of
the applied voltage, the fl ow rate of the liquid, the nozzle diameter, and the physical properties of
the precursors. As the applied fi eld strength increases, the atomization modes can be transformed
through a cone jet from a dripping mode to a multijet mode. The single cone-jet mode is the pre-
ferred spraying mode because it can offer a uniform, stable, and continuous spraying pattern with
a controllable jet size. An outstanding feature of electrospraying is its ability to generate monodis-
persed droplets, the size of which may vary from hundreds of micrometers to tens of nanometers
through optimizing the processing parameters.
11.2 ELECTROSPR AYING
11.2.1 D
EFINITION
Electrospraying is a process of generating very fi ne aerosol droplets through electrostatic charging
instead of mechanical forcing. During electrospraying, a liquid solution is pushed through a small
capillary or needle that is connected to the positive electrode of a power supply. The power supply
electrically charges the liquid to a high voltage and aggregates more and more charge on the drop-
lets. When this charge reaches a critical point, the droplets cannot hold any more electrical charge
and therefore become unstable. Eventually, the charged droplets are emitted out of the tip of the
capillary or needle and, due to the repelling nature of the charge, form a fl ume of fi ne aerosol drop-
lets. These fi ne droplets are less than 1 µm in diameter and are attracted to an oppositely charged
component. When the charged liquid droplets are attracted to the oppositely charged electrode, they
shrink rapidly as the solvent evaporates. Therefore, the charge density on the droplet increases to a
critical value at which the droplets break up into the tiny aerosol droplets because of the repulsive
Coulombic forces. This process can be repeated in the same way until the charge density on the
tiny droplets is no longer strong enough to break up the droplets. Figure 11.1 demonstrates the basic
principle of electrospraying.
Electrospraying in cone-jet mode is a process in which an electric stress resulting from the sur-
face charges on the liquid droplet transforms the droplet into a conical shape. This transformation
occurs when a liquid solution is pumped through a needle at an appropriate fl ow rate [14]. The liquid
droplets form a conical shape because a cylindrical shape can hold more charges than a sphere.
Cone jet
+
+
+
+
+
+
Charged liquid
Nozzle
Electrosprayed
droplets
FIGURE 11.1
An illustration of the basic principle of electrospraying.
