Biomedical Engineering Reference
In-Depth Information
Cone-jet mode
Ramified-jet mode
Dripping mode
Microdripping
mode
Multijet mode
Spindle-jet mode
FIGURE 11.4 Electrospraying patterns for different modes. (Reprinted from Cloupeau, M. and Prunetfoch,
B., J. Electrostat. , 25, 165, 1990. © Elsevier Science. With permission.)
properties of the solution, such as viscosity, concentration, electrical conductivity, electrical permit-
tivity, and surface tension. These atomization modes include microdripping, spindle, cone-jet and
multijet, dripping, and ramifi ed-jet modes, which are determined by the physical properties of the
suspension, the strength of the electric fi eld, and the fl ow rate of the liquid. Figure 11.4 shows different
modes of the electrospraying pattern. In the cone-jet mode, a jet forms at the tip of the capillary and
later propagates into spraying. The droplet size distribution produced in the cone-jet mode depends
on the diameter and the breakup of the jet. The applications of charged droplets with cone-jet spray-
ing pattern can be established to control the droplet size and particle transport by using a designed
and controlled external electric fi eld. For every solution with different physical properties, there is a
minimum fl ow rate at which the jet breaks up because of the axisymmetric instabilities [40].
When the fl ow rate is below a certain minimum value, a stable cone-jet mode cannot be formed.
However, at higher fl ow rates, the current through the cone-jet liquid will increase, resulting in an
increased surface charge on the jet. Moreover, the jet breakup will also be infl uenced by lateral or
azimuthal instabilities of the jet above a certain surface charge. When the infl uence of these insta-
bilities increases, the size distribution of the electrosprayed droplets becomes wide. Electrospraying
in a cone-jet mode can essentially be described in three different processes [27]. The fi rst process
is the acceleration of the liquid in the cone jet, which is a result of the balance of liquid pressure,
liquid surface tension, gravity, electric stresses in the liquid surface, the liquid inertia, and the liquid
viscosity. Figure 11.5 shows the various forces in electrospraying for a cone-jet mode. The second
process is the breakup of the jet into droplets. The third process is the development of the electro-
spray after the droplet formation in the cone-jet mode.
11.2.4 P ROCESSING P ARAMETERS
In terms of the application of electrospraying, a main objective of the research is to obtain the scal-
ing laws for the electrosprayed droplet charge and size as functions of the processing parameters,
which can guide the electrospraying process and the transformation of solutions into electrosprayed
 
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