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The real situation in the application is not in this case. Figure 5.14 shows that the
effective E field around the tip of the protrusion is 10 times higher than the average
value.
Further development of the model was of little importance, because the major
difficulty was geometrical. On the other hand, it can be inferred that large surface
deformation around the jet tip might conditionally play a notable role on the charge
quantity, because the local E field determines the charge quantity.
Figure 5.15 pictures the electrical field between the electrodes. The measured
charge quantity by Eq. 5.8 should be higher than the real value because of the
undesired electrical field between the charging ring and the electrodes. This error
should be nevertheless insignificant because of the strong concentration of the E
field between the electrodes. The CST field was calculated with the guidance of Dr.
W. Müller from TEMF (Institut Theorie Elektromagnetischer Felder, TU Darmstadt).
For the purpose of practical application, the diameter dependence of the charge
quantity was tested as Figure 5.16 shows. The quantity of charge was nearly linearly
proportional to the drop diameter.
5.3.2 Selective Deflection with Pulse Sequence
A sequence of voltage pulses was applied to the charging ring in order to selectively
deflect a portion of the drops while keep the rest drops uncharged and
fall straight into the observation region. Synchronization of the charging signal
with the excitation signal of the drop generation, as Figure 5.17 represents, was
compulsory for a periodic deflection.
The excitation signal served as a clock for the charging signal. The charging
signal had an idle state of “high” in order to deflect all the droplets by default.
The falling edge of the charging signal was triggered by 1 out of N pulses of the
excitation signal. The duration of the low voltage, specified as “low time”, was at
the maximum one period of the excitation signal. Longer “low time” led to charge
of multiple drops, while shorter in general led to less quantity of charge, as Figure
5.18 shows. The duration of the high voltage, specified as “high time”, is the rest
of the N periods of the excitation signal.
Essentially the charging signal was to be synchronized with the drop generation,
instead of the excitation pulses. Each pulse of the excitation signal corresponded
to one drop breakup from the liquid jet, but it was unknown at which phase of the
excitation signal the drop pinched off from the liquid jet. To examine the relation
between the jet breakup and the excitation signal, the charging signal was shifted
with a phase offset relative to the excitation signal as shown in Figure 5.19. The
144 5. Experimental Setup