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Figure 5.17.: Synchronization of the charging signal with the excitation signal.
Figure 5.18.: Influence of the “low time” on the amplitude of the deflection. The
“low time” were respectively 30% (left) and 80% (right) of the period
of the excitation signal. The drops were φ200 µm in diameter.
out by trial and error for each application. More comprehensive information on the
selective deflection can be found in the report of C. Guichet for this IREP summer
exchange program [44].
By keeping the charging voltage high for 10 s to 20 s, and thus deflecting all the
drops, the deposited water on the impact surface could be removed by centrifugal
force and aerodynamic shear stress effectively. The time for drying was tested out
by trial and error for each rotational frequency.
The deflected drops were forbidden to enter the cabin of the rotating target, because
the wind made them in a cloud form which continuously wetted the impact
surface. Therefore a piece of napkin paper was laid on the shield aluminum plate
as in Figure 5.21. The napkin paper served as a capillary pump, which drained the
water continuously away.
The drop deflection was imperfectly under control on account of the following
factors. The first was the undesired charge of the drop train. Frequently and
irregularly, the “uncharged” drop stream was deflected by the electrode. In fact
drop charging is a phenomenon that can not be safely avoided, as commented by
Lee, who proposed 5 different charging mechanisms that could happen in drop
and spray generation [75]. It was unknown which causes were responsible for
the awkward charging in our experiment, but evidently it was helpful to keep the
device dry. The second was the wind induced by the rotating target. The wind
5.3. Electrostatic Deflection of Charged Drops 147