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not listed, because it is dependent on the construction, and is therefore not meaningful
for comparison. Both the pressure and the duration of the pulse were set to
be slightly above the sufficient level, as suggested by the test results above.
T-Conjunction Exit tube Pinhole Drop diameter
Big 75 mm 500 µm 0.80 mm
Big 75 mm 800 µm 1.15 mm
Small 45 mm 500 µm 1.67 mm
Small 60 mm 800 µm 1.36 mm
Table 3.2.: Achieved drop diameter by the pneumatic drop generator of different
constructions.
The T-conjunction influences the pressure peak and the damping frequency on account
of difference sizes of the cross section, according to the Helmholtz resonator.
Smaller cross section led to a higher level and a longer duration of the first pressure
peak, which consequently squeezed more liquid out of the pinhole, forming bigger
drops. It was observed that the small T-conjunction was accompanied with larger
drops than the big T-conjunction.
Bigger pinholes tended to produce bigger droplets, as the results with the big T-
conjunction indicate. The results acquired with the small T-conjunction exhibit the
opposite direction, because the 500 µm pinhole required a longer pulse than the
800 µm pinhole, therefore more liquid was squeezed out, forming a larger drop. It
is expectable that if the pulse widths are equal and the pressure for the 500 µm is
increased, the drop diameter should be smaller.
Drops of φ1.5 mm was the most favorite compromise for both cooling and the observation.
Therefore the construction with small T-conjunction and 800 µm pinhole
was chosen for the application in the experiment.
3.3 Observation Plans
Three type of imaging were planned for the observation of the supercooled drop
impact, as Figure 3.20 depicts. The first was shadowgraph imaging, in order to
observe the dynamic motion of the drop during impact. The second was of similar
construction but with the infrared camera, in order to measure temperature of the
drop both before impact and during rebounding on hydrophobic surfaces. This
plan is named as the side-view infrared imaging. The third was to measure the
contact temperature between the substrate and the residual lamella. It is named as
the bottom-view infrared imaging.
68 3. Experimental Setup