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3 Experimental Setup
This chapter introduces the experimental setup and discusses the uncertainties
in the temperature measurement. Section 3.1 gives a detailed description about
the supercooling method, including calculation of the drop temperature and the
construction. Section 3.2 introduces the pneumatic drop generator. Section 3.3
describes the impact surface and the three imaging plans. The measurement of
the static contact angle of different surfaces is also in this section. Section 3.4 is
devoted to the infrared imaging, including the calibration of the camera, analysis
on uncertainties, and a demonstration of measurement of the drop temperature.
Section 3.5 briefly describes the synchronization and operation of the experimental
setup.
3.1 Supercooling Method
Supercooling is a metastable state and tiny thermal perturbations are able to
break this circumstance. Water is especially easy to nucleate. Various methods
are reported to create supercooled water in experiments. These methods can be
divided into two categories. The first is suspending small amount of water sample
in a cold atmosphere and cooling it by free convection. The water sample can be
suspended completely in the air for instance by acoustic levitation [12], or can be
hung at the tip of a thermal couple [4], a needle [64] or a thin wire loop [135–137].
The second category is cooling a small sample of water in an insulated container.
The container can be out of in a pyrex or fused silica [46], PMMA [90] or coated by
Teflon [172]. The cooling method in this case is arbitrary, such as free convection
[46], fluid refrigerant [90] or peltier element [172]. All these methods share two
aspects in common: the water sample is of tiny amount, and the cooling heat flux
is very low [52]. Large temperature gradient and high heat flux favors the thermal
perturbation, therefore detrimental for the existence of supercooling.
The vulnerability of the supercooled water forbids its application in the most
drop generators because they usually function with deformation of the liquid. Supercooled
water drop is conventionally created in the icing wind tunnel for aircraft
icing test. Tiny water drops (below 50 µm for conventional icing condition and
below 500 µm for SLD condition) are fed into cold airflow at icing temperatures
between 0 ◦ C and −20 ◦ C. After a long traveling distance (several meters at least),
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