  The photochemical haze produced in the upper atmosphere of Titan plays a key
role in various atmospheric and surface processes on Titan. The surface energy,
one important physical properties of the haze, is crucial for understanding the
growth of the haze particles and can be used to predict their wetting behavior
with solid and liquid species on Titan. We produced Titan analog haze
materials, so-called "tholin", with different energy sources and measured their
surface energies through contact angle and direct force measurements. From the
contact angle measurement, we found that the tholins produced by cold plasma
and UV irradiation have total surface energy around 60-70 mJ/m2. The direct
force measurement yields a total surface energy of ~66 mJ/m2 for plasma tholin.
The surface energy of tholin is relatively high compared to common polymers,
indicating its high cohesiveness. Therefore, the Titan haze particles would
likely coagulate easily to form bigger particles, while the haze-derived
surface sand particles would need higher wind speed to be mobilized because of
the high interparticle cohesion. The high surface energy of tholins also makes
them easily wettable by Titan's atmospheric hydrocarbon condensates and surface
liquids. Thus, the hazes particles are likely good cloud condensation nuclei
(CCN) for hydrocarbon clouds (methane and ethane) to nucleate and grow. And if
the hazes particles are denser compared to the lake liquids, they would likely
sink into the lakes instead of forming a floating film to dampen the lake
surface waves.
