Surface Energy of the Titan Aerosol Analog "Tholin"

TL;DR

This study measures the surface energy of Titan aerosol analogs ('tholin') produced by different methods, revealing their high cohesiveness and wetting properties, which impact particle growth, cloud formation, and surface interactions on Titan.

Contribution

It provides the first comprehensive measurement of surface energy for Titan aerosol analogs, linking physical properties to atmospheric and surface processes on Titan.

Findings

Tholin surface energy is around 60-70 mJ/m2.

High surface energy indicates high cohesiveness of haze particles.

Haze particles are likely good cloud condensation nuclei.

Abstract

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.