Surface energy of the Titan aerosol analog ’tholin’: implications on cloud formation and aerosol-lake interactions

The photochemical haze produced in the atmosphere of Titan plays a key role in various processes happening on Titan. The surface energy, one of the physical properties of the haze, is crucial for understanding the growth of the solid haze particles and can be used to predict their wetting behaviors with solid and liquid species on Titan. However, the surface energy of Titan’s haze is not well understood. 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 (Yu X. et al., in revision). From the contact angle measurement, we found that the tholins produced by cold plasma and UV irradiation have similar total surface energy. The commonly used two-liquid component method yields a total surface energy of tholin of ∼60-70 mJ/m. 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 (20-50 mJ/m2), indicating its high cohesiveness. Therefore, the Titan haze particles would likely coagulate easily to form bigger particles, while the haze-derived surface sand particles could need higher wind speed 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 most hydrocarbon and nitriles condensates to nucleate and grow into clouds (Yu Y. et al., in prep), which agrees well with Cassini observations of ice clouds (Anderson et al., 2018). Meanwhile, because of the low contact angle predicted between tholins and Titan’s lake end member species, 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 damp the lake surface waves.

1. Yu, X., Hörst, S. M., He, C., et al. in revision.

2. Yu, Y., Garver, J., Yu, X., et al. in prep.

3. Anderson, C. M., Samuelson, R. E., & Nna-Mvondo, D. 2018, SSRv, 214, 125.