The Surface Energy of "Tholin" and its Implication on Haze-Liquids Interactions on Titan

Photochemically produced organics are ubiquitous on Titan. They form the haze particles in Titan's atmospheres, and are believed to make up portions of the surface material. To better understand how the solid haze particles interact with the potential hydrocarbon liquids on Titan, characterization of the surface energy of the haze is needed. We used the sessile drop contact angle method [1] to measure the surface energy of the Titan aerosol analogs, "tholin". The surface energy of tholin is measured to be around 65-70 mN/m, indicating tholin's polar-dominated structures. We also calculated the contact angle between tholin and possible hydrocarbon liquids on Titan, and found out that most liquids on Titan would completely wet the tholin surface (contact angle<10°). Tholin is not soluble in most non-polar hydrocarbon liquids [e.g., 2], but insoluble particles could still serve as CCN if the solid-liquid contact angle is less than 10° [e.g., 3], which is the case between tholin and the hydrocarbon liquids on Titan. The low contact angle between liquids on Titan and tholin could also support the possibility of a film of organics floating on Titan's lakes as previously proposed [4].

[1] Yu, X., et al. (2017). Direct measurement of interparticle forces of Titan aerosol analogs ("tholin") using atomic force microscopy. JGR: Planets, 122(12), 2610-2622.

[2] He, C., & Smith, M. A. (2014). Solubility and stability investigation of Titan aerosol analogs: New insight from NMR analysis. Icarus, 232, 54-59.

[3] Mahata, P. C., & Alofs, D. J. (1975). Insoluble condensation nuclei: The effect of contact angle, surface roughness and adsorption. Journal of the Atmospheric Sciences, 32(1), 116-122.

[4] Cordier, D., & Carrasco, N. (2019). The floatability of aerosols and wave damping on Titan's seas. Nature Geoscience, 12(5), 315.