Direct Measurement of Interparticle Forces of Titan Aerosol Analogs ("Tholin") Using Atomic Force Microscopy

To understand the origin of the dunes on Titan, it is important to investigate the material properties of Titan's organic sand particles on Titan. The organic sand may behave distinctively compared to the quartz/basaltic sand on terrestrial planets (Earth, Venus, Mars) due to differences in interparticle forces. We measured the surface energy (through contact angle measurements) and elastic modulus (through Atomic Force Microscopy, AFM) of the Titan aerosol analog (tholin). We find the surface energy of a tholin thin film is about 70.9 mN/m and its elastic modulus is about 3.0 GPa (similar to hard polymers like PMMA and polystyrene). For two 20 $\mu$m diameter particles, the theoretical cohesion force is therefore 3.3 $\mu$N. We directly measured interparticle forces for relevant materials: tholin particles are 0.8$\pm$0.6 $\mu$N, while the interparticle cohesion between walnut shell particles (a typical model materials for the Titan Wind Tunnel, TWT) is only 0.4$\pm$0.1 $\mu$N. The interparticle cohesion forces are much larger for tholins and presumably Titan sand particles than materials used in the TWT. This suggests we should increase the interparticle force in both analog experiments (TWT) and threshold models to correctly translate the results to real Titan conditions. The strong cohesion of tholins may also inform us how the small aerosol particles ($\sim$1 $\mu$m) in Titan's atmosphere are transformed into large sand particles ($\sim$200 $\mu$m). It may also support the cohesive sand formation mechanism suggested by Rubin and Hesp (2009), where only unidirectional wind is needed to form linear dunes on Titan.