COAGULATION OF PARTICLES IN SATURNS RINGS - MEASUREMENTS OF THE COHESIVE FORCE OF WATER FROST

The cohesive properties of water ice particles may play an important role in the dynamics of ring particles as well as in the evolution of the particle size distribution. In this paper we present preliminary experimental data on the sticking force of water ice particles. The data indicate that the sticking force between smooth, frost-free ice particles is less than a dyne; however, the presence of a layer of water frost 10-100 μm thick on the ice surfaces increases this force up to 100 dyn. The resulting sticking force is dependent on the impact velocity of the particle and is maximized for some intermediate (∼0.1 cm sec-1) velocity. Consequently, a "Velcro" model is presented to describe the surface structure responsible for the sticking. The data indicate that there is a critical impact velocity of order 0.03 cm sec-1 below which cohesion of particles always occurs. After sticking, the ice particle undergoes a damped harmonic motion characteristic of a linear force law having a range of order 10 μm. A spring constant of k ≈ 104 dyn/cm is derived. We show that given the optical depth of micrometer-sized grains in Saturn's rings, particles are most likely coated with a significant layer of frost such that the cohesion of particles may be an important process in ring dynamics. Finally using the largest measured sticking forces we estimate the largest aggregate of ice particles capable of surviving tidal disruption to be about 10 m. © 1991.