Collisional properties of ice spheres at low impact velocities

In this paper we discuss the results of our experimental studies on the impact properties of water ice. The measurements were made using a new apparatus consisting of a compound disc pendulum and a stainless steel, temperature controlled cryostat. With this apparatus we have been able to achieve stable temperatures of 85 K and pressures as low as 10−5torr. Using a capacitive displacement device for accurately measuring the position of the pendulum during each collision, we have been able to obtain accurate measurements of the coefficient of restitution for ice spheres impacting with velocities in the range 0.015–2 cm s−1. The coefficient of restitution as a function of velocity, ε(υ), for ice spheres with four different radii of curvature and with a variety of surface conditions has been obtained. The coefficient of restitution data can be well fitted by an exponential law of the form ε(υ)= C exp (−γυ), for most measurements. We find, however, that the surface conditions can drastically alter the resulting value of ε. In particular the presence of frost or a roughened contact surface can lower ε at a given velocity by 10–30 per cent from that of a smooth sphere. We also show how the presence of frost can change the velocity behaviour of ε from an exponential to a power-law form. We briefly discuss the applications of our results to the dynamics of Saturn’s rings.