The present work investigates the mechanics of particle collisions submerged in a liquid. A simple pendulum experiment provided carefully controlled impacts in which measurements are made of the the position and the velocity changes of a set of different impacting particles immersed under a variety of fluids. The velocity of the approaching particle is measured using multi-exposure photographic techniques; the magnitude of the collision is quantified using a high frequency response pressure transducer at the colliding surface. Certain comparisons are drawn against predictions from the Hertzian theory of contact.
A simple control-volume model is proposed to account for the effects of fluid inertia and viscosity. When a particle approaches a planar surface or another particle, there is a fluid film that has to be ``squeezed'' out for contact to take place. By doing so a certain amount of the initial kinetic energy is dissipated or transferred to the fluid, resulting in the slow down of the particle. A model is formulated based on the momentum and mass fluxes that occur in this gap. The pressure profile can be estimated, which is then integrated over the surface of the particle to obtain a force. This force is the one responsible for the deceleration observed prior to impact. It is, in general, a function of the initial particle Reynolds number, $Re_o$, and the ratio of the densities of the particle and fluid phases, $\rho_p/\rho_f$. A comparison of the proposed model with the experimental measurements is presented.