Vibration-induced particle drift in a fluid cell under microgravity

Abstract

A theoretical investigation into the effect of small vibrations on the behaviour of a small particle contained in a fluid cell under microgravity is presented. Diffusion-controlled material processing such as protein crystal growth can be adversely affected by small vibrations called g-jitter, if a relative motion is induced between the particle and surrounding fluid. When a fluid cell containing a small particle such as a protein crystal is vibrated parallel to the wall nearest to the particle, the particle oscillates with a certain amplitude and a hydrodynamic force in the direction normal to the wall is induced. Theoretical models based on an inviscid fluid assumption are used to predict the particle amplitude variation and drifting motion. Due to an external vibration such as g-jitter, the oscillating particle is predicted to drift towards the wall and the particle oscillation amplitude to decrease slightly as the distance between the particle and wall is reduced. The reduction in particle ampitude also depends on the particle-to-fluid density ratio. The particle drift towards the nearest wall acclerates due to an increasing attraction force, and the drifting speed increases with both the vibration frequency and particle diameter. Even for small protein crystals with a density close to that of the fluid, the time required to drift from the center of the fluid cell to the wall is predicted to be much shorter than the growth time.