A Study on Vibration-induced Particle Motion under Microgravity

Production of protein and semi-conductor crystals with advanced quality and properties is considered to be possible under microgravity conditions due to the absence of natural convection effects. Such materials have several beneficial properties that can improve the human life. An example is the synthesis of protein crystals with improved structure that can be determined for the production of advanced drugs. In the past experiments conducted aboard several space platforms, however, g-jitter induced convective flow may have resulted in certain effects that reduced the quality of the produced crystals. To investigate the effects of g-jitter on the motion of small particles, experiments were conducted under normal gravity by suspending spherical stainless steel particles of different sizes with a thin wire or synthetic silk thread in a rectangular fluid cell. The fluid viscosities were 350 and 1,000 times higher than water. To produce the g-jitter induced motion in the fluid, the cell was subjected to horizontal sinusoidal vibrations with different frequencies and amplitudes. The focus of the experiments so far has been on vibration-induced force on the particle vibrating parallel to a near wall. Relatively low viscosity fluids such as water have been previously determined to produce a force on the particle which attracts the particle to the nearest fluid cell wall. The present experiments with a more viscous fluid have revealed an interesting change in the force from attraction in low viscosity fluids to repulsion in high viscosity liquids. Moreover, the repulsion force has been observed to increase with an increase in the fluid viscosity and the fluid cell amplitude. A numerical code, Partflow3d, has also been used to predict the vibration effects on the particle. Although, based on the objectives of this study, the numerical simulations were conducted only for a wire-free particle under microgravity, their results were qualitatively in agreement with the experimental results. The numerical simulations also revealed that the physical mechanism of the hydrodynamic attraction-repulsion force on the particle is related to Bernoulli’s principle of reduced pressure in high velocity zones in the fluid surrounding the particle. The results so far have shown new aspects of the g-jitter induced motion of the particle near a fluid cell wall. Better understanding of the forces affecting the particles in a fluid cell subjected to small vibrations, can reveal novel ways to produce new advanced materials and also improve material processing both in microgravity and normal gravity conditions.Ph