Experimental Study Of Clustering Behavior In Granular Gases

Granular system contains a great number of solid particles with diameter larger than one micrometer. Due to inelastic collisions between particles as well as frictions among particles and with the walls, granular system is dissipative in nature. Many interesting phenomena and complex dynamic questions exist in this special system that can behave like ordinary solids, liquids and gases under external forces. If particles in the system are sparsely distributed and only consider two-body collision between particles, this system is regarded as “granular gas”.Owing to the dissipative property of granular gas system, the kinetic energy of particles in the system is constantly consumed, and enough energy input by external forces is necessary to balance the energy loss. Local density increase will appear in initially homogeneous granular gas because of the fluctuation, and more inelastic collisions will occur in this higher local density part, so energy loss becomes more serious, which will lead to more particles getting together, so granular clustering emerges in the system; this clustering phenomenon in granular gases has been analyzed using phase-separation modeling of van der Waals-like molecules. However,the theory and simulations are discussed under zero-gravity environment and this condition is difficult to come true. In this work, we have performed the experiment in micro-gravity environment provided by the drop tower of NMLC. In the experiment we have observed the phase-separation clustering phenomenon for the first time.According to the experimental measurable parameter N and Lx we get N-Lx phase-separation curve. Comparing the observation condition with the model prediction, we are able to indirectly obtain the restitution coefficients of particles used in the experiment. It is found that the coefficient is larger with bigger particles. For d ?2.5mm titanium particles, the coefficient of restitution is higher than 0.8; for d ?1mm titanium particles, the coefficient of restitution is about 0.8, and for d ?0.5mm titanium particles, the coefficient of restitution is between 0.6 and 0.8.The experimental studies of three-dimensional granular gas clustering are meaningful for understanding and studying the knowledge of non-equilibriumcomplex systems, it also lay the foundation of granular gas-liquid phase separation experiment in microgravity environment.