Study On Freely Cooling And Phase Transition Of Granular Gas Under Microgravity

Granular matter can be seen everywhere in life or industry.Most of the solid matters on the earth are in discrete form.In past few centuries,human beings have achieved great breakthroughs in exploring both the microscopic and macroscopic world,but we have still known little about the laws of the complex granular matter consisted of huge number of discrete units.The nature of energy dissipation is an important characteristic of granular matter,so granular matter is a system far from the equilibrium state.The Dissipative Structure Theory points out that an open system which is far from the equilibrium state and exchanges energy or matter with the outside world continuously can reach a steady selforganized state under some certain critical conditions through interactions,and the system can transform from a disordered state to an ordered state in time or space.Granular matter can appear spatial-ordered phenomena,such as self-ordering,selforganization or patterns,under specific external conditions.Therefore,granular matter can be used to study the nonequilibrium mechanics.The granular particles will drop and lie on the substrate without energy input due to gravity on the earth.The gravity-free environment is very hard to achieve on the earth,hence most of previous researches conducted on the earth are influenced by gravity.However,it's vital to study the motion caused only by the nature of granular matter for exploring the nonequilibrium behaviors.Energy dissipation is the key factor that makes granular gas behaves unlike the classical ideal gas,thus it's of great significance to study the law of energy dissipation for understanding other phenomena of granular matter.We conducted experiments to observe the freely cooling granular gas under microgravity in Bremen Drop Tower,and verified that the kinetic energy decay as time follows Haff's law.We have also conducted numerical simulations to study the effects of restitution coefficient,friction,number density of particles and particles' size on the decay rate(or the characteristic decay duration).The number density in a local space will fluctuate for granular gas with continuous energy input.When the number density of a local area becomes large enough due to fluctuation,the intensity of energy dissipation in this local area will strengthen and the pressure will decrease.Then the particles in the area with large pressure will migrate to the area with low pressure due to the pressure gradient.Under some critical conditions,the process of particles migration can't be reversed.As a consequence,the cluster will form in granular gas.For granular gas in a double-cell connected container will behave like the Maxwell's Demon.We observed the Maxwell's Demon in granular gas under microgravity for the first time in the world in the Satellite “SJ-10”.We also studied the effects of the excitation intensity and the opening's asymmetry on the phenomenon of the Maxwell's Demon in granular gas under microgravity.