Particle movement in solids
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Particle Movement in Solids: Diffusion and Rigid-Body Motion
In solids, particle movement is primarily governed by diffusion and rigid-body motion. Diffusion involves the transport of atoms or particles through the solid matrix, often driven by concentration gradients. The velocity of particle movement in this context depends on the concentrations of the elements forming the compound within the solid solution, especially when the transport of these elements is the rate-limiting step. Experimental studies, such as those on oxide particles in internally-oxidized copper alloys, confirm that diffusion is a key mechanism for particle movement in solids .
Rigid-body motion, particularly during processes like solid-state sintering, also plays a significant role. Computer simulations show that translational rigid-body motion of particles promotes neck growth and pore shrinkage, which are essential for densification. The forces driving this motion depend on particle size and the configuration of neighboring particles. However, rotational rigid-body motion does not contribute to pore shrinkage, highlighting the importance of translational movement in the coarsening and densification of solids .
Particle Movement in Fluidized Beds and Solid-Liquid Interfaces
When solids are suspended in a fluid, such as in fluidized beds, particle movement becomes more complex. In gas-solid fluidized beds, particles exhibit both upward and downward movement, with upward movement often explained by diffusion and downward movement by convection. The overall motion is influenced by factors like gas velocity, particle size, and bed geometry. Particles tend to move upward in the center and downward near the walls, forming circulation patterns. These patterns can be characterized by different time scales—jump, idle, and relaxation times—which relate to the transfer of kinetic energy within the bed 3458.
Discrete particle simulations and experimental tracking techniques reveal that better mixing occurs with smaller particles, larger beds, and higher gas velocities. In systems with particles of different sizes or densities, mixing or segregation can be controlled by adjusting these properties. The movement of particles in these environments is not random; instead, particles often move as part of aggregates or clusters, which experience greater drag and thus move more slowly than individual particles 458.
At solid-liquid interfaces, particles adsorbed onto a solid surface can also exhibit lateral movement. This mobility is driven by forces that reduce the system's free energy, such as electrostatic interactions between charged particles and surfaces. The presence of a driving force is essential for this lateral movement to occur .
Particle Collisions and Transport Mechanisms
The dynamics of particle collisions, both dry and wet, are crucial for understanding particle movement in solids and at interfaces. High-speed imaging and modeling show that the translational and rotational motions of colliding particles can be measured and analyzed in three dimensions. The presence of a liquid can significantly alter collision dynamics compared to dry conditions, affecting how particles move and interact .
In specialized flows, such as peristaltic transport, particle movement is influenced by the shape and speed of the flow wave, particle size, and the gap width of the flow channel. The net displacement of particles can be positive or negative, and lateral migration can occur due to curvilinear flow regions, reducing overall longitudinal transport .
Conclusion
Particle movement in solids is a multifaceted phenomenon influenced by diffusion, rigid-body translation, and external forces in fluidized or interfacial environments. The mechanisms and rates of movement depend on material properties, environmental conditions, and the presence of driving forces or gradients. Understanding these processes is essential for controlling material behavior in applications ranging from metallurgy to ceramics and chemical engineering 13456789+1 MORE.
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