Examine the principles of fluid$-$particle systems in chemical engineering, focusing on particle characterization, fluidization, and particle$-$fluid interactions. Discuss the different types of particles, including size distribution, shape, and surface properties, and explain their behavior in fluid environments. Explore the applications of fluid$-$particle systems in processes such as powder handling, granulation, and pneumatic conveying, emphasizing their importance in industries such as pharmaceuticals, food processing, and mineral processing.