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Diffusion coefficient, often denoted by D, is a fundamental property in physics and chemistry that quantifies the rate at which particles spread out from a region of higher concentration to a region of lower concentration. This process, known as diffusion, is driven by the random thermal motion of molecules. The diffusion coefficient is a measure of how quickly this random motion leads to the net displacement of particles over time.

The unit of the diffusion coefficient is typically m²/s (square meters per second) or cm²/s (square centimeters per second). A higher diffusion coefficient indicates that particles move and spread out more rapidly. Factors influencing the diffusion coefficient include temperature, viscosity of the medium, and the size and shape of the diffusing particles. For instance, at higher temperatures, molecules have more kinetic energy, leading to faster diffusion and thus a larger diffusion coefficient. Similarly, diffusion in a less viscous medium will be faster than in a highly viscous one.

The concept of the diffusion coefficient is crucial in various scientific disciplines, including fluid dynamics, materials science, and biology. It plays a role in understanding processes like mass transport, heat transfer, and the movement of substances across cell membranes. The mathematical description of diffusion often involves Fick's laws, which relate the diffusion flux to the concentration gradient and the diffusion coefficient.