Difúzi

Difúzi, or diffusion, is the net movement of particles from regions of higher concentration to regions of lower concentration, driven by the random thermal motion of molecules. It occurs in gases, liquids and solids and can involve atoms, ions or molecules. Diffusion leads to the gradual mixing of substances and can occur across interfaces, such as membranes or boundaries between phases. In biological systems, diffusion provides a basic mechanism for the transport of substances without metabolic energy.

There are different forms of diffusion. Molecular diffusion refers to spontaneous movement due to kinetic energy. Facilitated diffusion uses membrane proteins to accelerate the process without expending energy, while active transport uses energy to move substances against gradients. In solids, diffusion often involves interdiffusion of species and defects movement, which is important in processes like annealing and alloying.

The rate of diffusion is described by Fick's laws. Fick's first law states that the diffusive flux J is proportional to the negative gradient of concentration: J = -D dc/dx, where D is the diffusion coefficient. Fick's second law describes how concentration changes with time in non-steady diffusion: ∂c/∂t = D ∂^2 c/∂x^2. The diffusion coefficient depends on temperature, the medium, and the size of diffusing particles; diffusion is typically faster in gases than in liquids, and increases with temperature.

Factors influencing diffusion include the concentration gradient, temperature, surface area, diffusion distance, and the viscosity of the medium. In biological contexts, diffusion underlies gas exchange in the lungs, nutrient uptake in cells, and neurotransmitter movement in synapses. In engineering, diffusion governs alloying, corrosion, and separation processes.