The AF4 system consists of a long, thin channel with a semi-permeable membrane at the bottom. The sample is injected into the channel flow, and a perpendicular cross-flow field forces the particles towards the membrane. The particles then redistribute based on their diffusion rates, which are inversely proportional to their size. Smaller particles, which diffuse more rapidly, get carried further down the channel faster than larger particles, resulting in effective size-based fractionation.
