Fractionation by Spatially\nHeterogeneous Diffusion:\nExperiments and the Two-Component Random Walk Model

The fundamental question regarding the fractionation\nphenomenon\nis whether diffusion alone is responsible for it or whether an additional\nadvection dynamic is involved. We studied the fractionation by diffusion\nof particles in spatially heterogeneous environments. By experimentally\nobserving the time-sequential fractionation patterns of dye particles\ndiffusing across a solid–solid interface of varying polyacrylamide\ngel densities, we found that the two-component diffusion model accurately\ncaptures the observed fractionation dynamics. In contrast, single-component\ndiffusion models by Fick, Wereide, and Chapman do not. Our results\nindicate that diffusion alone can explain the fractionation phenomenon\nand that additional advection dynamics are not involved. The underlying\nphysics in the fractionation phenomenon is discussed by using a two-component\nrandom walk model.