Figure 7.9 — Brownian particle in a diffusion gradient Fig. 7.9 — Brownian particle in a diffusion gradient. (a) Diffusion coefficient D(r), normalised to the bulk diffusion D(∞) (dashed line), and (b) drift C(r) for a particle performing Brownian motion next to a planar wall, assuming a hard-wall interaction. The lines represent the theoretical expectation and the symbols [Eqs. (7.36) and (7.37)] are calculated from a simulated series of data. Because no external forces are acting on the particle, C(r) coincides with the spurious drift.