LATERAL DIFFUSION IN AN ARCHIPELAGO - DEPENDENCE ON TRACER SIZE

In a pure fluid-phase lipid, the dependence of the lateral diffusion c oefficient on the size of the diffusing particle may be obtained from the Saffman-Delbruck equation or the free-volume model. When diffusion is obstructed by immobile proteins or domains of gel-phase lipids, th e obstacles yield an additional contribution to the size dependence. H ere this contribution is examined using Monte Carlo calculations. For random point and hexagonal obstacles, the diffusion coefficient depend s strongly on the size of the diffusing particle, but for fractal obst acles-cluster-cluster aggregates and multicenter diffusion-limited agg regates-the diffusion coefficient is independent of the size of the di ffusing particle. The reason is that fractals have no characteristic l ength scale, so a tracer sees on average the same obstructions, regard less of its size. The fractal geometry of the excluded area for tracer s of various sizes is examined. Percolation thresholds are evaluated f or a variety of obstacles to determine how the threshold depends on tr acer size and to compare the thresholds for compact and extended obsta cles.