In fact, it is possible for bulk diffusion to occur more rapidly than single-particle diffusion, when the concentration of particles used is high enough to induce crowding, according to a mechanism that has been outlined theoretically (shown schematically in Fig. ) . In the canonical case where particles are relatively dilute, then each particle can move with equal freedom in each direction (Fig ). This means that each particle executes a random walk, ultimately leading to diffusion with a bulk diffusion constant equal to the single-particle constant. However when the particle concentration is relatively high, then crowding effects mean that movement up a concentration gradient (towards the crowded region) is disfavoured, whereas movement down the concentration gradient, towards the less crowded region, is easier. This asymmetry means that the particles spread out into the lower concentration region faster than would otherwise be expected, meaning that bulk diffusion measurements such as we have conducted here will lead to higher measured diffusion constant, consistent with our observed results.