Figure 5.

Results from the pure diffusion, pure drift, and diffusion-and-drift models. (Left) Receptor density profile. (Right) Engulfment against time. (A) Pure diffusion model with D = 1 μm² s⁻¹. The receptor density, ρ, drops significantly just outside the cup so that ρ₊ < ρ₀ and evolves to the right as engulfment proceeds. Note that the parameters are such that ρ₊ ≈ 0. The engulfment increases as the square-root of time. Receptor profile shown at t = 3 s (solid) and t = 6 s (dashed). (B) Pure drift model with v = 0.1 μm s⁻¹. The receptor density has a completely different profile and decreases away from the cup, with ρ₊ now greater than ρ₀. Importantly, the engulfment now increases linearly in time. Receptor profile shown at t = 10 s (solid) and t = 20 s (dashed). (C) Diffusion and drift model with D = 1 μm² s⁻¹ and v = 0.1 μm s⁻¹. Engulfment now proceeds as a mixture of the pure diffusion and pure drift cases. Initially, when the receptor density is low, $a\sim \sqrt{t}$ as in the pure diffusion model. At later times, when the receptor gradient near the cup becomes approximately constant, a becomes more linear in time and behaves more like the pure drift result. Receptor profile shown at t = 3 s (solid) and t = 6 s (dashed). Parameters are ρ₀ = 50 μm⁻², ρ_L = 5000 μm⁻², $\mathcal{E}$ = 15, $\mathcal{B}$ = 20, R = 2 μm, and L = 50 μm. To see this figure in color, go online.