Fig. 4.

Results from numerical calculations of spatial dependence of the concentration field using the theoretically obtained velocity field. (A) Concentration, normalized by C_∞, near a perfectly absorbing [C(R) = 0] swimming spherical colony for various Péclet numbers, illustrating the development of a thin localized anterior boundary layer and long narrow posterior plumes at high swimming speeds. Colors represent the dimensionless concentration C/C_∞. The concentration fields near an immobilized colony are quantitatively very similar. (B) The relationship between boundary layer thickness and Péclet number. The thickness was determined from the computations as the distance from the front surface of the sphere to the point at which C/C_∞ = 0.1. When displayed as shown on a log–log plot (black line), the boundary layer thickness, divided by sphere radius R, is parallel to a line (dashed) proportional to Pe^−½, indicating that it also varies as the −½ power at Péclet numbers greatly exceeding unity.