A two-dimensional (2D) comb model is proposed to characterize reaction-ultraslow diffusion of tracers both in backbones ($x$ direction) and side branches ($y$ direction) of the comblike structure with two memory kernels. The memory kernels include Dirac delta, power-law, and logarithmic and inverse Mittag-Leffler (ML) functions, which can also be considered as the structural functions in the time structural derivative. Based on the comb model, ultraslow diffusion on a fractal comb structure is also investigated by considering spatial fractal geometry of the backbone volume. The mean squared displacement (MSD) and the corresponding concentration of the tracers, i.e., the solution of the comb model, are derived for reactive and conservative tracers. For a fractal structure of backbones, the derived MSDs and corresponding solutions depend on the backbone's fractal dimension. The proposed 2D comb model with different kernel functions is feasible to describe ultraslow diffusion in both the backbone and side branches of the comblike structure.

• Received 21 November 2019
• Revised 14 February 2020
• Accepted 20 March 2020

DOI:https://doi.org/10.1103/PhysRevE.101.042119