Diffusion-controlled formation and collapse of a d-dimensional A-particle island in the B-particle sea

Abstract: We consider diffusion-controlled evolution of a $d$-dimensional $A$-particle island in the $B$-particle sea at propagation of the sharp reaction front $A+B\to 0$ at equal species diffusivities. The $A$-particle island is formed by a localized (point)$A$-source with a strength $\lambda$ that acts for a finite time $T$. We reveal the conditions under which the island collapse time $t_{c}$ becomes much longer than the injection period $T$ (long-living island) and demonstrate that regardless of $d$ the evolution of the long-living island radius $r_{f}(t)$ is described by the universal law $\zeta_{f}=r_{f}/r_{f}^{M}=\sqrt{e\tau|\ln\tau|}$ where $\tau=t/t_{c}$ and $r_{f}^{M}$ is the maximal island expansion radius at the front turning point $t_{M}=t_{c}/e$. We find that in the long-living island regime the ratio $t_{c}/T$ changes with the increase of the injection period $T$ by the law $\propto (\lambda^{2}T^{2-d})^{1/d}$ i.e. increases with the increase of $T$ in the one-dimensional (1D) case, does not change with the increase of $T$ in the 2D case and decreases with the increase of $T$ in the 3D case. We derive the scaling laws for particles death in the long-living island and determine the limits of their applicability. We demonstrate also that these laws describe asymptotically the evolution of the $d$-dimensional spherical island with a uniform initial particle distribution generalizing the results obtained earlier for the quasi-one-dimensional geometry. As striking results we present a systematic analysis of the front relative width evolution for fluctuation, logarithmically modified and mean-field regimes and demonstrate that in a wide range of parameters the front remains sharp up to a narrow vicinity of the collapse point.