Phase separation of active Brownian particles on curved surfaces

The effect of curvature on an ensemble of repulsive active Brownian particles (ABPs) moving on a spherical surface is studied. Surface curvature strongly affects the dynamics of ABPs, as it introduces a new time scale $\tau =R\mathrm{/}{v}_0$, with curvature radius $R$ and propulsion velocity $v_0$, in addition to the rotational diffusion time ${\tau }_r$. This implies that motility-induced phase separation (MIPS) disappears for small $R$. Furthermore, it causes a narrowing of the MIPS regime in the phase diagram of P{é}clet number $\text{Pe}$ and particle area fraction $\phi$. Also, the phase-separation boundary at low $\phi$ attains a turning point at small $R$, allowing for the possibility of a reentrant behavior. These results characterize the effect of curvature on ABP dynamics and MIPS, and will help to better understand the preferred occupation of certain niches by bacterial colonies in porous media.