Boundary- and Screening-Induced Bubbly Phases in Autophoretic Active Matter

Abstract

Spatial confinement and chemical screening fundamentally reshape the non-equilibrium phase behavior of autophoretic active particles. Here, we present a systematic study mapping the collective dynamics of self-propelled particles governed by chemo-attractive translational forces (${\mu }_t<0$) and chemo-repulsive rotational torques (${\mu }_r>0$) across varying screening parameters $\kappa$, torque magnitudes ${\mu }_r$, and boundary condition coefficients ${\Lambda }^c$. Beyond standard chemotactic macro-phase separation and dynamic clustering, we report the emergence of novel boundary- and screening-induced bubbly phases, classified into boiling and bursting bubbles. Using a metric triad of steady-state cluster fraction $⟨S⟩$, temporal fluctuation magnitude ${\sigma }_S$, and coordination number $⟨Q⟩$, we draw phase diagrams to demarcate phases for no-flux boundaries (${\Lambda }^c=1$) and chemically permeable interfaces (${\Lambda }^c=0$) . Increasing chemical screening ($\kappa$) systematically suppresses long-range attraction, driving sequential phase transitions from macro-scale collapse toward bubbly states, dynamic micro-clusters, and homogeneous gas phases, while simultaneously inducing aggregate shape anisotropy. These findings provide predictive design rules for controlling active assembly and transport in microfluidic environments.

7 figures, 10 pages; supplemental movies at https://softmatter.gitlab.io/bubbles