  Activity and self-generated motion are fundamental features observed in many
living and non-living systems. Given that inter-particle adhesive forces are
known to regulate particle dynamics, we investigate how adhesion strength
controls the boundary growth and roughness in an active particle aggregate.
Using particle based simulations incorporating both activity (birth, death and
growth) and systematic physical interactions (elasticity and adhesion), we
establish that inter-particle adhesion strength ($f^{ad}$) controls the surface
roughness of a densely packed three-dimensional(3D) active particle aggregate
expanding into a highly viscous medium. We discover that the surface roughness
of a 3D active particle aggregate increases in proportion to the inter-particle
adhesion strength, $f^{ad}$. We show that asymmetry in the radial and
tangential active particle mean squared displacement (MSD) suppresses 3D
surface roughness at lower adhesion strengths. By analyzing the statistical
properties of particle displacements at the aggregate periphery, we determine
that the 3D surface roughness is driven by the movement of active particle
towards the core at high inter-particle adhesion strengths. Our results
elucidate the physics controlling the expansion of adhesive 3D active particle
collectives into a highly viscous medium, with implications into understanding
stochastic interface growth in active matter systems characterized by self
generated particle flux.
