Competing effects of activity and diffusive noise in collective ordering of rod-like particles

Self-organization and emergent order are hallmarks of active matter. Using large-scale Brownian dynamics simulations, we study a binary mixture of self-propelled and passive rod-like particles, representing bacterial cells or synthetic anisotropic colloids. The interplay between motility, diffusive noise, and shape anisotropy produces a rich spectrum of collective states, including clustering, demixing, and orientational ordering. We find that the degree of spatial and orientational order exhibits a non-monotonic dependence on both the Péclet number and the noise strength ratio. At intermediate activity and optimal noise contrast, passive particles form tetratically ordered domains accompanied by a pronounced decrease in configurational entropy of the entire system, indicating an entropy-driven ordering transition. At high activity or large noise disparity, orientational coherence and clustering are lost, restoring a homogeneous disordered phase. These results reveal the minimal physical ingredients, such as motility, noise asymmetry, and shape anisotropy, sufficient to drive large-scale organization in active-passive mixtures, offering new insights into the collective dynamics of dense active soft matter.