Motility allocation controls glass transition in size-asymmetric mixtures

Dense disordered active materials undergo rigidity transitions from fluid to solid states, a phenomenon observed in diverse biological and artificial systems. Their mechanical response, particularly in mixtures with size-asymmetric components, depends sensitively on how self-propulsion is distributed, yet the role of motility allocation remains unknown. Using simulations of three-dimensional size-asymmetric active-passive mixtures of soft Brownian particles, we show that activating small or large size particles governs steady-shear rheology and glass formation. When small particles are active leading to rapid stress relaxation and vanishing yield stress. Conversely, activating large particles leads to glassy arrest with finite yield stress and strongly skewed stress fluctuations. Flow curves fitted to the Herschel–Bulkley form reveal a re-entrant dependence of rigidity on size ratio and density, with maximal yield stress near the near-monodisperse limit. This shows that both who moves and how much activity is present critically determine whether a dense active mixture flows or becomes arrested. Our findings propose motility allocation as a powerful design principle for tuning rigidity in active soft materials and guiding the creation of reconfigurable, bio-inspired metamaterials.