Ultra-Low-Cross-Linked Microgels Reveal Unexpected Dynamics in Overcrowded Conditions

Ultralow-cross-linked microgels serve as powerful model systems for investigating structure-rheology relationships in soft colloidal suspensions. Using precipitation polymerization, we obtain both self-cross-linked microgels with a weakly cross-linked core, surrounded by an ultrasoft corona (ULC), and regular cross-linked (RC) microgels. ULC microgel suspensions exhibit distinctive rheological responses in crowded conditions. Their linear viscoelastic behavior shares features with critical-like gels, characterized by G' ∼ G″ ∼ ωⁿ. Large-amplitude-oscillatory-shear measurements reveal a solid-liquid transition reminiscent of polymeric networks lacking a G″ overshoot during yielding. Stress-shear strain rate measurements further reveal shear-thinning with a power-law behavior at low shear strain rates, σ ∼ γ̇^∼0.25. We attribute this behavior to a fine-tuned balance between polymeric and colloidal contributions. This rheological response to crowding establishes ULC microgels as emergent soft nanocolloids with potential biological relevance, particularly as analogues for the heterogeneity in mechanical softness (compressibility) observed in cell membranes.