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

## TL;DR

This paper studies how ultra-low-cross-linked microgels behave in crowded conditions, revealing unique dynamics that resemble biological systems like cell membranes.

## Contribution

The study introduces a new class of microgels with ultrasoft coronas and reveals their unique rheological behavior in crowded environments.

## Key findings

- ULC microgels show critical-like gel behavior with G′ ∼ G″ ∼ ωⁿ in linear viscoelastic measurements.
- Stress-shear strain rate measurements show shear-thinning with σ ∼ γ̇∼0.25 at low strain rates.
- The observed dynamics suggest ULC microgels could model mechanical softness in cell membranes.

## Abstract

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″ ∼ ω
n
. 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.