Direct link between mechanical stability in gels and percolation of isostatic particles

Abstract

Colloidal gels have unique mechanical and transport properties that stem from their bicontinuous nature, in which a colloidal network is intertwined with a viscous solvent, and have found numerous applications in foods, cosmetics, and construction materials and for medical applications, such as cartilage replacements. So far, our understanding of the process of colloidal gelation is limited to long-time dynamical effects, where gelation is viewed as a phase separation process interrupted by the glass transition. However, this purely out-of-equilibrium thermodynamic picture does not address the emergence of mechanical stability. With confocal microscopy experiments, we reveal that mechanical metastability is reached only after isotropic percolation of locally isostatic environments, establishing a direct link between the load-bearing ability of gels and the isostaticity condition. Our work suggests an operative description of gels based on mechanical equilibrium and isostaticity, providing the physical basis for the stability and rheology of these materials.

Emergent solidity of amorphous materials as a consequence of mechanical self-organisation

“…Stability of space-spanning mechanical network in amorphous solids should play a key role in various phenomena such as ageing and devitrification 65 and mechanical fracture. Recently, a similar scenario has been reported for the emergence of elasticity in colloidal gels due to the mechanical percolation 66,67 . These findings seem to indicate a general origin of apparent rigidity in disordered nonergodic materials made of particles covering the glassy and gel states.…”

Effects of Protein Unfolding on Aggregation and Gelation in Lysozyme Solutions

“…In this work we did not address the interesting dynamic properties of gels, which would require a more detailed DLS experiments and analysis. We just note that the correlation function of density/concentration fluctuations in gel can exhibit a compressed exponential decay [16,33]. The observed relaxation modes can be attributed to concentration fluctuations of the aggregates which are not a part of gel network, branching points, segments and restructuring of the gel [31,32,39].…”

“…As a result, normally buried hydrophobic residues may act as a crosslinker for intermolecular beta-sheet structures and lead to formation of aggregates of different shapes, globular or amyloid like, depending on the solution conditions [16,31,32]. Continuous heating leads to association of aggregates and subsequent formation of a gel network [16][17][18]32,33].…”

A unique route of colloidal phase separation yields stress-free gels

“…Next, we study how local structures can be stabilized in the postfolding regime. We recently found by in situ single-particle imaging that VPS-G gains macroscopic elasticity when rigid isostatic percolated structures form (44). The Maxwell criterion for isostaticity of 〈N C 〉 ≥ 6, where N C is the particle contact number, is a minimum requirement for mechanical stability (56).…”