Formation and Rupture of Ca$^{2+}$ Induced Pectin Biopolymer Gels

TL;DR

This study investigates how calcium ions induce gel formation and rupture in pectin biopolymers, revealing the microscopic dynamics, gel network structure, and the effects of salt concentration on gel properties using DLS and rheology.

Contribution

It provides new insights into the microscopic dynamics and mechanical properties of calcium-induced pectin gels, highlighting the strong link regime and power-law dependence on salt concentration.

Findings

Increased salt concentration leads to larger flocs and longer relaxation times.

Gels become more interconnected and less resistant to rupture with more calcium.

Elasticity scales with salt concentration, indicating a power-law relationship.

Abstract

When calcium salts are added to an aqueous solution of polysaccharide pectin, ionic cross-links form between pectin chains, giving rise to a gel network in dilute solution. In this work, dynamic light scattering (DLS) is employed to study the microscopic dynamics of the fractal aggregates (flocs) that constitute the gels, while rheological measurements are performed to study the process of gel rupture. As calcium salt concentration is increased, DLS experiments reveal that the polydispersities of the flocs increase simultaneously with the characteristic relaxation times of the gel network. Above a critical salt concentration, the flocs become interlinked to form a reaction-limited fractal gel network. Rheological studies demonstrate that the limits of the linear rheological response and the critical stresses required to rupture these networks both decrease with increase in salt concentration. These features indicate that the ion-mediated pectin gels studied here lie in a `strong link' regime that is characterised by inter-floc links that are stronger than intra-floc links. A scaling analysis of the experimental data presented here demonstrates that the elasticities of the individual fractal flocs exhibit power-law dependences on the added salt concentration. We conclude that when pectin and salt concentrations are both increased, the number of fractal flocs of pectin increases simultaneously with the density of crosslinks, giving rise to very large values of the bulk elastic modulus.