Fluid gels - rheology, tribology and thermal properties of hydrocolloid systems

Abstract

This work investigates the underlying gelation mechanism of hydrocolloid systems, with particular emphasis on agarose gelation under shear. The focus is directed at the physico-chemical properties of agarose fluid gels and their rheological, mechanical, and tribological behaviour. Fluid gels, based on gelling polysaccharides like agarose, undergo gelation at defined shear rates, resulting in suspensions of gel particles with controlled microstructures. These systems exhibit viscoelastic behaviour, characterised by the coexistence of solid and fluid-like properties over different time scales. Agarose, a strong gelling polysaccharide, typically forms firm, brittle gels through double-helix aggregation via hydrogen bonding while cooling. However, applying shear during gelation modifies this behaviour. The resulting agarose fluid gels display elastic, solid-like behaviour at rest that transitions to a fluid-like state upon exceeding a critical stress – attributed to the “hairy” microstructure of gels particles, which plays a role in the rheological, mechanical and tribological properties. A range of agarose concentrations (0.5 wt%, 1 wt% and 2 wt%) was investigated. Rheological measurements revealed that increasing agarose concentration resulted in higher storage moduli of the microgel particles. Notably, 1 wt% agarose fluid gels exhibit the lowest viscosity at low shear rates and the shortest linear viscoelastic range. Microstructure and particle size analysis indicated a decrease in particle size and unordered chains at the particle surfaces with decreasing agarose concentration. Based on these findings, we propose models to elucidate the effects of particle size, concentration, and “hairy” surface structure on the rheological and tribological behaviour of fluid gels. Atomic force microscopy revealed the inner network structure of gel particles, showing a dense core that decreases in density towards the periphery, emphasising structural gradients that affect elasticity and, consequently, bulk properties. The addition of co-solutes, such as sucrose, strongly influences interactions between agarose, water, and sucrose molecules during shear, impacting structure formation and network behaviour. Increasing sucrose concentration resulted in the formation of microgel particles with different sizes, shapes, and interconnected network structures. These microstructural changes arise from the dynamic competition between gelation and shear-induced disruption, directly impacting the rheological and tribological properties of fluid gels. The findings enhance our understanding of the structure-property relationship in fluid gels and broaden their potential in food applications – particularly in designing tailored textures for products targeting conditions such as dysphagia. This work contributes to advancing the functional design of agarose-based fluid gels by elucidating the molecular mechanisms underlying their formation and behaviour under shear.