Maria N. Charalambides

Rheology plays a critical role across science and engineering, affecting sectors from healthcare to energy. In the UK, rheological research supports innovation in both academia and industry. This review, coordinated by the British Society of Rheology (BSR), provides an updated overview of current research directions across the UK and beyond.

Closed-cell foams exhibit low density and high specific stiffness and strength. Their mechanical behaviour often deviates from classical Gibson-Ashby scaling due to geometric irregularities, heterogeneity and internal defects. These structural imperfections can promote stress localisation and premature collapse, suggesting that foam fracture is controlled not solely by the strength of the solid phase, but also by local structural instabilities. In this study, we characterise extruded food foams with different relative densities through mechanical testing, and their microstructures were reconstructed using X-ray microtomography. Finite element models from the reconstructed microstructures reproduced the experimental compression responses, enabling the solid-phase modulus (Es) and fracture stress (σs) to be identified through inverse calibration. The results revealed a nonlinear relationship between foam fracture stress (σ*) and σs. To investigate this behaviour, virtual structures with preserved topology but different relative densities were generated through cell-wall dilation and analysed using parametric simulations. For the first time, we report that σ* also depends on Es, through its influence on pre-fracture deformation, cell-wall bending, load redistribution and damage initiation. Based on this mechanism, the modulus-strength coupling was incorporated into a modified Gibson-Ashby relation through Es/σs. The proposed new relation provided reasonable predictions for extruded foams and other cellular materials.

Starch, a naturally abundant and widely consumed energy source, is biocompatible and extensively used in the food and pharmaceutical industries. Digestion of starch involves both the mechanical breakdown through mastication and peristaltic waves of the gastrointestinal wall as well as chemical breakdown through diffusion and reaction with saliva and gastrointestinal fluids. The mechanical properties of starch hydrogels significantly influence their breakdown and digestion speed, making it essential to investigate these properties. However, the effects of the degree of gelatinization (DOG) and degree of retrogradation (DOR) on the loading-unloading mechanical properties, which are essential for material model development and in silico digestion simulations, remain inadequately investigated. This study performed mechanical experiments to investigate the mechanical properties of starch hydrogels across different degrees of gelatinization and retrogradation, influenced by heating and storage conditions. Gelatinization was observed under optical microscopy by noting the loss of the spherical shape of the starch granules. The degree of retrogradation was measured using Differential Scanning Calorimetry (DSC). Starch hydrogels exhibit stress-softening behavior and viscoelasticity, varying with different degrees of gelatinization and retrogradation due to the crystallite melting and the recrystallization of the starch molecules. A pseudoelastic-viscoelastic constitutive law was developed to describe the mechanical behavior of the starch hydrogel, and the material parameters were calibrated based on experimental results. Such models are required for the simulation of the digestion of starch hydrogels. This study assists in understanding the influence of cooking and storage conditions on the mechanical behavior of starch hydrogels.

This study presents a rigorous mechanical characterisation investigation on milk chocolate with varying porosities, at different temperatures and strain rate levels.