Abstract
The effects of glucose and glycerol on the lubrication properties of agar fluid gels have been studied using soft tribology. A novel approach using the sediment and supernatant of centrifuged fluid gels has allowed investigation of the distinct contributions of both the gelled particulate phase and the continuous phase on fluid gel tribology. The friction coefficient of both the particulate phase and fluid gels was significantly lower than that of the continuous phase across the three lubrication regimes. This indicates that particle entrainment occurs at all entrainment speeds, enhancing lubrication by prevention of surface contact.Softer fluid gel particles produced with intermediate levels of glycerol (up to 30%) show increased friction as would be expected for an increased contact area between the tribological surfaces. At high levels of glycerol, the friction does not increase. It is proposed that soft particles are produced but the increasing friction is overcome with the increased lubrication from the more highly viscous continuous phase. In contrast, the presence of intermediate levels of glucose (up to 30%) increases the friction of the aqueous continuous phase but does not affect the particle properties. Texture analysis, rheology and light scattering techniques were used to elucidate the structural changes of the fluid gels induced by the addition of co-solutes and the influence this has upon lubrication.
Highlights
Fluid gels are concentrated suspensions of micro-gel particles dispersed in a continuous medium, typically water, and are produced by applying a shear field to the hydrocolloid solution while the solegel transition is taking place (Norton, Frith, & Ablett, 2006; Norton, Jarvis, & Foster, 1999)
In order to elucidate the effects of glycerol and glucose on gel particle stiffness, compression tests on quiescent gels were carried out
We have introduced a new approach to investigate the tribological behaviour of fluid gels using the continuous phase and the particulate gelled phase
Summary
Fluid gels are concentrated suspensions of micro-gel particles dispersed in a continuous medium, typically water, and are produced by applying a shear field to the hydrocolloid solution while the solegel transition is taking place (Norton, Frith, & Ablett, 2006; Norton, Jarvis, & Foster, 1999). We used fluid gels produced from agar which is a seaweed hydrocolloid composed by mixtures of neutral agarose and charged agaropectin, in proportions that vary depending on both the production process and the raw material (Araki, 1956; Phillips and Williams, 2009). The gelling temperature of agar varies depending on the degree of methoxylation of the agarose chain which is characteristic of the seaweed speces from which the agar has been extracted (Armisen & Galatas, 2000)
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