Viscoelastic Lubrication of Model Emulsions in Shear-Thinning Matrix*

Soft tribology is used as an analogue for mechanical movement of lubricated biological surfaces such as those encountered in oral processing and cartilage motion, which has been found to be useful in the design of food, cosmetics and biomedical devices. However, these applications often incorporate multiphase fluid with complex rheological behaviour, where the tribological response deviates from the typical Stribeck curve. It is hypothesised that oil ‘plating out’ effect at a critical speed is dependent on the rheology of emulsion. We studied soft lubrication of model emulsions with oil droplets dispersed in non-Newtonian hydrocolloid solutions designed to be iso-viscous at low- and high-shear rates respectively. The switchover from oil- to aqueous-dominated tribological response is observed at a critical speed for emulsions with an oil: aqueous (high-shear) viscosity ratio above 4. Above this limit, the critical speed for oil-dominated contacts is strongly dependent on both low- and high-shear viscosity. Below the limit, lubrication of emulsions follows the behaviour of non-Newtonian aqueous phase. This work enables development of a framework explaining the drivers for deviation from the Stribeck curve for complex multiphase fluids.