Rheology and tribology of dextran/ polyethylene oxide-based water-in-water emulsions.

This study investigated the microstructural, rheological and tribological properties of model W/W emulsions composed of dextran (D) and poly(ethylene oxide) (P) at a fundamental level. Rheological analysis revealed that increasing the P concentration, [P], resulted in increased viscosity (η), whilst increasing the D concentration, [D], intensified shear-thinning behaviour, likely due to changes in the size of D-based droplets. Confocal laser scanning microscopy (CLSM) demonstrated a significant increase in the average droplet size with higher [D] or [P]. A striking tribological result was that the W/W emulsions demonstrated an unusual speed-independent regime, with a coefficient of friction (μ) < 0.01 over a considerable range of sliding contact speed (~10 to 100 mm s^-1, of physiological relevance) before the onset of the elastohydrodynamic lubrication (EHL) regime. This was not observed for solutions of the individual polymers on their own. Such composition-dependent behaviour may be due to W/W emulsion droplets entering the tribological gap, flattening and reducing the η of the entrained lubricants, thus delaying the formation of a fluid film. Overall, this detailed study shows how fabrication of W/W emulsions via phase-separating polymers can offer unique lubrication characteristics that could provide advantageous aqueous lubricants for biomedical applications.