Tuning Lubrication Performance of Phase-Changing Emulsion-Filled Gels by Sugar Alcohols

Abstract

Lubrication by hydrogels is an emerging area in surface science. Although oil-driven friction reduction is predominant in literature, emulsion gel-based lubrication has attracted very little attention to date. We hypothesize that an interplay of viscoelasticity and oil volume fraction may modulate the lubricity of emulsion-filled gels. Herein, phase-change gelatin-based emulsion-filled gels with different sugar alcohols and oil droplet concentrations (0–30 wt%) are tested for their lubrication performance. The rheological and tribological tests are analyzed in conjunction with spectroscopic and structural characterizations to reveal structure–function relationship. Structural characterizations demonstrate that hydrogen bonding is enhanced in pure gelatin gels with the increase of sugar alcohol (glycerol)-replacement time, which enhance the storage modulus (G′) of the gel networks. Emulsified droplets serve as active fillers further strengthening the G′ and strikingly the presence of glycerol reduced the thermoresponsiveness of the emulsion-filled gels. Emulsion-filled gels containing sugar alcohols, irrespective of their type, can greatly reduce the friction coefficients (μ) between hydrophobic surfaces in the boundary and mixed regimes versus the systems without sugar alcohols. In the hydrodynamic regime, the friction coefficient of the system is proportional to the second plateau shear viscosity, regulated by the oil content.