Friction, lubrication, and in situ mechanics of poroelastic cellulose hydrogels

The tribology between biphasic materials is challenging to predict and interpret due to the interrelationship between mechanical properties, microstructure and movement of the fluid phase contained within. A new approach is presented to deconvolute these effects for cellulose hydrogels, which have a fibrous network that is akin to the microstructure of articular cartilage and plant cell walls. This is achieved by developing a tribo-rheological technique that uniquely incorporates in situ mechanical characterisation (compression-relaxation and small amplitude oscillatory shear) immediately prior to measuring the tribological response between pairs of hydrogels. A radial pressure gradient is generated upon compression-relaxation of the poroelastic hydrogels that results in a non-uniform film thickness at the interface between them. Simulations of this process show that contact between gels occurs in an outer annulus region. Accounting for the predicted contact area between hydrogels varying in cellulose density and pectin solution viscosity causes measured tribology data to collapse onto a single curve; the apparent static friction between hydrogel tribopairs increases with the storage modulus of the hydrogels according to a power law with exponent 0.67. The method is used to compare the influence of plant cell wall polysaccharides, xyloglucan and arabinoxylan, on the interactive forces between cellulose fibres; xyloglucan is found to reduce the static friction between the hydrogels while arabinoxylan had no significant effect. The methodologies presented should provide a new framework for studying the friction between gels and other biphasic soft materials and polymeric surface films.