Friction between like-charged hydrogels—combined mechanisms of boundary, hydrated and elastohydrodynamic lubrication

Abstract

The friction between two like-charged polyelectrolyte gels in pure water is measured by using a normal strain-controlled rheometer with a parallel-plates geometry. The effects of normal stress, gel elasticity and sample thickness on the velocity dependence of friction between the gels are investigated. The frictional stress demonstrates strong velocity dependence (liquid-like) when the gel is soft and thick, while it demonstrates a weak or even no velocity dependence (solid-like) when the gel is rigid and thin. The former is interpreted by a combined mechanism of boundary lubrication and hydrated lubrication, wherein the thickness of the lubricating layer is velocity-independent, due to the formation of an electric double layer at the soft and repulsive interfaces. On the other hand, the latter is interpreted by a combined mechanism of boundary lubrication and elastohydrodynamic lubrication, wherein the thickness of the lubricating layer is velocity-enhanced by the water entrainment during sliding. The friction of the soft, thick sample is related to micro-contact while that of the rigid, thin sample is related to macroscopic geometric effect. This work may contribute to the science of friction between two soft and repulsive interfaces in water.

Surfactant-induced friction reduction for hydrogels in the boundary lubrication regime

“…The σ in water showed a slight velocity-weakening over a wide velocity range, indicating that the gel friction was dominated by the boundary lubrication, that is, the first term of Eq. 1, in agreement with the results in the previous observation [19]. In the low velocity region below In order to elucidate whether this surfactant effect also exists for substrates with different hydrophobic levels, two additional substrates, silicon wafer and a glass substrate treated with dansyl chloride (D-glass), were used.…”

“…A recent study on the friction between two like-charged polyelectrolyte gels in pure water showed that the frictional behavior is also dependent on the elasticity of the gel and its thickness [19]. The frictional stress demonstrates strong velocity dependence (liquid-like) when the gel is soft and thick, but demonstrates weak or even no velocity dependence (solid-like) when the gel is rigid and thin.…”

“…, where  is the viscosity of water,  v is the sliding velocity, and h is the apparent thickness of the liquid film formed at the interface [19]. The boundary lubrication component 0  , which is the predominant term at low velocity regime, is strongly dependent on the interfacial contact and interaction.…”

“…On the other hand, the latter is interpreted as the combined mechanisms of boundary lubrication and elastohydrodynamic lubrication, wherein the elastohydrodynamic lubrication is due to the velocity-enhanced water entrainment that occurs during sliding. In either cases, the frictional stress  can be analyzed phenomenologically by considering two components: a velocityindependent component 0  in the boundary lubrication regime, where the highly compressed asperities 3 come into contact; and a velocity-dependent component   vis as a result of the viscous energy dissipation, i. e., lubrication, by the water layer that exists interstitially at the repulsive interface [19]; that is,…”

“…In order to highlight the frictional reduction effect of a surfactant, we chose a "rigid and thin" gel sample (elastic modulus, 0.8 MPa; thickness, 2.75 mm), for which the boundary lubrication component 0  was predominant over the whole velocity range in pure water, according to the literature [19]. Furthermore, we chose negatively charged surfactant molecules to avoid the unnecessary adsorption of the surfactant molecules on the gel, which might have caused large changes in the degree of swelling, sample size, and elastic modulus of the gel in the surfactant solutions.…”