The friction between two chemically cross-linked polyelectrolyte gels carrying the same sign of charges has been investigated in pure water as well as in salt solutions using a rheometer. It is found that the friction was largely dependent on the charge densities of the gel surface and the ionic strength of the aqueous solution. The chemical structures of the polyelectrolyte gels also play an important role. The friction is described in terms of the hydrodynamic lubrication of the solvent layer between the two gel surfaces, which is formed due to the electrostatic repulsion of the two gel surfaces. The thickness of the solvent layer has been estimated using the Poisson-Boltzmann equation supposing that the ionic osmotic pressure is balanced by the normal pressure applied on the gel. The friction values have been calculated by considering the shear flow of solvent in gel region using the Debye-Binkman equation. For strongly charged polyelectrolyte gels swollen in pure water, the theoretical analysis shows that the friction coefficient almost has no dependence on the water content of the gel, which well agrees with the experimental observations.

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The velocity dependence of gel friction was investigated in pure water and in salt solutions to elucidate the effect of interfacial interaction with substrates. When the gel and the substrate were repulsive, the frictional force depended strongly on the sliding velocity, whereupon the higher the normal compressive strain, the stronger the velocity dependence of the friction. The frictional force per unit area, f, was found to follow a power law as f ∝ v β, where the exponent, β, depends on the normal compressive strain. This result shows that the gel friction in the repulsive case cannot be explained in terms of the simple hydrodynamic mechanism, from which f ∝ v 1.0 is predicted. On the contrary, in the attractive case, the frictional force showed a maximum value with increase in the sliding velocity, which qualitatively coincides with our repulsion-adsorption model proposed previously.

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.

An extensive static friction has been observed when two like-charged polyelectrolyte gels are slid over each other in water. The two like-charged gel surfaces could not slip with each other at the initial shearing until the shear stress acting on the interface exceeded a certain critical value. The critical yield shear stress (static friction) and strain did not show a distinct dependence on the shearing rate but decreased with increasing temperature. The value of the static friction also increased with the increase in the normal pressure and effect of pressure becomes more substantial at a higher temperature. Furthermore, the static friction decreased 3-4 times when the counterions of the polyelectrolyte gels were changed from Na + to Cs + . The observation of the static friction indicates that our previously proposed repulsion-adsorption model, which predicts a hydrodynamic mechanism with no static friction for two like-charged hydrogels in pure water, requires modification. Possible origins of the static friction are discussed.