Hydrogels are often used as model systems for articular cartilage due to similarities in their tribological properties. However, neither the structures nor the friction mechanisms of either system are fully understood. A key aspect of hydrogel lubrication is the nature of the polymeric structure at the surface, and the lubricating water film. A combination of neutron reflectometry and infrared spectroscopy is used to probe polymer volume fraction from the interface into the bulk hydrogel and its dependence on the molding material. The depth dependence of the polymer‐network density influences the compressibility of the hydrogel surfaces, as demonstrated by both atomic force microscopy (AFM)‐ and micro indentation. By changing molding materials, substantial differences in the gradient of polymer‐network density are observed with depth. The lower the volume fraction of polymer at the hydrogel surface, the more water it can maintain at its interface as a substantial water film that is stable even under static conditions. Such films render the hydrogel highly lubricious, with a speed‐independent friction coefficient of μ = 0.01, measured in gemini contact. This result provides experimental evidence that the presence of these highly lubricious water films is strongly dependent on the polymer‐network structure at the surface.

The polymeric structure of hydrogels is commonly presented in the literature as resembling a fishing net. However, this simple view cannot fully capture all the unique properties of these materials....

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Molecular dynamics (MD) simulations present a data-mining challenge, given that they can generate a considerable amount of data but often rely on limited or biased human interpretation to examine their...

Divalent ions, such as Mg, Ca, and Zn, are being considered as competitive, safe, and earth-abundant alternatives to Li-ion electrochemistry. However, the challenge remains to match elec- trode and electrolyte materials that stably cycle with these new formulations, based primarily on controlling interfacial phenomena. We explore the formation of electroactive species in the electrolyte Ca(BH4)2 in THF through molecular dynamics simulation. Free-energy analysis indicates that this electrolyte has a majority population of neutral Ca dimers and monomers, albeit with diverse molecular conformations as revealed by unsupervised learning techniques, but with an order of magnitude lower concentration of possibly electroactive charged species, such as the monocation, CaBH4+, which we show is produced via disproportionation of neutral Ca(BH4)2 complexes. Dense layering of THF molecules within 1 nm of the electrode surface (modeled here using graphite) hinders the approach of reducible species to within 0.6 nm and instead enhances the local concentration of species in a narrow intermediate-density layer from 0.7-0.9 nm. A dramatic increase in the monocation population in this intermediate layer is induced at negative bias, supplied by local dimer disproportionation. We see no evidence to support any functional role of fully-solvated Ca2+ in the electrochemical activity of this electrolyte. The consequences for performance and alternative formulations are discussed in light of this molecular-scale insight.

Divalent ions, such as Mg, Ca, and Zn, are being considered as competitive, safe, and earthabundant alternatives to Li-ion electrochemistry. However, the challenge remains to match electrode and electrolyte materials that stably cycle with these new formulations, based primarily on controlling interfacial phenomena. We explore the formation of electroactive species in the electrolyte Ca(BH4)2 in THF through molecular dynamics simulation. Free-energy analysis indicates that this electrolyte has a majority population of neutral Ca dimers and monomers, albeit with diverse molecular conformations as revealed by unsupervised learning techniques, but with an order of magnitude lower concentration of possibly electroactive charged species, such as the monocation, CaBH + 4 , which we show is produced via disproportionation of neutral Ca(BH4)2 complexes. Dense layering of THF molecules within 1 nm of the electrode surface (modeled here using graphite) hinders the approach of reducible species to within 0.6 nm and instead enhances the local concentration of species in a narrow intermediate-density layer from 0.7-0.9 nm. A dramatic increase in the monocation population in this intermediate layer is induced at negative bias, supplied by local dimer disproportionation. We see no evidence to support any functional role of fully-solvated Ca 2+ in the electrochemical activity of this electrolyte. The consequences for performance and alternative formulations are discussed in light of this molecular-scale insight.