title: Polymer fluctuation lubrication in hydrogel gemini interfaces.
abstract: Interfacial sliding speed and contact pressure between the sub-units of particulate soft matter assemblies can vary dramatically across systems and with dynamic conditions. By extension, frictional interactions between particles may play a key role in their assembly, global configuration, collective motion, and bulk material properties. For example, in tightly packed assemblies of microgels - colloidal microspheres made of hydrogel - particle stiffness controls the fragility of the glassy state formed by the particles. The interplay between particle stiffness and shear stress is likely mediated by particle-particle normal forces, highlighting the potential role of hydrogel-hydrogel friction. Here we study friction at a twinned "Gemini" interface between hydrogels. We construct a lubrication curve that spans four orders of magnitude in sliding speed, and find qualitatively different behaviour from traditional lubrication of engineering material surfaces; fundamentally different types of lubrication occur at the hydrogel Gemini interface. We also explore the role played by polymer solubility and hydrogel-hydrogel adhesion in hydrogel friction. We find that polymer network elasticity, mesh size, and single-chain relaxation times can describe friction at the gel-gel interface, including a transition between lubrication regimes with varying sliding speed.
authors.affiliation: Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida 32611, USA.
authors.forename: A A
authors.initials: AA
authors.lastname: Pitenis
authors.forename: J M
authors.initials: JM
authors.lastname: Urueña
authors.forename: K D
authors.initials: KD
authors.lastname: Schulze
authors.forename: R M
authors.initials: RM
authors.lastname: Nixon
authors.forename: A C
authors.initials: AC
authors.lastname: Dunn
authors.forename: B A
authors.initials: BA
authors.lastname: Krick
authors.forename: W G
authors.initials: WG
authors.lastname: Sawyer
authors.forename: T E
authors.initials: TE
authors.lastname: Angelini