Electrotunable interfacial friction of polyacrylamide composite hydrogels

Hydrogels exhibit the significant application potential in flexible electronics, biomedical engineering, lubrication technology, robotics, and related fields. The electrotunable interfacial friction of hydrogels provides a conceptual framework for developing intelligent materials. Here, the polyacrylamide (PAAm) composite hydrogels were synthesized via photopolymerization using acrylamide as the monomers. The tribological tests under applied electric potential were conducted using PAAm composite hydrogel discs and silicon nitride balls as the friction pairs, showing the effects of sliding velocity and lubricants on the electrotunable interfacial friction behaviors of the PAAm composite hydrogels. The applied electric potential serves as a critical determinant governing the friction coefficient which is primarily attributed to (1) the differential diffusion kinetics of weakly bound water and non-bound water within the cross-linked polymer networks, and (2) the formation of hydrated cationic layers via strong electro-adsorption of hydrated cations at the interfaces. The findings provide the profound insights into the electrically modulated lubrication mechanisms of the soft contact interfaces from a combined perspective of intrinsic polymer network properties and lubricating molecule-surface interactions.