Relaxation and Recovery in Hydrogel Friction on Smooth Surfaces

Background Hydrogels are crosslinked polymer networks that can absorb and retain a large fraction of liquid. Near a critical sliding velocity, hydrogels pressed against smooth surfaces exhibit time-dependent frictional behavior occurring over multiple timescales. The origin of these dynamics is unre...

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Bibliographic Details
Published in:Experimental mechanics 2021-09, Vol.61 (7), p.1081-1092
Main Authors: Wu, B., Harper, J. S. Méndez, Burton, J. C.
Format: Article
Language:English
Subjects:
Biomedical Engineering and Bioengineering
Characterization and Evaluation of Materials
Coefficient of friction
Control
Dynamical Systems
Engineering
Friction
Hydrogels
Lasers
Optical Devices
Optics
Photonics
Polyacrylic acid
Polymers
Polymethyl methacrylate
Recovery
Research Paper
Shearing
Sliding
Solid Mechanics
Special Issue on Tribology of Advanced Materials
Time constant
Time dependence
Vibration
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Summary:Background Hydrogels are crosslinked polymer networks that can absorb and retain a large fraction of liquid. Near a critical sliding velocity, hydrogels pressed against smooth surfaces exhibit time-dependent frictional behavior occurring over multiple timescales. The origin of these dynamics is unresolved Objective Here, we characterize this time-dependent regime and show that it is consistent with two distinct molecular processes: sliding-induced relaxation and quiescent recovery. Methods Our experiments use a custom pin-on-disk tribometer to examine poly(acrylic acid) hydrogels on smooth poly(methyl methacrylate) surfaces over a variety of sliding conditions, from minutes to hours. Results We show that at a fixed sliding velocity, the friction coefficient decays exponentially and reaches a steady-state value. The time constant associated with this decay varies exponentially with the sliding velocity, and is sensitive to any precedent frictional shearing of the interface. This process is reversible; upon cessation of sliding, the friction coefficient recovers to its original state. We also show that the initial direction of shear can be imprinted as an observable “memory”, and is visible after 24 hrs of repeated frictional shearing. Conclusions We attribute this behavior to nanoscale extension and relaxation dynamics of the near-surface polymer network, leading to a model of frictional relaxation and recovery with two parallel timescales.
ISSN:0014-4851
1741-2765
DOI:10.1007/s11340-021-00748-z