# Mesoscale Modeling of Hydrogels Under Frictional Shear Stress

## TL;DR

This paper uses computer simulations to study how hydrogels behave under friction, revealing how their structure affects lubrication.

## Contribution

The study introduces a computational model using dissipative particle dynamics to explore hydrogel friction at mesoscale.

## Key findings

- Hydrogel friction is governed by the interplay between polymer relaxation and viscous shear.
- Friction increases linearly with the Weissenberg number at higher values.
- Friction coefficient decreases with increasing normal load.

## Abstract

Hydrogels are three-dimensional
networks of hydrophilic
polymers
often used as a simplified model of hydrated biological materials,
from cartilaginous joints to the ocular tear film. However, the lubrication
mechanisms of hydrogels remain poorly understood, partly due to their
complex polymeric structure, which creates blurred interfaces during
sliding that are challenging to study experimentally. In this study,
we employ dissipative particle dynamics (DPD) to investigate the frictional
behavior of a polymeric hydrogel network sliding against a solid wall
in an explicit viscous solvent. This computational approach enables
us to model hydrodynamic interactions and mesoscale polymer dynamics,
capturing key aspects of hydrogel friction. Our simulations reveal
that hydrogel friction is governed by the interplay between polymer
relaxation and viscous shear, characterized by the Weissenberg number
(Wi). At low Wi, friction coefficient
remain nearly constant, dominated by polymer relaxation. However,
at higher Wi, friction is dominated by viscous drag
within a near-wall solvent layer, leading to a linear increase in
friction coefficient with Wi. Furthermore, our results
demonstrate an inverse relationship between the friction coefficient
and the applied normal load, consistent with experimental observations.
This work provides new insights into the fundamental tribological
properties of hydrogels, shedding light on the micromechanics of hydrogel
friction. Improving our understanding of hydrogel structure and dynamics
under friction advances our knowledge of the mechanisms regulating
biological lubrication in health and disease.